Finding six synchronised sub-Neptunes orbiting HD110067

We have a big press release today on the discovery of the HD110067 system, involving more than a dozen institutions across the US and Europe. That focuses on the science and the broader picture of these new planets. But I thought I would share a more personal viewpoint of the idiosyncratic (and somewhat unique way) of exactly how we found these planets.

The story begins in lockdown 2020. The Transiting Exoplanet Survey Satellite (TESS) had launched 18 months previously and I was at MIT working with the TESS science team. This space telescope scans a large fraction of sky for a month looking for the hints of transiting exoplanets – planets whose orbits pass between us and their stars, causing a brief dimming in their starlight which we can detect. The depth of the dip tells us the size of the planet and, when we have consecutive transits, we also get the orbital period of the planet.

One of my roles at MIT was to help run the candidate vetting process. Two separate automated pipelines churn through the data from all the stars TESS has observed and spit out planet-like signals. Our job is to look through the reports and determine which ones really were planetary (to become “TESS Obejcts of Interest”) and which ones weren’t. In late April 2020, the first hints of planets orbiting HD110067 arrived in the form of the PDC vetting report, and it immediately piqued my interest given the star was extremely bright. The data also showed a handful of clear U-shaped transit signals – good signs of planets – but the automatic pipeline itself didn’t do a great job linking up the various transits. One unfortunate problem with TESS’ observations is that it typically only looks at a patch of sky for 27 days before moving on, so even planets with 15 day orbits might not show two consecutive transits in each given sector. However, ESA’s CHEOPS satellite (the other half of my two-mission fellowship) is a targeted telescope, so we tell it where to point, and in this case I immediately wondered CHEOPS could help.

The photometry from TESS in 2020 – The purple transit marks a 9-day candidate, while the yellow triangles mark our guess for a 5.4 day signal (with a ? where one is missing). The other blue dips marked with “?” have unknown origins.

But the first thing to do was to model the data and see if I could figure out exactly what was going on… There seemed to be five clear transits, but from two or more planets. and so we needed to play a game of “match the dips” – which transits are from the same planet? Two relatively shallow dips 9 days apart seemed to link together well, as did two other deeper dips 5.4 days apart (although they should have produced a third transit, I thought this might have erased by noise). The fifth and deepest dip didn’t seem to link with anything else, so I thought this might just be a single (or mono-) transit. With this solution in hand, we got TESS PI George Ricker’s approval to speed up the TOI announcement process, and immediately gave the transit periods to CHEOPS for observations which… rejected them. It turns out we were about 10 days too late and HD110067 had just slipped out of the observability region of CHEOPS… Dammit!

Two years later

In the mean time, I worked on other projects involving TESS & CHEOPS, finding a bunch of other interesting planets. But by the time spring 2022 came around, I realised that TESS was about to observe HD110067 again! I was interested to see if our friend the monotransit would show her shadow again, as these planets with two transits separated by a large gap (which I have coined “duotransits”) were exactly the types of planets my CHEOPS programme was designed to follow. Luckily the guys at MIT had managed to create a rapid release pipeline for TESS data called TICA, and thanks to some other CHEOPS projects, we had even managed to convince them to release each TESS orbit extremely quickly. This was just the thing we needed, as HD110067 would set from CHEOPS’ view only a few weeks after TESS finished observing.

So I grabbed the TESS data from the new sector, and I have to say I was shocked! There were so many transits! I thought there could be three planets in this system, but it looks like I had been very wrong… After looking a bit more, I found our friend the planet at 9.11 days – so we had at least one planet with a confirmed orbit. But the 5.4 day planet was conclusively ruled out. And there were another four transits in these 14 days of TESS data which appeared new!

The first half of raw TESS data for HD110067 in March 2022. The 9 day planet is marked in yellow, but the origin of the other four transits are unknown…

Time to play a “Pair Matching” game…

Single transits are extremely tricky to deal with – the potential periods are effectively unlimited. But if you can find a second transit, then you drastically reduce the possible periods from infinite down to a few dozen of “aliases”. This is the game I had been playing with other TESS candidates, and it was the game I wanted to play with HD110067. But it’s a lot easier when you have only 2 transits as opposed to 12… How can we figure out which transits are from the same planets? Well, the best way was to play another “pair matching” game, just like for kids. Take the shape of a transit in 2020 and match it with the shape of a transit in 2022. Of course, I did this by fitting models to the transits and comparing the resulting parameters rather than eye-balling it, but the end result is the same – three of the 2020 transits seemed to match with three of the 2022 transits, while two more didn’t seem to match at all.

Playing “match the pair” with the transits. The right figure shows the transits & best-fit models. The left figure shows duration & depth for the unidentified dips. Red clouds are transits seen in 2020 & green clouds are transits seen in 2022 (both ignoring planet c – see next). Stars represent our guess for the transit shapes of 4 planets (2 pairs, 2 singles)

So, now we had three “duotransits”, we could compute those possible periods, and start using ESA’s CHaracterising ExOplanets Satellite (CHEOPS) to hunt for the true periods. Thanks to my position within the CHEOPS team, I could start checking possible periods immediately after TESS stopped observing. We also got the second orbit of TESS data, which didn’t bring any more unexpected transits (we had enough), but solidified our knowledge of the 9d planet. A little bit later we were also able to improve our lightcurve using algorithms developed by Andrew Vanderburg, and this revealed that one of the planets we thought was a duotransit (i.e. with only two transits) actually had another transit hiding in noise, turning it into a planet with a 13.5d orbit – two planets solved, four more to go!

After many observations with CHEOPS for these possible aliases, we finally got lucky in late April and caught a third transit of planet d, with an orbit of 20.5 days! Half of the planets now had orbits – we were sure three more planets were hiding in the system with periods beyond 20 days, but for the moment we weren’t sure where they might be. However, it was at the point that I got the CHEOPS data for this new planet and solved it’s period that a cog clicked into place – the periods for planets b, c and d all appeared to increase by 50% each time – a clear hint for “resonance”.

ESA/CHEOPS transit of planet HD110067d which confirmed a 20.5 day period.

Resonance is like a gravitational dance – the subtle influence of interior and exterior planets (or, in the case of Io, Europa & Ganymede, moons) lock their orbits into tight ratios and maintain this intricate balance, sometimes over billions of years. The planets may slightly oscillate around these precise integer period ratios (2:1, 3:2, & 4:3 being the strongest), but each time their periods drift the gravitational pull of a neighbour brings them back into lock-step. We know of a few key systems like this – TRAPPIST-1 & TOI-178 for example – but even they have certain planets which break from a perfectly “first order” resonance.

I also very quickly realised that the last remaining “duotransit”, which had more than 30 possible periods, was surely at the next position in the chain. And, sure enough, one of those resonances matched almost exactly (with a precision of <0.1%) a 3:2 orbital resonance with planet d. My colleague at the Observatory of Geneva, an expert in resonances, confirmed that the inner three planets were a stable resonance, and that planet e with a 31 day period was also stable and by far the most likely.

So, with the detection planet d, the dominoes started to fall and the next planet in the chain was solved. But the next two candidates didn’t have the luxury of producing two transits in the TESS data… So how to proceed. Luckily, Adrien came up with a plan to test every possible combination of resonant orbits for the outer two planets in order to test two things. The first was whether a planet at that orbit would have been spotted by either TESS or our subsequent CHEOPS observations, and the second was how far that planet was from a resonant equilibrium – effectively planets far from equilibrium are unstable over long timescales, and therefore far less likely to persist.

After trying 2:1, 3:2, 4:3, 5:4 & 6:5 orbits for each of the two outer planets (a total of 25 cases), we found that, remarkably, if the planets were resonant then there was a single likely solution – an orbit of 43 days for planet f and an orbit of 54 days for planet g. All other possible orbits were either ruled out by our data, or appeared far from the stable resonant equilibrium. At this point, we thought it was extremely likely that we had solved the whole system. But convincing the community (and a referee) was not going to be so easy… So we needed independent evidence for the planetary orbits.

Predicted orbital period ratios for planets f & g (with respect to the inner planet), alongside whether they are ruled out by past data or by unstable orbits (typically Laplace angles>70 degrees).

Ground-based telescopes pick up the baton

By this point, unfortunately, HD110067 was no longer visible to CHEOPS. But there was a chance that we might be able to spot the outer planets in data from ground-based telescopes via two methods. The first was to look at radial velocities, which our Spanish colleagues had been gathering using the HARPS-N and Carmenes telescopes. These spectroscopic measurements can precisely measure the tiny push-and-pull caused by a planet orbiting a star, so maybe we could find planets f & g in that data. However, HD110067 is a pretty active star, and the signal from stellar activity dwarfs that of the small planets in this case. However, thanks to some state-of-the-art activity modelling from Oscar Barrangan and Andrea Bonfanti, we were able to recover the signals from three of the six planets, including planet f at 43 days. This was a clear, independent sign that there was a planet at the orbit we predicted.

Radival velocity observations and models. The upper left plots show timeseries for RVs and FWHM (which traces stellar activity), the top right plots shows phase-folded RV signals for each of the six planets, while the lower six plots show the histograms for the amplitude of the planetary signal for each of the six planets.

The second independent sign came via a global campaign of ground-based telescopes at the time of a predicted transit of planet f, which I coordinated. Thanks to colleagues with access to the MuSCAT-2 & -3 telescopes, the Saint-Ex telescope, the Tierras observatory, the NGTS telescope array of 10 telescopes, and a DDT on the LCO network. All these multiple observatories spread across the globe sent their data back to me and, although no single observatory found a clear-cut transit signal, careful analysis of the entire dataset showed a clear hint of a transit at the expected time! So it looked like the resonant orbit of planet f was a certainty.

Detrended ground-based photometry from each of the telescopes used in the global campaign.

But planet g still only had a single TESS transit, and no hint in RVs… So we were resigned to publish the paper like that: effectively with five and a half planets. Until, at a CHEOPS meeting, Rafa Luque, who we had tasked to lead the paper, showed us a plot that Josh Twicken had made, which had re-analysed some “missing” TESS data from 2020, right when our orbits predicted transits of both planets f & g… And there they were – two transits perfectly matching those which we had seen in 2022, but which had been masked due to poor-quality. This was a wonderful confirmation of Adrien’s predictions, with the two planets being once again less than 0.1% away from perfect resonance orbits.

The full TESS & CHEOPS photometry, with the additional “missing” TESS data which revealed transits of f & g in green (and highlighted in yellow)

With that additional TESS data, we had a confirmed system of six resonant transiting planets. The system is by far the brightest resonant system of transiting planets on the sky. The architecture of the system is also unique – it is only system with six planets all in first-order resonances (the strongest), and their orbits are the closest to perfect resonance of any such system. HD110067 is also now the brightest star to host a transiting multi-planet system with four or more planets, and it could even be the brightest we will ever find. The six planets are all sub-Neptunes with radii between 1.9 and 2.9 times that of the Earth. We also know they are low-density, likely with large amounts of hydrogen-helium gas (or possibly steam). This makes them ideal candidates for JWST spectroscopy, although the star is if anything too bright for JWST making many instruments saturate!

But I am most looking forward to the next few years – CHEOPS will continue to monitor the system looking for transit timing variations – the hints of gravitational tugs between planets which can help tell us the masses. I also lead a successful monitoring proposal on the star with CARMENES and HARPS-N which should independently improve our precision on the planetary masses. There is also a very real possibility that the resonance chain is not finished at six planets. And, with our current data, we do not really know what might be lurking beyond planet g on orbits of 75 or even 150 days, although we know they should transit as the system is extremely edge-on. And if that is the case then each additional planet will be cooler and closer to earth-like temperatures… Only time will tell for this brightly shining new star of exoplanetary science.

I created a detailed animation of the system, as well as annotations describing the discovery.
I also created a new mug for the office, featuring the six HD110067 planets.

Four new planetary systems from TESS & CHEOPS

Today we announce four new planetary systems which were discovered via a combination of TESS and CHEOPS photometry through a project I started on the CHEOPS GTO (see our ESA and Swiss press releases).

The problem with TESS

TESS is surveying the whole sky to find new transiting planets around bright stars, and it has done a fantastic job in the past few years – the vast majority of characterisable planets around stars brighter than V=9 come from TESS. However, because its observing strategy involves observing much of the sky for only 27 days every two years, planets on periods longer than around 20 days are unlikely to produce consecutive transits. Instead, what we have seen with TESS, is that many planets produce single transits during TESS observations with large (often 700+ day) gaps between transits. This is hugely problematic as it means the orbital period is still poorly constrained and follow-up characterisation efforts – for example constraining planet mass through radial velocities or atmospheric composition through transmission spectroscopy – are impossible.

This is where ESA’s CHEOPS satellite can step in. When two transits are observed this constrains the orbital period to a range of periods, usually 30-40. By modelling those transits and extracting as much information as possible from the star, transit shape, and planetary system (using my MonoTools code), this can be further reduced to a handful of possible so-called period aliases – we know that one of them must be the true period, but not which.

CHEOPS to the rescue

So how can we recover these lost planets? By using ESA’s CHEOPS satellite. This 30cm ESA telescope is far more precise than TESS, but targets specific stars. But we can target these interesting stars with long-period planets during the times when we expect the transit of one of the possible periods. We chose a selection of candidate planets best-suited to CHEOPS – planets producing shallow transits impossible to spot in ground-based telescopes (i.e. planets smaller than Neptune) orbiting bright stars.

This began a game of hide and seek between the planets and CHEOPS – we monitor the stars and check the data hoping to see a transit which would confirm the planet and its period… but more often than not we would see a flat line, ruling out that orbital period, allowing CHEOPS to move onto another one.

The four systems:

HD 15906 c

This star, much cooler than the Sun, was seen to have a single transiting planet on a 10d orbit in TESS. However, looking at the data, I spotted the sign of a second transiting planet producing two transits. We hunted the true period, and revealed an outer planet on a 21d orbit. This paper was led by Amy Tuson and published in MNRAS.

HD 22946 d

This bright sunlike star was already confirmed to host two transiting short-period planets in a previous paper. They also identified a single transit of an outer planet, but once again we spotted a second transit in the data, and went after it with CHEOPS to find the true period. We were able to confirm it as 47d for the outer planet d, as well as improving the radii of the inner planets (which due to a bug had incorrect values in the previous paper). This paper was led by Zoltan Garai and published in A & A.

TOI-5678 b

In this case, we had been successful with previous systems and therefore decided to try the same technique with slightly larger planets but using less CHEOPS data – hoping to rely on only a fraction of the transit to confirm the planetary period. So we scheduled short observations at the aliases for the Neptune-sized planet TOI-5678, and managed to catch the ingress – the first half of the transit – confirming a 48 day period for this planet. Solene Ulmer Moll, who led the paper, was also able to observe the planet with high-resolution HARPS spectra, with the radial velocities providing a mass. The planet was published in A & A.

HIP 9618 c

For this bright star, TESS initially spotted two transits which were thought to be from an 11d planet, but those transits were actually from two different planets. The inner planet transited again three times in two later TESS sectors, revealing it to have a 21d period, but the outer (and slightly smaller) planet transited only once more. As for the other three systems, we quickly turned to CHEOPS to seek the true period, finding a beautiful transit on the fourth observation. This confirmed a 52.5d period. HARPS-N, SOPHIE and CAFE radial velocities also constrained the masses for these planets, showing them to be low-density mini-Neptunes about ten times the mass of Earth and 3-3.5 times the radius. The RVs also revealed that the system has an outer brown-dwarf companion in the outer reaches of the system. I lead this paper in MNRAS.

Uploading an overleaf paper to arXiv

It’s not always obvious exactly how to proceed, so here’s a quick guide.

1) Fix compilation problems on overleaf

Even though overleaf might still be able to compile a pdf with a few orange/red warnings, arxiv definitely will not. So click on “logs and output files” and go through the bugs & warnings one-by-one fixing them.

2) Download the source

You’ll need a .zip file for arxiv

3) Recompile the source on your computer

Unfortunately that .zip file isn’t usually enough (especially if, like most of us, you use bibtex). So you’ll have to unzip the and recompile the source. Of course, in order to compile the paper, you will need to have latex locally running on your computer, so if you dont (i.e. if running “pdflatex” in the command line give you nothing) then download latex from here

If you now have latex, get the command line up, cd into the (unzipped) source folder, and now run pdflatex [your .tex file], followed by the bibtex [your .bib file], and once again pdflatex [your .tex file]. Verify that the pdf that is generated looks good and has the correct references, and that a .bbl file has been created. If so, you need to re-zip the folder.

4) Upload to arXiv

Now you can start a submission to arxiv. You’ll need to choose the license – I haven’t read much into this, but I usually choose arxiv’s own license for this. Then upload the source and hope that nothing breaks. For the abstract you can copy it from overleaf (most peoples computers should compile any latex maths in the abstract) but make sure to remove any macros and comments.

For the comments, it’s typical to put the paper status (is it submitted, accepted, etc?), the number of tables and figures, and any other accompanying links/data/etc.

Some extra points:

  • Do not upload the proof-corrected version. The journal you submitted to has rights to that – you only have rights to the pre-corrected (i.e. preprint) version.
  • Go through the paper comments and remove anything… silly. Otherwise the overheard on astroph bot might find it.

So you think you’ve found an exoplanet…

One of my day-to-day jobs as an exoplanet astronomer is to look through transiting exoplanet candidates and find those that are actually from exoplanets. However, scrolling through Kepler, K2 or TESS lightcurves is something anyone can do – professionals and amateurs alike. Therefore, it may be useful for me to go through a few of the steps we use as professionals to determine what makes, and what kills, a good planet candidate.

First some key points:

  • The majority of “dips” found by photometric surveys are not planets – they are false positives like eclipsing binaries (stars crossing stars), or even just junk noise from the detector.
  • Even good-looking planet candidates are not, on their own, interesting or publishable. To confirm a planet there needs to be a lot of follow-up time on ground-based telescopes, as well as a lot of complex data analysis.
  • But you can still make a contribution to exoplanet science by finding previously-missed planet candidates and passing them to astronomers.

So let’s get started…

1. What makes a transiting planet candidate?

Planets can be found by searching the shadow they cast as they pass in front of their parent star – effectively blocking a bit of starlight for a few hours. This is the famous “transit” effect. However, we don’t see the shadow directly (except during transits within our solar system – i.e. the transit of Venus which is shown below). Instead, the star is seen as a “point source” with all the light from the star smeared onto a few pixels on a detector. But, by adding up the light from the star across those pixels and plotting it against time (this is “photometry” and produces a so-called “light curve”) we can see the “transit” in the data – a U-shaped dip. We can immediately throw out signals that jump up (instead of down), or asymmetric events which drop down without ever jumping back up again – planets only ever cause symmetrical dips.

The 2012 transit of Venus and associated transit light curve (from JR Davenport)

Planets are also strictly periodic – their signals repeat precisely once every orbit. Therefore, good planet candidates should have repeating dips as is shown below. However, this isn’t strictly necessary – long-period planets may transit only once. But we can still assess these monotransits with the same rules.

A periodic planet seen in TESS

This strictly periodic nature means we typically search do not find planets by looking at lightcurves – instead we search through a range of potential periods and stack data together (e.g. using BLS or TLS). If the period matches the real period, each transit stacks together, and their combined signal is strong, producing a spike on a periodogram. This lets us delve into the noise and find shallower transits.

The phase-folded photometry and periodogram of MASCARA-4b

2. Information from the transit

So you have found a dip… The most useful information now comes from the depth of the dip. The reason for this is obvious when you think about the geometry of a transit – the bigger the transiting object is, the more light it blocks.

Using geometry, the area of the star emitting light is πRs2, while the area of that star being blocked by the planet is πRp2, so the depth is the ratio of these, i.e. (Rp/Rs)2.

Transit depth is influenced by both planet radius and stellar radius (from A. Vanderburg)

Planets are, by definition, objects smaller than stars. Typically, giant planets top out at about 20 earth radii, or about 0.2 solar radii. So, for a star like the Sun, the maximum possible depth from a planet transit is 0.22 = 0.04. So if you found an interesting dip which seems to drop by greater than 4%, then it’s not a planet – it’s one star orbiting another, or what we call an eclipsing binary (or EB). And even dips between 1% and 4% are far more likely due to EBs than planets.

Not every star is like the Sun. But thanks to Gaia there are estimates of stellar radii for almost every star in the sky. If you’re looking at TESS data you can check the TESS ID in ExoFop-TESS, while MAST has data for K2/EPIC IDs and for Kepler IDs. Now you have a transit depth and a stellar radius you can estimate a planetary radius (Rp= Rsdepth).

Giant stars 10 times the radius of the Sun will effectively never have detectable transiting planets around as the limit drops from (0.2/1)2=4% to (0.2/10)2=0.04%. Smaller M-dwarf stars could host planet that are almost as large as their stars themselves, however giant planets are extremely rare around M-dwarfs, and typically the <4% rule holds for all stars smaller than the Sun too.

3. Transit Shape

It’s not only the depth of the lightcurve which contains information about the transiting object – the shape of the transit does too. Typically, us astronomers would model the lightcurve with a transit model, and this would produce parameters including a better estimate of planetary radius, period, as well as all the transit geometry. For this modelling, programs like Jason Eastmans’s exofast2 are good places to start.

However, for just a simple sense of what else the transit tells us, here’s a couple of points:

  • V-shaped dips are caused by objects that do not fully traverse the stellar disc. This means their radius is larger than the radius calculated using (Rp/Rs)2. The exact boundary depends on many parameter, but even a 1%-deep V-shaped dip (i.e. one that does not have a flat bottom) is far more likely to be a binary than an exoplanet.
For the same sized object, a grazing eclipse (turquoise) hides the true depth & size.
  • Transit duration (tdur) should match the stellar density (ρs). There is a bit of weird geometry to do before it’s obvious why this is the case, but transit duration is proportional to the stellar density & planet period where P/tdur3 ~ 5000 ρs. So for a P=10d transiting planet with a 0.1-day transit duration (2.4hrs), the computed density roughly suggests a star slightly denser than the sun (with ρs~2). If this is hugely discrepant with the actual density of the star (as provided on ExoFop or MAST), it suggests the object is in fact an EB. However, the transit duration can vary by a factor of 2-3 here, so only really large discrepancies (a factor of >10) are useful.

4. Out-of-transit features

Transiting planets can be thought of as simple black spheres which are only seen when they cross the star and block it’s light. Eclipsing binaries, however, are due to stars of their own and emit their own light, and therefore we often see their effects out of transit as well. So if there are any of the following features in the lightcurve, we can safely call this candidate an eclipsing binary and not a planet:

Secondary Eclipses

While planets cause one dip per year, eclipsing binaries often cause two – one when the dimmer star passes in front of the brighter star (the “primary eclipse”), and another smaller dip when the smaller star passes behind it’s larger host (the “secondary eclipse”). This might look like one of two things:

1. A smaller dip seen consistently at the same place between each “transit”:

2. A slight difference in depth between odd transits and even transits (caused by two similar-sized stars):

For circular orbits, the secondary eclipse should happen when a star is half way round its orbit (i.e. at phase 0.5). But this is not always the case – eccentric orbits mean that often that the position of the secondary eclipse is offset from this mid-point.

Synchronised sinusoidal variations

Stars are typically variable due to starspots & other effects. However, for planets, those up-and-down wiggles are never linked to the planet’s period. If the wiggles in the lightcurve are synchronised with the transit period, then that’s a sure sign that you’re looking at an eclipsing binary.

That’s because, when the transiting object is massive enough to influence the primary star and bright enough to produce its own light, they can produce sinusoidal signals linked to the binary period due to:

  • tidal effect of the smaller star synchronising the stellar rotation of the primary star so starspots spin with the companion;
  • a hot-spot located at the noon position of the smaller star;
  • the gravitational pull of each star on the other stretching the stars into elongated ovals creating two cycles of “ellipsoidal” variation per period;
  • the velocity of the smaller star’s orbit causing a “beaming” effect where light is red- & blue-shifted into & out-of the colour range that the spacecraft observes;
  • some combination of all of these effects.
Synchronised ellipsoidal variation at the candidate period
Out-of-transit variability at the candidate period

5. Extra Checks

There are a couple of final checks which require a little more effort than merely looking at the lightcurve.

One of the key things to check is: is this already a known candidate? For TESS & K2, this should be obvious on the ExoFop page for that target. For Kepler you can check if it’s on the Kepler candidate list. In both cases, these catalogues should also contain a disposition – i.e. whether the Kepler or TESS vetters have also determined it to be a planet (PC), or a false positive (FP, NTP, EB, etc). If it has already been detected (and at a period and epoch that matches), there is not much that you can do – astronomers already have this on their lists and it will be (or will have been already) published without your help. That’s the way it goes sometimes!

If it’s not known, then there’s a chance you have an interesting target that astronomers missed!

One of the key points is that the dip should be occurring on the brighter target star in the field you’re looking at. If a star which only contributes a small fraction of the light is causing the dip, it must be dimming proportionately by a large amount. For exmple, if you see a 1% dip but it’s actually coming from a nearby star which only provides 4% of the light, then that star must have dimmed by 25% – far too deeply to be a planet! The best way to do this check is to:

  1. Check the lightcurves of nearby stars to see if they are causing the dip.
  2. Check the “centroid position”. The centroid is a weighted average of the flux coming from the star onto the detector, and any shift in this between the in-transit images & out-of-transit images moves is a sign that a nearby unresolved star is causing the dip and not the target star.
  3. See if the depth of the transit increases as a function of the “aperture” size (i.e. the number of pixels used to compute the lightcurve). A larger depth at larger apertures suggests the star is a blended nearby star not at the position of the target star.

Another useful check is using Gaia’s radial velocity and astrometry error – if these are high it’s typically because the target star is in fact two stars which are constantly in motion. This can be done by typing the coordinates into a Gaia cone search and scrolling down to the astrometric_excess_noise_sig and dr2_radial_velocity_error parameters. If either of these are higher than about 5, it’s a sign that the object is a binary (though this is not always true).

6. What Now?

So you’ve found a candidate which seems to pass most of these tests… What now?

As I mentioned at the start, it isn’t just a case of “upload it to the exoplanet database and move onto the next one” – these databases only contain planets that have been published in the academic literature, and have been independently & statistically verified.

So, if you’re simply an amateur scrolling through lightcurves for fun, what do you do now? One nice next step would be to share it on the planethunters message boards as the interesting cases will make their way to astronomers. Alternatively you can directly email the candidate to an exoplanet astronomer (like me), and we’ll let you know if we think the candidate is interesting, and likely pass it on to someone better suited to studying (and eventually publishing) that candidate as a planet. But – fair warning – even if you think your candidate is awesome, and even if an astronomer agrees that it’s likely a planet, that doesn’t necessary mean that it’s easy or interesting enough to get published. Some cases are very tricky and require a lot of extra observations – something that typically only happens for interesting cases.

Another avenue, only to be taken for candidate you are sure are real, is to propose it as a Community TOI through ExoFop-TESS. However, it’s best to get confirmation from an exoplanet astronomer and TESS member before doing this.

I hope this proves a useful guide! Happy hunting.


For more basic info on transiting exoplanets, check out Andrew Vanderburg’s light curve tutorial and Paul Wilson’s exoplanet transit method page.

Astronomers’ Favourite Exoplanet

Occasionally I have wondered which exoplanets are most loved by astronomers. One way to find the answer is to pit planets against each another and see which one survives… something my podcast exocast does every year in the #ExoCup!

But another way to answer this question is to look at the trends in the published literature – which planets get cited, mentioned and observed most often? But doing that would mean scraping through every single published astronomical paper for the last fourty years for every single one of the 4000+ exoplanets…

So, with the help of ADS, that’s exactly what I did. And here is the result, plotted using bokeh as a function of time vesus combined exoplanet mentions. (Click here, or scroll below, for the full interactive version)

And the most-cited exoplanet? It’s a classic – the first world ever to be seen transiting, and the first to have its atmosphere detected – HD209458b. And, as always just following it, is HD189733b – the second brightest transiting hot Jupiter, and the first planet to have its surface mapped.

There are other interesting battles for the Top-10. For example, which is the best planet around an M-dwarf? GJ1214b and GJ436b (both transiting Neptunes) are locked in a tight battle for third place!

And what is the best directly-imaged planet? After a long timebehind Kappa And b and HR8799, Beta Pic has spent the last 5 years as the king of Imagers. But 51 Eri b and PDS 70b are steeply rising to challenge for its crown…

There’s a cluster of planets apparently referenced pre-1995, but the majority of these are simply errors in my paper-scraping code. For example a paper with the sentence “stirrups and coat” would flag the planet “Ups And c” (41 mentions since 1993).

Bokeh Plot

The legend is interactive, and if you want to best compare the paths of two planets, you can use the legend labels to “mute” all the planets and then select only those you are interested in. Click here for a full-screen version.

Why the “Random Transiter” is now the most mysterious star in the Galaxy

Move aside Tabby’s star – a new enigmatic star observed by K2 looks to have taken the title of the “most mysterious star in the galaxy” – HD 139139. Here I’ll try to explain why it’s just so weird.

The Basics

First, a few basics – we use satellites like NASA’s Kepler mission to find exoplanets, planets orbiting other stars, by monitoring the brightness of stars. In some systems, the planets pass in front of their stars, blocking some starlight from reaching our telescopes – a “transit”. The amount of missing light (the transit depth) tells us the size of the object crossing the star, and the length of that dip (the transit duration) is usually related to the speed of the body – i.e. its orbital velocity.

The key thing about these transits it that they are periodic – every single orbit of the exoplanet, it produces a transit. So when we go hunting for these systems, we find repeated dips, and the “light curve” (the change in light over time) looks like this:

K2-3’s lightcurve

Here you can see the biggest planet transiting, like clockwork, every 10 days. There are also two more planets – one causing four transits, and one producing just two. But this light curve shows unmistakable periodicity. Of course, we don’t check that by eye – we run a periodic search on each light curve which tests each period and checks for regular dips.

Of course, when a telescope is staring at hundreds of thousands of stars, it also spots other things passing in front of stars – starspots on a distant star’s surface for example. Or so-called “exo-comets” where we see the comet’s tail cross the star producing an assymettric dip. In very rare cases, planets themselves can end up mimicking comets as we watch them get evaporated by their parent star, jetting dust around the small planet, and causing occasional periodic transits which come and go as dust get created and then burned off.

Other interesting “dips” occur for young stars, which are still in the process of forming. Here there is gas and dust aplenty being thrown around by various processes (planet formation, stellar wind, accretion of material onto the star along magnetic field lines). These produce bizarre assymettric and irregular dips in the light curves of young stars (we call these “dipper” stars), and are something that I have experience in studying.

HD 139139

So you’ve seen what a typical exoplanet lightcurve looks like above. It’s time to show you HD 139139’s lightcurve. It was observed for 80 days by NASA’s K2 mission, with exquisite precision – the typical errors on each point is 30 parts per million (0.003%), and was published by Saul Rappaport et al in June 2019 after being detected initially by citizen astronomers.

HD 139139 / EPIC 249706694

It looks extremely similar to those I showed above – a forest of dips. 28 in total, most with similar depths of around 200ppm. And the dips themselves look similar too – they have the same distinctive U-shape as planets:

But, the more you look and play with the lightcurve the more you see something troubling: they are not periodic. AT ALL. Here’s what happens when you run a “period match” test against a typical planet (K2-110, left), compared what happens when you run it on this lightcurve (right)…

There’s really nothing. There is no way to link these dips, even slightly. Rappaport et al even tried to relax the necessity of perfect periodicity (sometimes so-called TTVs between neighbouring planets pull them away from strict periodicity), but even this seems to not work… In fact, when you compare the timings of the dips, instead of matching regular periodicity of a planet, they match very well what might happen if random times were simply being pulled out of a hat – it really is a “Random Transiter”.

Also interesting is that most of the dips appear similar sizes (200ppm means objects about 50% larger than Earth), except for a single observation of roughly double this. This suggests that the objects have mostly the same size. The duration, however, seems to vary wildly.

The team also did extensive work to make sure these were real signals, and not some kind of bizarre instrumental problem. But the dips really do appear to come from this star.

I say star, there are in fact two stars superimposed together in the data – a solar-like G-type star about 1.5Gyr old, and a slightly smaller star about 3 arcseconds away which seems likely to be connected to the brighter star in a binary system.

So what off Earth is going on? In the paper, they go through a series of ideas for what could be happening, but the clearest answer is: nobody knows…

Possible explanations

A K2 multi-planet system

A plethora of planets? – if enough planets orbited the star, they could all be responsible for a single (or possibly two) dips. However, not only is it extremely improbable that 14+ planets would all be in-transit and all have similar radii, they would be certainly on unstable orbits to produce such similar transit durations. This is also true if we suggest some planets are orbiting the smaller star B. And even if we assume the planets “wander” around a true period due to TTVs, they simply cannot wander far enough or be numerous enough to explain the data. So we can rule out classical planets.


A disintegrating planet? – As I mentioned above, a handful of objects have been found with dips that come and go, caused by a small planet in the process of being evaporated sending out a cloud of dust which is eventually blown off. However, in these cases, there is still some degree of periodicity – the body may not cause a transit every orbit, but every time it does it should happen at the same point in the orbit. Also, HD139139’s dips are seen to happen at a minimum 5 hours apart… but such an orbit is likely unstable, and also incompatible with dips that last longer than 5 hours.

The dips of WD-1145

Dust-emitting asteroids? – In some systems, bizarre aperiodic dips have been spotted that appear to be due to planetesimals that are undergoing evaporation (much like our disintegrating planet idea, but with multiple bodies). This is true for the young star RZ Psc and the old white dwarf WD-1145. To me, this is the scenario that looks most similar – the dips can be transit-like but are not periodic (although in the case of WD-1145, they are often followed for more than one orbit, giving a periodicity). However, there are some problems with this idea – namely the fact that all of the transits here are the same depth! These clumps of asteroid should not all be producing exactly the same dust clouds in terms of both size and density – they should be far more variable. Similarly, how are these asteroids (all on different, wide orbits) at just the right orbit to produce so much light-blocking dust?

The varied transits of Kepler-16b

Planets in a Binary system? – One way to avoid periodicity of planets is to place them in a binary star system. I guess in this case we mean in a triple system (with one of the two stars we see being a tight binary & a planet and the other not involved). In this case, the stars are moving, therefore not every orbit produces a transit, and they can have different durations (but often the same depth). Sounds familiar right? There are two ways to do this – place one planet around both stars, or place a planet around a single star. However, both options would require extremely short periods for both the planet and the binary to make so many dips, and the team could not find a stable system for either case which matched the data, and the radial velocity measurements rule out this being a binary system.

K2 Dipper stars

A young “dipper” star? – These are young stars with random clumps of dust raised from the circumstellar dust disc into our line-of-sight, blocking up to 50% of the starlight. However, even if this dipper behaviour is happening on the fainter companion, it do not resemble at all what is observed from the Random Transiter – there is no periodicity, no out-of-dip variability, and all evidence suggests the star system is old and doesn’t have any obvious dust disc (which would show up as excess infrared light). Also, the assymetric dips of these “dipper” stars really shouts “clumpy dust” to me, as does the lightcurve of the bizarre Tabby’s star. The dips of HD139139, on the other hand, appears far more ordered and “planet-like”. However, these dipper stars are poorly understood (as I know well) and it is entirely possible we have found a bizarre new member of the group.

Short-lived star spots? – Another poorly-understood part of exoplanetary science is that we don’t know exactly what stars are capable of. On the Sun starspots last weeks, and sometimes multiple stellar rotations. In this system, the star appears to spin every 15 days, but maybe there’s some rare process where a starspot could bubble to the surface, depress starlight for a few hours, and then dissipate entirely.

SETI? – This option is not mentioned in the paper, but it is a system that is sure to interest those who are certain Tabby’s star is actually an alien megastructure (it’s not). If anything, the coherent dips in this system look more like solid structures than the incoherent wandering of flux during the eclipses of Tabby’s star (though the orbital periods the transit durations suggest don’t really make sense for any solid structure). Some also suggested searching for pi or prime numbers in the signals, but if an alien was trying to get our attention and had the ability to build structures 1.5 times larger than Earth at random orbital periods why wouldn’t they just, you know, send us a radio or laser pulse? To me, it makes far more sense that this system is some previously-unknown natural phenomenon instead of relying on an “alien of the gaps” logic, but there will always be those who want to believe.


The only thing we know for sure is that HD139139 (or its neighbor) has something weird around it. Exactly what that is, we can’t yet say.

The remarkable thing is that, unlike the Tabby’s star case (which imho always seemed like it would be clumps of dust), the symmetric and similar-depth transits here could be equally stellar activity, dust clumps, or bonafide terrestrial planets…

For me, I would lean towards one of the “evaporating asteroids” or the “starspots” hypotheses. But exactly how those scenarios would produce such similar depth eclipses is a real mystery… One thing that will help is, as always, more data – we really need photometry in different colours, which will tell us something about the composition of the objects crossing the star.

But until a definitive theory arrives, we are going to live with that thing every scientist both loves and dreads – the uncertainty of not knowing.

From Planets to Exoplanets – An Updated Animation

My most-cited work is probably not a peer-reviewed paper (to my shame), but a simple gif I made a few years ago. I was asked by to update it recently, and I gladly took the chance to run it forward to the present day. So here are the direct links to a few versions updated in December 2020:

The following embedded version is compressed and therefore not necessarily the best one to download:

Let me know on twitter if you would like any custom versions!

NB: All data from the NASA exoplanet archive. Where masses are unknown, they are converted from radii (e.g. in the case of many transiting planets) using Weiss+Marcy mass-radius relations. While Ceres and Pluto both appear and dissappear from this plot as their planetary status became updated, disproved former exoplanets are not included in this way.

What TESS will do for Exoplanets

The Transiting Exoplanet Survey Satellite is a rather impressive mission! Only approved in 2013, in just 5 years the team from MIT have built an astounding little satellite. I won’t talk about it’s funky lunar-resonant orbit, about its internal hardware, or even about it’s launch on a Space X Falcon 9. Instead I’m going to talk Science. What will it find? Why is that important?

TESS has four cameras, each around 10cm in diameter. These will scan tens of thousands of stars hunting for the blip in light as a planet transits across its star, just like NASA’s Kepler mission did before it. However, TESS’s cameras are about 10x smaller than Kepler and that means TESS will collect around 100x fewer photons. And, given photons are built up over time, consider also that TESS will stare at most stars for only a month where Kepler looked at its field for 36! In both these cases, TESS will capture far less light from the stars it studies, and therefore far more noise. For any given Kepler planet, TESS will see between 20 and 60 times less signal, making easily-spotted giant planets & neptunes disappear into the noise of the TESS data.

However, all is not lost. TESS searches an area around 5.5% of the sky in one pointing  (20x more than Kepler) and will eventually cover 60% after its 2 year mission (100x more than Kepler!). That wide field also means that TESS can focus on closer and brighter stars. In fact the average TESS planet host will be about 3 magnitudes brighter than a Kepler planet. Some of them will even be naked-eye visible! That’s a factor of 16 in photons, which helps claws back some of the losses from its reduced size. Those brighter stars also suddenly mean that following up targets from the ground (either to hunt for more transits, or to measure a planet’s mass with RVs) is so much easier! So this strategy should pay off, in terms of the sheer number of planets (which should dwarf Kepler), and in terms of the number of those planets which will be characterisable afterwards.

The sheer number of TESS planets! From Zach Berta-Thompson

So what will it spot? Well, planets. A lot of them. The Sullivan et al yield paper (which has recently been updated by Barclay et al) showed that, from the 200,000 stars it will study in detail, we can expect nearly 2000 planets. But in the 2 million or so bright stars that are also observed (in the full-frame images it sends back), that increases to a whopping 25 thousand! They includes something like 17,000 hot Jupiter candidates, meaning TESS will detect almost every single transiting hot Jupiter within a few hundred lightyears (and help put out-of-business the ground-based observatories like WASP, KELT, HAT, etc).

TESS’s planet haul will also include a few thousand Neptunes & Super-Earths, and a few dozen Earth-size planets (~50). These small planets, unlike Kepler, will mostly be around small stars. That means, if your personal definition of “earthlike planet” requires a sunlike star (and maybe it should), than TESS is incapable of finding any Earthlike planets. It will find a handful of ~1Re planets with similar surface temperatures to Earth around M-dwarfs, however.

I’ll finish off with some personal thoughts. I have to admit, I was a big sceptic of TESS until a couple of months ago! I thought its tiny cameras would only find M-dwarf planets & hot jupiters. However, having run my own yield simulations, I realise I was wrong! TESS is more than capable of finding interesting small planets around stars larger than M-dwarfs! For example, it should find hundreds of 1.5-4Re short-period planets around bright K and G-stars in the full frame images. Far brighter stars than for Kepler! The mass-radius plot after TESS is going to be far better filled-in than it is now!

Despite the fact it will find lots of interesting rocks planets, the recent news articles calling TESS a “habitable planet hunter” are simply bad science journalism (although something we’ve come to expect from the NASA press-release hype-machine). The coolest thing about TESS is not that it will find a few flare-scorched M-dwarf worlds, but instead that it will find so many other new exoplanets, from super-Earths to hot Jupiters. Those sheer numbers will allow statistical samples of well-characterised planets in ways we’ve never even previously considered.

Something I think we should not expect from TESS, however, is huge bulk announcements of “validated” TESS planets. TESS’s wide field means it has enormous pixels compared to Kepler. That means there is far higher chance that there’s another star (such as a binary) hiding within the pixels, something Kepler did not have to worry about so much. That might mean that many more of TESS’s candidate planets remain candidates (my guess), or maybe that, thanks to our ability to follow-up these candidates from the ground, we get far more planets and far more information on those planets than we did with Kepler. Only time will tell!

All that remains is to say good luck to TESS on its SpaceX launch & it’s journey out to its resonant orbit (via the moon)!

On a side/personal note – what was supposed to be a “birthday present” for me (launching on 16th April) is now more of a “memorial” to my grandmother Alice, who passed away on the afternoon of the launch (18th April 2018) at the age of 97. RIP. 


Every day I check Cornell’s now-universal arXiv for new exoplanet papers, and scan the most interesting ones to get a gist of the new research happening. So why not share those short summaries as a Hogg-style research note! (

I have decided to coalesce relevant images into a single metaplot. I’ll use U and L for Upper and Lower, and L and R for left and right)

Friday 15th Dec 2017

Shallue & Vanderburg: Identifying Exoplanets with Deep Learning: A Five Planet Resonant Chain around Kepler-80 and an Eighth Planet around Kepler-90

NB. I have scanned this paper but, given that I am studying this exact area, I will leave an in-depth review for a later date…

This paper covers the use of a deep convolutional neural network to search for Kepler planets. Despite the impression the press release gave, this is far from the first use of machine learning in exoplanet astronomy. But it is certainly an interesting one! They used the Kepler TCE (threshold crossing event) list as training set and set about classifying these transit signals as either planets or false positives, punishing the NN when it classified each target wrongly. The result was a DCNN that correctly identified planets 98.8% of the time.

One interesting technique they used was to send both “global” (whole phase-folded lightcurve) and “local” (transit-only) flattened lightcurve data into the neural net seperately, thereby insuring the small percentage of time spent at transit was adequately important to the NN. The model was also tested with injected transits & eclipses to simulate real data, and inverted lightcurves to simulate systematics. The Kepler team’s RoboVetter still outperforms this approach, though, especially for non-standard vetting approaches (such as faint secondary eclipses).

When searching known multi-planet systems, the neural net found 30 new signals that it said were more-likely-than-not to be planetary. Given they already have confirmed planets, these are highly likely to be real, allowing Kepler-90i and Kepler-80f to be validated as planets, making Kepler-90 a solar-system-twin* with 8 planets!

*Not actually that solar-system-like

Meisner,+: A 3pi Search for Planet Nine at 3.4 microns with WISE and NEOWISE

Does Planet 9 exist? If you asked me in late 2016 I would have said “probably” and yet here we are in late 2017 and I’m seriously questioning it. One major reason would be the Shankman paper which suggested the clustering of eccentric minor planet’s orbits was not due to a massive ninth planet but just because of biases in how we look for such bodies.

Although lack of evidence is not evidence in itself, the lack of any detection after two years of frenetic searching also does not help. This paper added to past infra-red searched using WISE and NEOWISE data to search nearly the entire sky for faint bodies. They found no hint of planet 9 down to a magnitude of 16.7, and fainter than some (optimistic) models of the planet. Those models do suggest, for the majority of cases, that Planet 9 would be undetectable with WISE, leaving much room for doubt. The existence of P9 remains very much in the balance.

Thursday 14th Dec 2017

Li,+: A Candidate Transit Event around Proxima Centauri 


It is what it is – A candidate transit signal of a planet around Proxima Cen (figure L). Except a) the timing and duration don’t fit with the detected RV planet. b) If they come from a new short-period planet, it likely should have been spotted in the RV data (unless it’s unusually low-density). c) If it comes from a short-period planet it certainly should have been spotted by David Kipping’s MOST search. d) It’s ground-based photometry that looks through the notoriously disruptable atmosphere and with notoriously systematic-happy telescopes.

It’s also the fourth (or maybe fifth?) such candidate event, none of which seem particularly good to me (eg the afore mentioned Kipping search, C & TL, and Liu et al’s search from Antarctica, LR). If this weren’t such an interesting and nearby star, we would likely never see papers on such low-significance transit events, which surely happen a lot, especially for notoriously spotted M-dwarfs.  My take: nothing to see here. Proxima b almost certainly doesn’t transit.

Ziegler,+: Robo-AO Kepler Survey IV: the effect of nearby stars on 3857 planetary candidate systems

High-resolution imaging with a big telescope is a necessary step to figuring out whether a candidate transiting planet is real or not. A faint second star might be the source of the eclipse signal (a “Blended Eclipsing Binary”), and even if it is the target star with a planet the light from this companion could make the transits shallower and therefore give inaccurate planetary parameters.

The Robo-AO guys have done a seriously impressive survey of thousands of Kepler planets and candidates to check just that. Of the 3300 candidates studied so far, around 13% showed another nearby source. This paper sums up the stats for the survey so far and adds 84 new companions. For 35 planetary candidates, the re-defined planetary radii push their planets from being terrestrial / earth-like worlds into the non-rocky gaseous regime.

Wednesday 13th Dec 2017

Henning,+: HATS-50b through HATS-53b: four transiting hot Jupiters orbiting G-type stars discovered by the HATSouth survey

Following KELT & Qatar hot jupiters this week, we have 4 more from HAT-South! These guys benefit from (if the weather is good) near-continous coverage of the southern sky thanks to observatories across three continents! I should say that I like the look of HAT-S’s photometry here- very decent quality for such faint stars.

These four are all hot (1000-1500K) planets with super-Jupiter radii, although the masses vary from 0.3 to 2 times that of Jupiter. They’re also ludicrously faint for hot jupiters. I remember teasing a colleague for publishing a V=13.5 magnitude WASP planet but three of these are fainter than that (sorry Jess)! So don’t expect to see HST pointing at these any time soon.

HATS-50 seems by far the interesting of the bunch, as there are hints that it hosts a hot Neptune planet interior to the Hot Jupiter – which would make it WASP-47-like and an extremely rare and interesting system (these are proof that, at least some of the time, planets get swept close to their star in discs and not by dynamics). The signal (lightcurve on the right above) is not too strong though, and it will take TESS observations to tell for sure…

Petigura,+: The California-Kepler Survey. IV. Metal-rich Stars Host a Greater Diversity of Planets

There have been a few papers looking at what happens to planet occurrences when stellar metallicity changes, and they have suggested that metal-poor stars host small planets and metal-rich stars host all kinds of planets (eg Buchhave). With all the time spent with 10m Keck telescope getting spectral information on more than 1000 Kepler stars, the Californian Kepler Survey would seem like the perfect dataset to check this correlation. And, indeed, Eric Petigura finds that this trend is still present – metal-rich stars host both more hot planets, and more giant planets.

Unexpectedly, this paper also compiles standard occurrence rates for all the Kepler planets, using LAMOST stellar parameters for all the Kepler field stars, to give a detailed break down of the average number of planets as a function of size and period, updating the Fressin result of 2013 from 85d out to 1 year. This shows, for example, that there are ~15 warm super-Earths per 100 stars, that hot Jupiters are rare (~0.85%), that mini-Neptunes are very common, that giant planets are more common further from their star, and a whole host of other interesting things! Go hunting through the data if you have the time. I certainly will be using it!

Mroz,+: Can gravitational microlensing detect extragalactic exoplanets? Self-lensing models of the Small Magellanic Cloud

In a twist on the old Betteridge’s law, the answer is yes! Once LSST is running in the early 2020s, we will be able detect “a few dozen” extragalactic (eg SMC) exoplanets through their microlensing events per year. It’s also expected to provide transiting Hot Jupiters too. Although if we thought V=13.5 was faint…

Tuesday 12th Dec 2017

Zwintz: The exoplanet host star beta Pictoris seen by BRITE-Constellation

As some of you may know, I have an interest in exorings, and for that reason this paper piqued my interest. The giant planet that orbits Beta Pic passes close to its star in 2017, therefore photometry could reveal the presence of the rings of an extrasolar planet! So what do these three little space telescopes (BRITE) find… nothing. The 200 days of observations ended in mid-June (before the expected time of Beta Pic’s closest approach), but “no decrease in intensities caused by the transit was observed”. As they say round here, quelle dommage.

Johnson,+: KELT-21b: A Hot Jupiter Transiting the Rapidly-Rotating Metal-Poor Late-A Primary of a Likely Hierarchical Triple System

The KELT team are really ramping up their output, with as many as 10 new planets in the last year. Their wide-angle telescopes mean they focus on the brightest stars, and therefore have found a lot of planets around hot F- and A- type stars. This is often problematic as planets produce shallower depths, RV follow-up is more difficult, and stars have more binary companions. KELT-21b is no exception, with only a 1% transit for a super-Jovian world, and in (at least) a triple system of stars.

To confirm the planet, rather than using RVs (the star rotates too fast!), they used doppler tomography, which recovers the velocity of the region of star covered by the planet (LR). This also automatically tells us the path the planet takes across the star, and can tell us whether its orbit is “aligned” with its star’s rotation (star in UR). Unusually for such a hot star, KELT-21bn seems perfectly aligned!

Monday 11th Dec 2017

Alsubai,+: Qatar Exoplanet Survey: Qatar-6b — a grazing transiting hot Jupiter

This looks like a pretty standard ground-based Hot Jupiter to be honest. Publishing only a single new hot jupiter in a paper is actually getting quite rare these days.

It is grazing though, which is in some ways interesting (not many grazing planets), but in other ways annoying (reduces the atmospheric signal observable with transmission spectroscopy and increases the false positive likelihood).

The detection lightcurve, of course, looks awful (as most ground-based transit surveys do), but the follow-up lightcurves are great. These “grazing” planetary systems often worry me as they are most often mimicked by EBs (even with a seemingly planetary RV curve, see OGLE-TR-33b and WASP-9b). But given there seems to be no variation in the BIS (bisector span – a measure of the shape of the absorption lines), I’d say this is far more likely a planet than something else.

Shkolnik & Llama: Signatures of Star-planet interactions

Can large, close-in planets have a direct observable impact on their stars? In this review chapter, the two authors cover all the relevant results on this, especially the spectropolarimetric studies which might show the strength of hot jupiter magnetic fields, which could even be intertwined with those of their parent stars. Personally, I’m not entirely convinced that any direct “interaction” is there, although clearly some statistical anomalies exist. The significance of the HD 179949 signal (left) seemed to shrink away with re-observation, just like the old claims that HD189433 flared each planetary orbit. But it’s an interesting idea, and I await the paper that conclusively shows evidence for a hot jupiter’s magnetic fields with bated breath.