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.

KIC1255

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.

Summary

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. 

Exo-Arxiv

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.

 

Every Exoplanet – An Animation

This animation shows a zoom out from 0.015 au to 25 au in a hypothetical solar system containing all the exoplanets we have detected so far, plus the planets in our own system. Each planets colour indicates the type of star they orbit, from cool, M-dwarf-orbiting planets in red to planets around hot, F-types in blue. Solar system planets (and their orbits) are in green. (Click here for a 75mb .gif version, and here for a stremable version).

I hope what this animation shows is that we’re getting really good at finding planets close to their star, but when it comes to more distant planets, even just those as far out as Mercury or Venus, we’ve still got much work to do!

Also fun to spot:

  • PSR J1719-1438 b which literally grazes the Sun here, orbiting its tiny neutron star parent once every 2 hours!
  • Trappist-1‘s seven planets are noticeable because they’re on very short orbits and also move really slowly because their star is so low-mass.
  • Highly eccentric planets like which swing through the inner system rapidly are cool. Try to spot HD80606b, the most eccentric exoplanet known!

Exoplanet orbits and radii are taken from NASA’s exoplanet archive. Planetary orbits were generated with PyAstronomy’s Keplerian motion package. Orientations were randomly generated.

Saving Britain’s Global Reputation

One of the things that makes me most proud about Britain is how internationally-facing it is. I’ve met Swedes who go to London more than Stockholm, scientists who long for an international conference in Cambridge, Americans who go to the Edinburgh festival, Chileans saving for a trip to London and the Harry Potter Studio Tour, etc. And I love that for decades we’ve attracted the smartest and best people from around the world to work in our banks, on our health service, in our universities. And I don’t think anyone can argue that’s not a good thing. How can taking clever, motivated people from a different background and putting them into your town or city not contribute to society more than detract from it?

But you know that image we cultivated? The one of an outward-facing, forgiving nation? Of a global hub mid way between Paris and New York? Of a liberal yet enterprising island? It is dead. A knife has been taken and thrust into the heart of the identity we projected to the world. Over the past year what I have heard, from foreigners in the UK and people I meet around the world, is somewhere between sorrow and confusion. Suddenly their image of a global Britain has faded. In place of the positives is bland food, rain, and resentment.

And all that was done in the space of one day in June. Damage that will probably take a decade to fix. European doctors, nurses, engineers, scientists, students who would once have jumped at the chance of a year or a life in Britain, are staying away. Those already here are thinking of going home.

And it need not be because anything has changed (although it has). Even if the mood amongst the British towards foreigners hadn’t degraded (and it has – xenophobic resentment is being voiced far louder than before), the spectre than 52% of the population would face economic hardship just to be rid of you looms close. Even if Brexit didn’t effect the sources of money and jobs the previously drew in international workers (and it has – EU science collaborations are shunning UK members, banks & car manufacturers are leaving), the threat of a cut-crazed Tory government going havoc on those industries is enough.

When I meet people abroad and Brexit comes up, I apologise. I try to heal the wound; stick paper over the cracks. But a sceptical eyebrow remains raised. And so it should. The only way Britain can prove that we are still a global nation is with actions. Tearing down meaningless financial and bureaucratic barriers in the way of international immigration. Increasing the funding in science, technology, research, etc, beyond what the EU was previously contributing. Becoming a brain sink rather than a start brain drain. Condemning and stop the acts of xenophobia and racism. And remaining open-for-business to international collaborations (and pay our fair share for the privilege). Only then, and with many years of hard work, can Britain fix its international reputation.

But that can’t happen with a government that’s hell-bent on creating an introspective, austerity-ridden tax haven. The 8th of June is our last chance to save Britain’s global reputation. Lets take it.

300 years of Planetary Discovery in 30 seconds

I have updated my popular exoplanet graphic (which got to 1 million views in 2015) to 2017. And made it far prettier, I’m sure you’ll agree. As before, the x-axis shows orbital period (which can also be thought of as “distance from star”), the y-axis shows planet mass, and the colour shows how the plants were found.

(Click the image for a more high-resolution version, and feel free to use it in any way you see fit.)

 

 

 


And, as a bonus for finding this page (I wont be publicising this anywhere): this is what the future of exoplanet detection (probably) looks like:

This includes estimates of exoplanet hauls from new ground-based detections such as transits, RVs & direct imaging, and the space missions TESS, Gaia, PLATO, WFIRST; all capable of finding tens of thousands of planets.

Astronomy Flight Survey

As astronomers we study the cosmos from distant observatories and travel to far-flung locations for international conferences. But we, more than most, know the impact such long-haul flights will have on our planet’s climate.

Please fill out this survey, so that we can gauge astronomers’ travel habits and figure out just how much damage our field’s addiction to travel is causing. With this data we can lobby large observatories and conferences, and help change our field for the better.


Please wait while the form loads…