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…
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).
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.
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.
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:
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.
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.
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…
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.
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?
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.
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.
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 onto a white background, and I gladly took the chance to run it forward to the present day. So here it is up to July 2018:
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!
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.
It’s now more than a year since the Kepler space telescope renewed its search for transiting exoplanets. It has spent that time looking at a series of parts of the sky for 80 days at a time, substantially shorter than the continuous 4-year view of the original mission. However, the number of planets it should be able to spot as they cross their star remains the same. So, with K2’s limited campaigns, it should see many long-period planets that transit only once.
In fact, using a couple of different estimate for how many planets are out there, we expect to see a detectable single transit around every few thousand stars in K2. Given that it looks at nearly 20,000 at a time, there should be a lot out there to find!
But without seeing a second or third transit, there would seem no way to estimate the orbit of the planet. However, with such good in-transit data from Kepler and knowledge of the planet’s parent star from it’s colours, we can begin to estimate such planet’s orbits. To do this, I developed the transit modelling code (Namaste: An Mcmc Analysis of Single Transiting Exoplanets), which estimates the planetary velocity across the star, and uses Kepler’s laws to turn this into an orbital period. To test this code, we tried it out on six known Kepler planets, which gave extremely promising results, with orbital period estimates often within 10%!
We then had to go hunting for them in K2!To do this we ran a transit model along the curve, storing wherever it found a good. We also checked the resulting transits by eye, and found a few more it inexplicably missed along the way. The results of that were the seven dips you see below; our best single transit candidates.
Each of these dips could be caused by a planet crossing in front of it’s star. Alternatively, something else could be cause the signal, such as the eclipse of a star in front of another star. For three of these transits, we think this scenario is more likely, either because the radius given by Namaste is too big to be a planet, or because the period estimated is just too long. For example, EPIC203914123 gave a whopping great period of 200 years! As the probability of seeing a transit scales with distance, we are very unlikely to be seeing such a planet in transit; instead this is much more likely to be a small star crossing a giant star on a shorter orbit
Three more of these single transits are ambiguous – they could come either from binaries or planets. The uppermost signal, however, is exceptionally well-constrained and very likely to be a planet. We estimate that this object, which crosses the 11th magnitude star EPIC203311200, has a circular period of 540 +410/-230 days and a radius 0.51±0.05 times that of Jupiter. If it turns out to be planetary (and I’m speculating here), this would make it a mid-sized gas giant placed at the outer edges of its star’s Habitable Zone. And while gaseous planets are inhospitable to life, the possibility of habitable or even inhabited moons around EPIC203311200b cannot be ruled out.
As with all planet candidates, however, we need to do more work to confirm this hypothesis. This includes weighing the planet with radial velocity observations and searching for contaminating stars with high-resolution images of the star.
But with many similar missions to K2 (for example, TESS and PLATO) on the horizon, finding and analysing the single transits of long-period exoplanets is likely to become extremely important. It could even let us find planets that can be confirmed by the Gaia mission or future direct imaging. Alternatively, such planets could let us use JWST to sample the as-yet unstudied atmospheres of warm- and cold-jupiters for the first time.
I should point out that this paper is under peer review and may change somewhat on publication. We decided to put on arxiv for a few reasons; the first being that, as evidenced by two candidate papers last week, K2 papers just seem to be going that way. The second being that the excellent Planet Hunters team released a similar technique for single transiting Kepler planets last week, and it felt logical to try to release them close together.
I will also be releasing the transit analysis code Namaste on my github this week (it needs a few tweaks and additions before it’s useable by anyone else unfortunately).
I also happen to enjoy photography (some of my work is here), so turned my camera to the skies most nights during both my two-week stays. This video is the result of nearly 10,000 photos taken on those trips. Hope you enjoy it! Feel free to share it far and wide.
I was using a Nikon D3200, a remote shutter (although when this broke, an elastic band over the shutter did the job), a Hama tripod and both the basic Zeiko 18-52mm lens and Sigma 12-24mm wide-angle lens. I also used my GoPro occasionally for the twilight sequences.
In total, I took around 10,000 photos totally around 200GB of photos. Exposure times were from 25 to 40 seconds and ISO levels from 400 to 1600. In total, I ended up with about 60 sequences of ~1hr in length, totaling 1.5 days of shutter open time! The first batch (from December 2014) went on a hard-drive that later broke, so a few dozen other timelapses were lost. I used UFRaw to reduce the images convert to jpg, Panolapse to create the moving timelapses and Lightworks to assemble the video.
I’m not happy with all of the images; some have been poorly reduced, some were set at too high an ISO rate (made worse by the poor noise properties of the D3200). But this is my first (or maybe second) venture into Astrophotography, so I figure I’m allowed to make a few mistakes!
EDIT: The video was, unfortunately, taken down by Vimeo for not constituting ‘Fair Use’ (I believed that a non-profit, educational video such as this adhered to Fair Use, but apparently vimeo disagree). It is now up on Youtube who have a less stringent fair use policy.
The number of worlds discovered around other stars is now counted in the thousands. But, if you were to go out on a dark night and try to spot those planet-hosting stars with your own eyes, you would struggle – only 6% of planets orbit stars bright enough for our eyes to pick out. This is especially true of transiting planets; those that pass in front of their star relative to our line of sight. Of more than 1000 such planets known, only one (55 Cancri) is bright enough to see in the night sky. That is, until today…
HD 219134, nestled between Cassiopeia and Cephus, is remarkable in so many ways. It was first studied with HARPS-N, during it’s Rocky Planet Search. This instrument, a spectrograph on the TNG telescope in the Canary Islands, is able to measure the motion of stars so precisely that it can spot the to-and-fro wobble caused by planets.
Amazingly, this instrument found not just one but four planets around this star; a mini solar system just like our own. The outermost is a gas giant on a 3-year orbit, while the inner three are between the size of Earth and Neptune orbiting once every 3, 7 and 47 days.
At this point, astronomers had no idea if these new worlds transited. But a planet on a 3-day orbit has pretty good odds to pass in front of its star so, taking control of the Spitzer space telescope, they pointed it and hoped. And sure enough, exactly when predicted, the innermost planet blocked out 0.036% of starlight. This fraction is just the surface area of the star covered up, giving a precise measure of the radius of the planet.
Now, with the mass of the planet measured by HARPS and the radius of the planet measured by Spitzer, it’s density can be found. While many similar sized worlds have turned out to be fluffy gas-balls rather than true super-Earths, a density of 5.89gcm-3 puts HD 219134b bang on Earth-like composition. If there was a surface, it’s gravity would be just under twice what we experience on Earth (18.8ms-3). With an orbit of only three days, though, the planet’s star-facing surface is likely to be hot enough to melt!
At only 20 light years away, the newly-discovered solar system around HD219134 is also the closest transiting exoplanet ever found, and one of the 20 closest bright star systems to our Sun. With transiting planets extremely rare, there’s even a chance that this could actually be the closest transiting planet around a bright star (K & G-type).
HD219134’s brightness is also important for astronomers. The brighter & closer a planet, the more interesting ways we can study it. For example, this new world has jumped to the top of the list for those trying to study exoplanet atmospheres. We can also measure the path it takes as it crosses it’s star to determine just how the planet orbits. The outer 3 planets might peturb the orbits of the inner one, causing detectable variations in transit timing (TTVs).
It has truly been a remarkable week for exoplanet astronomy, beginning with the discovery of habitable-zone super-Earth Kepler-452b, and now the detection of the brightest, closest, awesomest transiting planet ever found. And, thanks to a huge array of exciting follow-up options, this will not be the last you’ve heard of HD219134b,
Here’s how you can find the star in the sky (and a very neat animation of the transit):
Last week I gave my first proper talk to a conference of PhD students from nearby universities. Being an easily distracted man, rather than actually write my talk, I decided to spend the day before putting together an animation of the entire history of Planet Detection, from 1750 to 2015. It shows the orbital period (x-axis), planet mass (y-axis), radius (circle size)* and detection method (colour) of the 1800+ planets now known.
The idea of this plot is to compare our own Solar System (with planets plotted in dark blue) against the newly-discovered extrasolar worlds. Think of this plot as a projection of all 1873 worlds onto our own solar system, with the Sun (and all other stars) at the far left. As you move out to the right, the orbital period of the planets increases, and correspondingly (thanks to Kepler’s Third Law), so does the distance from the star. Moving upwards means the mass of the worlds increase, from Moon-sized at the base to 10,000 times that of Earth at the top (30 Jupiter Masses).
The colours are also important – dark blue shows the solar system planets (which include Ceres and Pluto for a few deacades each); In light blue are RV planets, which began the gold rush in 1995 with the discovery of 51 Peg; In maroon are Direct Imaging planets; in orange the microlensing discoveries; and in green those planets found by the transit method.
You might see a few patterns beginning to emerge:
The top left has a dense cluster of large worlds. These are the Hot Jupiters. We know of loads of these, even though they’re pretty rare, simply because they are easiest to find. Being so close to their star they produce the biggest radial velocity signals (light blue) and are most likely to transit (green). Ground-based transit surveys like WASP cant find anything beyond ~15 days, causing the sparse region to the right of this group.
The top right cluster is a population of Jupiter-like worlds that Radial Velocity is best at finding – anything beyond 10 years is too long at the moment to have a full signal.
The bottom group is from the Kepler space telescope. This clustering is the only one that’s actually real and not just a systematic effect. This is because Kepler was capable of finding every type of planet down to ~1 Earth radius. So this clustering shows that there are more Earth and super-Earth sized planets than any other. Hopefully we can begin to probe below it’s limit and into the Earth-like regime, where thousands more worlds should await!
Hope you enjoy it, and feel free to borrow it for your own use!
*Where Mass or Radius were unavailable I used the Mass-radius relations of Weiss & Marcy. Information from exoplanet.eu, so it might be a bit wrong. Thanks to Matt Kenworthy for suggestions. Pulsar planets are not plotted.
73%. That’s what former Minister for Science and Chairman of Lunar Missions Ltd Ian Taylor reckoned the chance of success of Lunar Mission One would be. This number, on the face of it, appears to be reasonably precise. Assembled from a detailed analysis of all the risks, you might think? No. In reality, like much of his talk this evening, it was a fudge – pieced together on the fly, with little scientific substance to back it up.
Lunar Mission One is a crowd-funded space mission. Started in late 2014 by a wide array of collaborating UK institutions which Ian Taylor listed with pride. It plans, in the early 2020s, to send a probe to the Moon and perform cutting-edge scientific research. They pitched the idea to the public via Kickstarter and, by the skin of their teeth, made it to the £600,000 goal needed to start developing the idea. Their ultimate goal is to produce a missionfor everyone. (Where, as far as I can work out, ‘everyone’ is those who have donated sums of money to the cause).
Their scientific principle, at least, has merit. They will fly a large probe to the unexplored Shackleton Crater at the Lunar South Pole and use cutting-edge drilling tools to make a ~100m hole in the lunar ice and rock. The geology of this borehole could reveal untold secrets about the history of the Earth-Moon system and make literally dozens of British Lunar geologists quite happy.
The team will then fill in this hole with “Memory Boxes” – digitised containers that members of the public can, for a small fortune, fill with their most treasured memories and unwanted iTunes collections. These will then sit under the lunar crust for 4 billion years before either being fried by an expanding sun or rescued by some helpful intelligent alien species.
For those of you worried that there might be a limit to the market of moon-based hard-drive storage space, fear not – the Lunar Mission One team might have something else up their sleeve: Someone will take it over! Ian Taylor banded around a couple of names. Lockheed Martin for instance. For too long have these private contractors worked for contracts based on money – the time is right for them to start backing space missions for free…
Despite lots of long-winded answers-that-weren’t-quite-answers, Ian Taylor did not really fill us in on just how such a mission might be funded. The £600,000 raised so far is only 0.1% of the budget needed to get a space probe to the moon. He suggested international collaborations could easily get the funding necessary, but with most western countries already invested in their own agencies (eg ESA), where and why would any extra funding end up in a British company’s hands?
The Google X-Prize contenders, who set to reap a $35million bonus for landing on the moon, tell a cautionary tale. Backed by a combination of crowd-funding and business investments, not one of the half-dozen teams involved made it to the Moon by the 2012 deadline. Twice this has been extended, and twice the teams have failed.
Even private investors such as Virgin Galactic or Space-X, who have a profitable business model, have struggled with the costs and timescales involved with spaceflight. And these, which Ian Taylor was so quick to draw comparisons to, are profitable ventures. Lunar Mission One has even less potential for generating income than Mars One, another independent space mission that looks destined for failure. Most would agree that crowd-funding has its limits, and £600million is above that limit. Way above.
Another question that springs to mind is why, if the scientific concept is so good, did the institutions involved go down a private route? Why not propose directly to ESA, and face the potential of €650million reward to build the space mission? I put this to Ian, and he tried to convince me that ESA (and NASA) was hell-bent on Mars and not on the Moon, whereas the scientists he had spoken to were adamant that lunar missions are more important. (NB: Well of course they were, Ian; you spoke to Lunar Scientists. If I only spoke to the dozen White Dwarf astronomers in my department then I’d probably get the idea that the only necessary mission was to send a craft to Sirius B!) And ESA missions to Jupiter, a comet, the Sun as well as two exoplanet satellites prove that is not really the case. The selection process is done by numerous committees that select programs based almost entirely on their scientific potential. That lunar geologists cannot get their missions selected in telling.
In reality, most planetary scientists would agree that there are still other more interesting places in the solar to explore than the Moon. Many of these destinations, such as our planet’s near-twin Venus, are also relatively thin on the ground when it comes to future missions. The Moon, however, is an easy target for newly-emerging space agencies such as India and China. One can even imagine a manned mission before Lunar Mission One even launches.
I was cautionary optimistic when before hearing Ian Taylor and the Lunar Mission One concept. Now, after an hour of name-checking and avoiding difficult questions, I feel the opposite. The whole mission seems to lack any clear sense of direction. It seems like they caught their £600k target almost by surprise, like a dog chasing a car. Now, from that unlikely position, they must raise £599million more (£15 for every working adult in the UK) for a mission that, compared with the exploits of Rosetta, sounds uninspiring. 73% suggests Ian? My guess would be more like 0.7%.