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.
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.
After a week of controversy and embargo-breaking, the actual science behind the detection of Proxima Centauri b is finally here (published in Nature). And it, honestly, is a breathtaking discovery. A terrestrial planet around the closest star to our sun. It proves what Kepler showed: Earth-like planets really are everywhere, including around the star next door. But should we believe it? And is it all that it is hyped up to be?
As you can probably tell from the name, Proxima is the closest star to Earth. Located only 4 lightyears away in the Alpha Centauri system, it is a tiny red speck of light, only visible in a telescope. The reason for it’s lackluster brightness is that the star itself is dimunative. Only 12% of the size of the Sun, it is also 100 million times fainter. Although that may sound bizarre, M-dwarfs like it are the most common type of star in our galaxy.
Many people have hunted for planets around Proxima before. These usually involve monitoring the star’s radial velocity, it’s to-and-fro speed, and searching for the tell-tale tug of a gravitationally bound exoplanet. But until 2016, there had been no luck. That’s when the Pale Red Dot team decided to throw everything they could at the star to try to do what others had not.
Using the HARPS instrument on La Silla (which I am currently sat only 50m from), they took observations nearly every night for 3 months. And, as we found out yesterday, that kitchen-sink technique paid off. They found a 1.5m/s (that’s brisk walking pace) with an 11.2 day signal. And it had a 99.9999% chance of being real. And they found the same signal, hidden just below detectability in the past data too.
Activity and detection:
When the rumours were flying, I urged caution on this potential discovery. One of the reasons being that Proxima is not a quiet sun-like star. It is instead a turbulent M-dwarf. That manifests itself in large star-spots, strong stellar flares and varying shapes in the spectral lines (the bar-code like lines we observe in the colours of the star). All of these cause confusion in the radial velocities, and there have been a few planets (some of which were discovered by this very team) which are now assumed to be simply variability.
But, they have convinced me. One way they have done this is with simultaneous photometry. That means not just observing the star with a spectrograph, but also simultaneously measuring its brightness with an imaging telescope. This photometry also gives a view of the activity of the star, but without any of the doppler signal from the planet. And what the team see is that the photometry (the trends in brightness) matches up perfectly with the activity that is suggested by certain features in the spectra. And that this signal is completely different to that from the planet.
So, I have to say it: it seems unlikely that the strong signal comes from the star itself, and much more likely that we are indeed seeing the gravitational tug of an orbiting planet.
Firstly, we only have a minimum mass for the planet. What this means is that, it could not be less than 1.3Me, but it certainly could be more. That is because the signal from a small planet with its orbit observed edge-on has the same signal as a larger planet observed more obliquely (pole-on). So do not be surprised if it turns out to be larger than this first measurement
M-dwarfs and habitability:
Another caveat is that the planet probably isn’t habitable. I know that flies in the face of every news headline, but hear me out. Firstly, as I’ve said before, Proxima Centauri throws out an abundance of flares. These are so numerous and so strong that they are clearly seen four times in the ~80 nights of photometry. With a planet only 0.05AU away (1/20th the distance of Earth), these flares would have the potential to do damage to any organic molecules on the surface. The paper itself suggests the sterilising X-ray flux could be 400 times that experienced by Earth; and are likely to have been much higher in the past.
Another problem is that any body that close to another, larger body is likely to be tidally locked. Just look at the moon. This proves problematic for habitability. The large temperature gradient from day to night a tidally locked planet sucks the atmosphere (with supersonic winds) to the cold side of the planet. There, atmosphere can gets frozen and be lost. You can break this cycle, but that involves having a very thick (and equally un-earthlike) atmosphere.
One interesting remark was that there seems to be another signal in the data from a more massive outer planet. Now, this signal might be closer to the rotational (and therefore activity) cycle of the star so could more easily be a false positive. But it would not be surprising if, like our own terrestrial planet, it had bigger siblings lurking slightly further out.
As with any exoplanet result, it seems like everything besides a few key details is speculation ( I have even seen some press speculating on the number of continents proxima has!). In fact, details such as its true mass aren’t completely tied down just yet. And even 1.3Me planets can still be un-earthlike; look at KOI-314c for example.
But, unlike most of the ‘earthlike’ planets we have found, there’s a pretty good chance we could actually answer these questions directly. And I don’t just mean with giant telescopes (although those would obviously work too) – I mean actual in situ observations. Crossing 4 lightyears of space currently no more than a pipe-dream, but it’s not inconceivable to think that, within our lifetimes, a probe might set off to see just how earthlike these exoplanets really are. And there’s no question where it will be going first; towards a Pale Red Dot…
How do planets form? Can they migrate through their solar system? What are they made of? What can modify a planet over time? Is Earth, or our solar system, special?
These are all questions that those in our field seek to answer. And there seems, to me at least, to be an easy way of figuring them out: Find More Planets.
As last week’s news of 1200 new planets showed, the Kepler spacecraft is an excellent way of doing that. Even in it’s new and slightly more limited mode of “K2”, nearly 200 planet candidates and at least 50 bona fide planets have so far been detected.
I am involved in a collaboration between 7 European universities to search for and confirm planets in K2. So far this has resulted in half a dozen papers & planets including the 2-planet K2-19 system. Today I can add one more to that tally: EPIC212521166 b (or 1166 for short).
Finding and Confirming EPIC-1166 b
Initially, we searched the 28,000 stars observed by K2 in field 6; scouring the lightcurves with computer programmes and by eye to spot the repeated dips that might be the tiny signals of planets passing in front of their stars. A handful of candidates including 1166* stood out as promising targets, and we took those few stars to the next stage: radial velocities.
Using the high-resolution spectrograph HARPS, we searched for the star’s to-and-fro motion that orbiting planets should create. In the case of 1166, we saw a strong signal on the same timescale as we expected from the transits.
Then, using a code called “PASTIS”, we modelled the radial velocities, the transit lightcurve and information about the star it orbits simultaneously to pin down exactly what 1166 could be. Almost unquestionably, it was a planet, which was a relief. But we can also tell the size of this planet: it has a radius of only 2.6±0.1 times that of Earth, but a mass a whopping 18±3 times our planet. Combined they give EPIC-1166 b a mass similar to Neptune but a radius more than 30% smaller.
Super-Earth or mini-Neptune?
This makes 1166 b a member of an interesting group of planets: between the size of our solar system’s largest terrestrial planet (Earth) and it’s smallest gas giant (Neptune). So which one of these does our planet most closely resemble?
From it’s density (5.7g/cm3), EPIC-1166b might seem to be closer to Earth than the puffy Neptune (1.64g/cm3). However, densities are misleading for objects so large. The high pressures in the interior of an 18 earth-mass (Me) planet are enough to crush rock and iron to much higher densities than their terrestrial values. This effect is so large that, for a 2.6Re planet to have earth-like composition (70% rock, 30% iron), it would need to be around 50 earth masses! That’s a density nearly three times higher than Earth’s, and clear evidence that 1166 b is not quite as Earthlike as first impressions.
Instead, it seems like our planet must contain something other than just rock and iron. The most obvious candidate is hydrogen gas. This is so light and fluffy that at atmosphere consisting of only 1% the mass of 1166 b (0.2Me) is enough to cover an 18Me earth-like core in a 0.4Re-deep atmosphere, and produce the mass and radius that we see. Alternatively, water could be another component that could drag the density down. For example, if 1166 b was 50% water and 50% rock, it could also explain the composition perfectly. However, this scenario is unlikely, and a hydrogen-dominated atmosphere seems to be the more likely option.
Getting a handle on the interior composition of a planet is interesting, but in EPIC-1166 b’s case it is especially perplexing. Planet formation models show that, once a planet grows to around 10Me, it should begin to rapidly draw in gas from the surrounding gas disc until it becomes a gas giant like Jupiter. In the case of 1166 b, we also have reason to think it likely migrated inwards to its current position through that very gas disc. This is because it is not close enough for tides to affect its position, and orbits in a circular (rather than eccentric) orbit; both pointers to disc migration.
So how did it avoid becoming a gas Giant? One way might be if EPIC-1166 b was a gas giant, but lost all its atmosphere due to UV and X-Rays emitted from its star. However, at 0.1AU and with a surface temperature of 600K (much less than many exoplanets), 1166 b is too far away to have been affected by activity.
My favourite way of solving this puzzle (and it is pure speculation) is through giant impacts between planets. This could both grow a large planet at 0.1AU after the initial planet formation stage, and also blast away a large hydrogen atmosphere. The fact that the star is much older than the Sun (8±3 Gyr) and that we do not see any other planets in the system, further adds to the possibility that this was once a multiplanet system (like K2-19b and c), which destabilised, crashed together, and resulted in a single dense mini-Neptune.
The jury is still out on it’s precise formation. But with EPIC-1166 b orbiting a bright star, there is hope that we can re-observe the planet and tie down it’s size, composition and history even further. And, together with the diverse and growing crop of exoplanets, this new mini-Neptune will surely help to answer those important open questions in our field.
And if that fails we can always fall back on the exoplanet mantra: Find More Planets.
The paper was submitted to A&A and released onto arXiv (http://arxiv.org/abs/1605.04291) on May 13th 2016.
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).
“Why do astronomers look for Earth-like planets in their hunt for life in the universe. Why couldn’t life exist in all manner of ways that we are incapable of contemplating?”
I see this question, put in a variety of different ways, all the time. And to first glance, it is a valid question. Why should we focus on finding Earths? What if, by searching for only the ‘twin’ of Earthlife, we’re missing a plethora of friends and family? I hold the opinion that we’re not. That life formed of carbon, oxygen & hydrogen atoms suspended in water is in all likelihood the most common, and maybe the only, way that the universe is able to ‘do’ life. And that, while the habitable zone (the zone where an Earthlike planet would have liquid water on it’s surface) is not the only place in the universe that these ingredients can get together, it is both most likely and most likely to be detectable there.
So I will lay out my argument here in a series of posts that are almost certainly better explained elsewhere.
What is life?
A question famously posed by Schrödinger in the 1940s, it is also the first key question on our path: To find life, we need to know what we’re looking for…
Defining life may, to first glance, seem obvious. But the boundary between the living and the lifeless is hard to define – just ask any virologist. Definitions of life tend to focus on the fact that all lifeforms are structures capable of self-replication (ie reproduction), movement, growth, and the ability to evolve. But these properties, themselves, are not unique among the natural world. Crystals can replicate themselves and even respond to the environment. Bubbles of fat can inorganically grow and replicate. And anything capable of near-perfect self-replication is likely to feel the process of natural selection.
One of Schrödinger’s great insights about Life is that it does not die. It “evades the decay to equilibrium”, instead exchanging material with the environment to keep alive for much longer than an ordinary lump of matter might. In Schrödinger’s words, life feeds on ‘negative entropy’ (order in the environment) and produces positive entropy (in waste and heat).
But just how valid is this definition? Are we focussing too much on earth-life and leaving out a whole universe of possibilities. Could life be simple, rather than complex; electrical rather than chemical; or even gaseous rather than semi-solid? Give each point a bit of thought and the alternative, to my mind, sound implausible:
Complex – Picture an airless world covered in fine, unstable sand. One day, maybe thanks to a meteorite hit, a cavity is created and a small cascade begins, slowly eating its way into the surface like a wave. I can see this cascade steadily feeding off the gravitational energy of the raised sand, increasing entropy in the low sand left behind. It might even grow over time and then split into two or more such cascades. But is this system alive? My thought would be ‘no’, because of the final argument for life – that it must undergo evolution by natural selection. A single simple process such as this, while ticking all the boxes for life, cannot adapt. The cascade of sand is not determined by any in-built mechanics, but simply the result of disequilibrium.
There is also no easy way to turn one molecule into another and energy. Even the simplest chemical pathways have an energy barrier to get over, such that stored energy and catalysts are needed just to make more energy and avoid death. Then there is the apparatus needed to reproduce not just all that energy-making machinery, but also the structures that hold this all in one place. Life, by its very definition, seems to need to be more complex that the environment it feeds on to evolve.
Chemical – The universe has plenty of other ways of making and transferring energy, as the example above shows. But life requires not only creation of energy, but also to store and use that energy too. While electrical, thermal or even light energy could likely be used (indeed life on earth does), this energy cannot itself form its own storage facility; Light cannot conjure its own mirror. For that, it needs to interact with chemistry. Only the chemistry of complex molecules has the dexterity to perform these storage and reproduction tasks.
Semi-solid – Specifically, life must have a barrier to it’s environment. Just to avoid death and decay, all life on earth continually cycles through its own body weight in energy and resources once every day or so. This is only possible thanks to the fact that our bodies have a semi-solid barrier to the surroundings, with the molecules of metabolism able to leave by diffusion. If metabolism excreted solids (like FeS or SiO), these products would build up and suffocate the organism. If an organism were fully liquid or gaseous, all the important self-replicating machinery would drift away due to diffusion. Life needs to be self-contained, and it likely needs a liquid solvent in which to live…
I hope I’ve shown you that, by simply starting at the definition of life and applying physics, we can get a decent handle on what life in the universe must be like. It seems like that life must be more complex that it’s surrounding; it must be a chiefly chemical system and it must be self-contained. That is a surprisingly narrow conclusion, but surely there could be a myriad of ways to assemble a chemical system such that it fits the criteria for life? We shall explore that next in “The Ingredients of Life”.
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):
Scanning the list of new planets WASP has found (a large proportion of which are unpublished), it occurred to me that we are getting very close to 150 planets! It also occurred to me while making the Underground Map of Wasp planets (see next post), that our planet names are really boring.
So, to both fix the naming problem and celebrate the number of WASP planets, I have decided to turn all Wasp planets into the 150 original Pokemon! Working on Wasp-12 b? Nope – You’re working on Butterfree. Wasp-6? Charizard. Wasp-64? Machoke. Wasp-135 b? Jolteon. IAU eat you heart out…
And while I know this will be a difficult thing to achieve politically, we can at least achieve it indirectly, thanks to the magic of Chrome Apps! Unfortunately I don’t have time to make a completely self-contained app for this, but here’s 4 quick steps to follow to add a bit of early-naughties humour to exoplanet science:
Open Chrome and go here to download ‘WordReplacer’
Find Chrome’s ‘Settings’ menu, then the ‘Extensions’ tab, then find ‘Word Replacer’ and click on options.
Open up this pastebin in another tab, and copy the text (it’s easiest from the “Raw paste data” box at the bottom). [BONUS: Kepler names replaced by 1920s baby names with this pastebin]
Back on Word Replacer, click ‘Import’ and paste the text in. Finally, click Save Settings and you’re good to go!
Then you get to enjoy lists such as this; or papers such as this: . . . . EDIT: Bonus update. Now replace all Kepler planet names with the top 2000 baby names… from the 1920s! Say hello to planets Gertrude (Kepler-127), Salvatore (312) and Ruth (Kepler-11). Use this pastebin in place of the WASP-only one above to get both!
A new analysis of Kapetyn’s Star by Paul Robertson at Penn State University suggests that Kapetyn b, the innermost and most Earth-like of two planets detected in 2013, is not a planet but rather an artefact of sunspots on the star’s surface.
The two planets were detected by Anglada-Escude using the radial velocity technique. This involves tracing the spectrum of the star, the light from which is imprinted with a barcode of absorption lines, to detect minute changes in the velocity of the star. The team used this to spot the to-and-fro (Doppler) motion of the star due to gravitational pull of two unseen planets.
This also allowed Anglada-Escude to place the innermost planet in the Habitable or “Goldilocks Zone”, the region around the star where temperatures might be just right for liquid water to exist on the planet’s surface.
But planets are not the only thing that can influence a star’s spectra – Robertson’s reanalysis of the spectra found tracers for starspot activity which varied on a 143 day period. This caused an artefact signal at 143/3 days, or 48 days: precisely the supposed orbital period of Kapetyn b.
This latest result is the third skirmish in a bitter war between the two teams with three habitable-zone planets detected by Anglada-Escude all now refuted by Robertson.