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…

Proxima Centauri b

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?

The star:

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.

The signal:

HARPS at the 3.6mMany 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.

PCb_singal
A strong signal in the HARPS data

 

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.

Caveats:

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:

kepler_438bAnother 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.

Further planets:

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.

Exploration:

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…

For more information visit the Pale Red Dot website: http://www.palereddot.org
For more information visit the Pale Red Dot website: http://www.palereddot.org

EPIC-1166 b: a Neptune-mass planet with Earth-like density

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.

NASAK2As 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.

Transit Lightcurve EPIC1166
Transit Lightcurve of EPIC1166

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.

Radial Velocities of EPIC-1166
Radial Velocities of EPIC-1166

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?

Mass-Radius diagram showing EPIC-1166 compared to other exoplanets
Mass-Radius diagram showing EPIC-1166 compared to other exoplanets

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.

EPIC1166_Compositions

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.

impactMy 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.

*EPIC-1166 b was initially (and independently) detected by Suzanne Aigraine and released on twitter.

Single Transit Candidates from K2

Our paper entitled “Single Transit Candidates from K2: Detection and Period Estimation” is out on arXiv today (and under review at MNRAS)! Here is a brief overview:

NASAK2It’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.

NumberSTEsIn 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.

K2SinglesEach 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

coldPlanetThree 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).

Looking for Life – Part I

“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”.

 

Further Reading:

Erwin Schrodinger – What is Life (pdf version here)

Nick Lane – The Vital Question (review)

Southern Skies Above La Silla

In December 2014 and June 2015, I was lucky enough to visit La Silla in Chile. Part of the European Southern Observatory, La Silla is home to dozens of working telescopes. My baby for both occasions was the 1.2m Swiss Telescope, designed specifically to search for exoplanet via the radial velocity technique.

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.

 

Technical details:

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!

The music is Modern Drift by Efterklang, all rights for which are the property of 4AD & Rumraket (used under fair use guidelines).

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.