The Solar System’s has Four New Neighbors

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…

Position of HD219134 in Stellarium

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.

And the prize for funkiest Colour scheme goes to...
And the prize for funkiest Colour scheme goes to…

ssc2015-02b_Inline[1]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!

HD219134dens

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,

 

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Here’s how you can find the star in the sky (and a very neat animation of the transit):

 

The paper by Montelabi can be found on arXiv here

Other coverage includes:

Elizabeth Tasker’s piece on “The closest rocky planet ever has been found… so what?

Sci-News: HD 219134: Three Super-Earths Found Orbiting Star 21 Light-Years Away

Daily Mail: Star discovered with THREE Super-Earths, and one is the closest rocky planet ever found outside our solar system Read more: 

Wasp Planets… as Pokemon: A Chrome App

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.
Pokeball
An exoplanet Transit

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:

  1. Open Chrome and go here to download ‘WordReplacer’
  2. Find Chrome’s ‘Settings’ menu, then the ‘Extensions’ tab, then find ‘Word Replacer’ and click on options.
  3. 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]
  4. 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: PokemonPlanets PokemonPlanets2       . . . . 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!

Kapetyn b – Another One Bites the Interstellar Dust

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.

Kapteyn_b[1]
Kapetyn’s star compared with Earth. Credit: Habitable Exoplanet Catalogue
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.

Activity Tracer showing Rotation Signal & alias
Activity Tracer showing Rotation Signal & 48-day alias

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.

(paper: Stellar activity mimics a Habitable-Zone planet around Kapteyn’s Star )

Shifting Eclipses – K2’s Second Multi-planet System

On March 20th this year, the moon will pass between Earth and the Sun sending a slither of Northern Europe into darkness. For those in the UK, this partial eclipse will be the most impressive eclipse until three minutes of totality at 4:56pm on September 23rd, 2090. Calculating something so far ahead seems like an impressive feat but in fact astronomers can precisely work out exactly when and where eclipses will occur for not just the next hundred, but the next million years. Such is the way for most transiting exoplanets too, the calculations for which could probably be valid in thousands of years.

477859main_KeplerSinglePanelStill[1]But a new planetary system, discovered by a team that includes Warwick astronomers (including me), doesn’t yet play by these rules. It consists of two planets orbiting their star, a late K star smaller than our sun, in periods of 7.9 and 11.9 days. The pair have radii 7- and 4-larger than Earth, putting them both between the sizes of Uranus and Saturn. They are the 4th and 5th planets to be confirmed in data from K2, the rejuvenated Kepler mission that monitors tens of thousands of stars looking for exoplanetary transits. (36 other planet candidates, including KIC201505350b & c, have been released previously).

But it is their orbits, rather than planetary characteristics, that have astronomers most excited. “The periods are almost exactly in a ratio of 1.5” explains Dave Armstrong, lead author of the study. This can be seen directly in how the star’s brightness changes over time. This lightcurve appears to have three dips of different depths, marked here by green, red and purple dips. ”Once every three orbits of the inner planet and two orbits of the outer planet, they transit at the same time”, causing the deep purple transits.

K2 Paper Lightcurve

But this doesn’t just make for an interesting lightcurve; the closeness of these periods to a 3/2 ratio also causes other weird effects. “The planets perturb each other and change their period every orbit, so they never quite transit when you expect”, explains Arms. These shifts are called Transit Timing Variations (or TTVs).

An Example of TTVs in a 2-planet system
TTVs in a 2-planet system (credit: Eric Ford)

The size of these TTVs is related to the mass of the planets, and some previous multi-planet systems have been weighed in this way. When the team went back to observe the larger planet less than 9 months later, they found that the transit time had shifted by more than an hour. And their period ratio of 1.5035 means the resulting TTVs are likely to continue increasing over a few years, potentially shifting the system more than a day from it’s current rhythm.

These TTVs also help prove that the planets are real. Their presence means that both objects are interacting with each other, so the planets must orbit the same star rather than being, say, two different background binaries. The team also used these shifts in transit time to constrain the planet masses, showing them to be less than 1.2 and 2.04 times that of Jupiter.

rvgif2[1]Not only is this one of the most interesting multi-planet systems yet discovered by Kepler, it is also one of the brightest (12th magnitude), making ground-based follow-up much easier than many Kepler systems. Most interestingly, precise spectrographs like HARPS and SOPHIE will be able to measure the tiny to-and-fro shift in the star’s velocity caused by the gravitation pull of planet on the star. This radial velocity would give a precise mass for the planets in the system and for the first time allow masses found by TTVs to be directly compared to those from RVs.

Fomalhaut_planet_341px[1]Examples of 3:2 resonance can be found everywhere in planetary science, including between Pluto & Neptune’s orbits, in the Kirkwood gap of the Asteroid Belt, and even between the planets around pulsar PSR1257+12. It is also thought that Jupiter and Saturn may have, at one point, become caught in a 3:2 resonance as they migrated inwards. This scenario, of planets caught in 3:2 resonance migrating inwards, could explain how these two sub-Jupiter sized planets came to be in such an unusual orbit.

These two planets could also help settle other dilemmas. “We’d like to answer questions like ‘Did they form there?’, ‘Did they migrate there and get stuck?’ and ‘will they eventually get ejected from the system, or crash into the star?’” suggests Armstrong. The best way to do this is simply by watching future transits and monitoring just how in-sync the planets really are. And maybe one day we could even begin to predict their eclipses as confidently as we can with those happening here on Earth.

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The paper, submitted to A&A, can be found on ArXiV here. My work on the paper involved developing the tools to find the transiting planets in the K2 lightcurve.

A History Of Planet Detection in 60 Seconds

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.
Made by Hugh Osborn
Made by Hugh Osborn

More Details

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.

Lunar Mission One – Can It Succeed?

10349900_1523807731203424_1290925223437780697_n[1]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 mission for everyone. (Where, as far as I can work out, ‘everyone’ is those who have donated sums of money to the cause).

lunar-mission-one_3110612k[1]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.

Lockheed build the Phoenix Lander
Lockheed build the Phoenix Lander

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?

google-lunar-x-prize[1]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.

China's Moon Probe, Chang'e
China’s Moon Probe, Chang’e

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%.

An Open Letter to the RAS – The Writings of Adrian Berry

Dear Members of the RAS council,

I wish to bring to your attention the writings of RAS fellow Adrian Berry. You may know him as the Telegraph’s astronomy columnist, writing once a month in Britain’s most-read broadsheet and reaching a potential audience of more than half a million people.

However, his recent column seriously calls into question his scientific judgement, to the extent that I believe his membership to this esteemed society should be reviewed.

AdrianBerry

He states that Global Warming is a myth, that 500 million years of geological record have proved this to be so, that Carbon Dioxide has no effect on surface temperatures and that the un-peer reviewed writings of two authors (Svensmark and Calder) are somehow more credible than the tens of thousands of peer-reviewed science (see attached article).

As a society, we should be all for scientific debate and should not just discard unpopular hypotheses. However, an unbalanced piece in the Astronomy column of a national newspaper is neither the way nor the place to do it. Instead it is a reckless attempt to discredit climate change in the eyes of the public. For a public-facing astronomer with a huge audience and the credibility of this society behind him, stating these concepts as facts is disgraceful.

His article also completely disregards the charter and bye-laws of this society. This includes: “Act with skill and care in all scientific work”. “Take steps to prevent corrupt practices and professional misconduct.” “Declare conflicts of interest“. And, most pertinent of all: “Not knowingly mislead, or allow others to be misled, about scientific matters.”

Standing by such members damages the reputation of our society and I believe (along with many other fellows I have spoken to) that his membership should be reviewed and revoked.

Yours sincerely

————————————————————-

Hugh P. Osborn
Fellow of the Royal Astronomical Society (FRAS)
Research Student (Ph.D)
Astronomy and Astrophysics Group,
Department of Physics,
University of Warwick

UPDATE 1: A copy of this letter was forwarded to the members of the RAS Council. I shall update this post with any decisions that are made.

UPDATE 2 (26/02/2015): The issue was extensively discussed at the most recent RAS general council. The final conclusion was that the specific case of Adrian Berry would not be taken any further. This was in part due to issues with free speech and the fact that, while being a member of the RAS, Berry did not specifically state his membership in the piece, so was not directly using it as credibility. The current process would also require a general vote of fellows to remove a member of the society.

However, with the RAS bye-laws being updated this year, a new Code of Conduct will be created that fellows will be expected to ratify on a yearly basis. This code, which can be viewed on the RAS website, will more easily allow the RAS to discipline fellows breaking this code and take action more easily.

I was not expecting the issue to even be discussed, so full credit to the RAS for listening, especially to President Martin Barstow who contacted me directly with the result. I understand the council’s decision in this case, especially as I feel any disciplinary action on Berry would probably serve to merely increase his (false) belief of a global warming conspiracy. But the new code of conduct is certainly a step in the right direction. Whether the RAS, which has never historically removed a member from it’s ranks despite worse cases than that of Berry, will actually discipline those breaking this code remains to be seen.

Frequently Asked Questions.

For anyone wondering, in the most basic terms, what it means to find new planets and how we are able to do it, here’s a short blog I put together based on what people seem to want to know…

So what do you do?

I use telescopes to hunt for planets around other stars.

How does that work? Do you just find new dots of light in the sky?

Kepler-62f_with_62e_as_Morning_StarStars are really bright. The Sun, for instance, shines tens of billions of times brighter than any of the planets. Not only that, but the stars we see in the night sky are also incredibly far away. Imagine holding the Earth and the Sun within your palm. (By the way, the Earth in this scenario is the size of a human red blood cell). On that scale, Mars is just beyond your wrist and Saturn is by your elbow. The nearest star, however, is still more than 7km away! And what we want to do is spot a fleck of dust around that. It is as impossible as it sounds – even with the best telescope on Earth, we can’t spot planets directly in this way.

So how do you know if a planet is there?

While we may not be able to spot the planet’s light directly, there are hints in the starlight itself.

Orbit3

Everything that has mass has a gravitational pull; and while most people know that the mass of the Sun keeps the Earth in orbit around it, few people know that the Sun is also in motion around the Earth. That orbit is a tiny circle: only 450km in radius compared to the 150million km orbit the Earth follows once a year. Larger planets like Jupiter cause correspondingly larger effects. So, by detecting the motion of a star, we can figure out how big a planet is, and how far away from the star it must be.

This was how the first planets were found 20 years ago; by splitting starlight into its colours and using tiny shifts in the ‘barcode’ of elements to determine the motion of the star. It’s also how the new Gaia satellite will hopefully find tens of thousands of new planets, by precisely measuring their locations and spotting these tiny wobbles.

eso_planet

Another method is to hunt for the rare occasions when planets pass in front of their stars, like the moon during an eclipse. However, even the largest planets only block less than 1% of the starlight during these transits. So to find planets this way we not only need telescopes that can look at thousands of stars at a time, they also need to be really accurate at measuring the star’s brightness. This is something we have been able to do from the ground, with surveys like WASP and HAT, and from space with telescopes like Kepler.

So which method do you use?

I hunt for planets using the transit method. At the moment I am trying to find long-distance Jupiter-sized planets using the WASP survey. This uses 16 relatively small cameras in South Africa and Chile to follow the brightness of millions of stars. So far we’ve found over 100 planets this way, with dozens more in the pipeline.

How many have you personally found?

Finding planets really is a team game. I have helped out at every stage from identifying the first signs of planet transits, to following up transits with better telescopes to check they’re real. So, depending on how you count, somewhere between zero and half a dozen.

Do you get to name them?

Nope :(. Any planets we find get named after the telescopes, eg WASP-134b. Not exactly the most captivating names, I know!

Have we found any planets like Earth?

EarthMoonSmaller planets, especially ones far away from their star, are a lot more difficult to detect. To detect such planets using the radial motion of a star requires measuring the star’s velocity to better than 10cm/s. That’s slower than your average tortoise! Remarkably, there are some instruments being built that could do it.

As for the transit method, Earths crossing their star dim the light about 0.008%. That’s about the same as watching for a piece of dust crossing in front of a lightbulb. But, remarkably, we can do it! The Kepler space telescope was able to monitor the brightness of more than 100,000 stars down to a few parts per million. And it found dozens of Earth-sized worlds that look very similar to our own.

Will there be Aliens on them?

Maybe. Thanks to missions like Kepler, we now know that there are hundreds of billions of planets like the Earth in our galaxy. And we know that life sprung into existence on at least one of them. So there’s no reason to think it wouldn’t have done so elsewhere else too.

Whether intelligent life is common is another question – we’ve been looking for decades and found absolutely no evidence. My own feeling is that, while simple microbes might be common in the universe, technologically advanced life probably isn’t. It took 4 billion years and a handful of chance events for intelligence to evolve at all. And the way our species is going, surviving even for a few thousand years more (~0.0001% the lifetime of the planet) seems unlikely.

Please feel free to ask any more questions in the comments! I will add some if and when I think of them.

Gliese 581d is an ex-planet

Gliese581TopTrump
Exoplanet poster child

If, in 2009, you asked 18-year-old me to name an exoplanet, then Gliese 581d would have been it. Discovered by an American team of astronomers in 2007, it was, for a long time, the poster child for exoplanetary science. Not only was the first rocky world ever found in the habitable zone of its star where life-friendly temperatures are found, it was also relatively nearby (for astronomy standards) at only 20 light years.

Astronomers used the radial velocity technique to find the first planet around Gliese 581 as far back as 2005. This method relies on the gravitational pull that a planet has on a star as it orbits. This wobble is detectable in the spectra of the starlight, which gets doppler shifted as the star moves back-and-forth, allowing the period and mass of an orbiting planet to be determined. While the first planet, ‘b’, orbited close to the star with a period of only 5.4 days, it was joined by two cooler (and more habitable) planets, ‘c’ and ‘d’ in 2007. This was soon followed in 2009 by Gliese 581e, the smallest planet in the system on an even shorter (3.1d) orbit.

RVgif
Movie credit: ESO

Things started to get even more confusing in 2010 when observers at the Keck observatory announced two more planets (‘f’ and ‘g’) orbiting at 433 and 37 days respectively. This would put ‘g’ between ‘c’ and ‘d’ and right in the middle of the star’s habitable zone. However, new observations of the star with a Swiss telescope showed no such signal. Was there a problem with the data, or could something else be mimicking these planets?

article-2003824-006ED8C000000258-933_634x565[1]
Other stars, just like our sun, have extremely active surfaces
One problem comes when we consider the star itself. Just like our own sun, most stars are active, with starspots skimming across the surface and convection currents in the photosphere causing noise in our measurements. These active regions can often mimic a planet, suppressing the light from one side of the rotating star and shifting the spectra as if the star itself were moving back-and-forth. Add to that the fact that, like planets, activity comes and goes on regular timescales and that cool stars such as Gliese 581 are even more dynamic than our pot-marked sun, and the problem becomes apparent.

The first planet to bite the interstellar dust was ‘f’. At 433 days, its orbit closely matches an alias of the star’s 4.5-year activity cycle, and it was quickly retracted in 2010. Similar analyses with more data also suggested Gliese 581g was also likely to be an imposter, but the original team stuck by this discovery. For the last 3 years, this controversy has simmered, until last month all the data available for Glises-581 was re-analysed by Paul Robertson at Penn State. This showed that not only is Gliese 581g not a planet, but that the poster child itself, Gliese 581d, was also an imposter.

CorrectedPeriodogram
The signal strength of any potential planets with (red) and without (blue) activity correction.

To do this, the team took all 239 spectra of GJ581 and analysed not just the apparent shift in velocity, but the atomic absorption lines themselves. Using the strength of the Hα absorption line as an indicator for the star’s activity, they compared this to the residual radial velocity (after removing the signal from planet b). This showed that there was a relatively strong correlation between activity and RV, especially over three observing seasons when the star was in a more active phase. They also found that this activity indicator varied on a 130 day timescale.

581_orbits[1]
The new system with only 3 planets
When the team removed the signal from stellar activity, they found that planets ‘c’ and ‘e’ were even more obvious than in previous searches. However the signal for planet ‘d’ dropped by more than 60%, way below the threshold needed to confirm a planet. Even more remarkably, ‘g’ does not appear at all. So what exactly caused this ghostly signal. The planet’s orbital period of 66 days gives us a clue -it is almost exactly half that of the star’s 130 day rotation cycle, so with a few fleeting starspots and the right orientation, a strong planet-like signal at 66 days results.

This case of mistaken identity is a sad one, but thanks to the incredible progress of our field in the last 5 years, their loss barely makes a dent in the number of potentially habitable exoplanets known. Instead, it acts as a warning for planet-hunters: sometimes not all that glitters is gold.

The results are also explained in exquisite detail at Penn State University’s own blog, including an excellent timelapse showing how our understanding of the Gl 581 system has changed over time