Tag Archives: detection

Proxima Centauri b

Astrobiology, Exoplanet Detection, , , , , ,

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

Gliese 581d is an ex-planet

Exoplanet Detection, , , , , , , , , , , , , ,
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.

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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

A Planet for Every Star?

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Astronomers have now found an astonishing 1000 exoplanets. But that pales in comparison to the 100 billion stars in our galaxy. So how can we say whether planets are the norm? And is it possible to find a star that is definitively a planet-free zone?

The current crop of alien worlds comes from a limited selection of well-studied stars. Rather than try to directly spot what is the equivalent of a fleck of dust in a spotlight, astronomers use changes in the light of the star itself to tease out the signal of a planetary companion.

eso_planet

This can be done in a variety of ways, each of them with their own shortcomings. Often the method of discovery itself means that only a tiny selection of flukily-aligned planets will have the potential of being discovered.

For example, the Kepler spacecraft was staring at over 100,000 stars to try to detect the drop in light as exoplanets crossed their star. However, the probability of the average planet making this crossing is extraordinarily low. A planet orbiting at 1AU (the same distance from its star as Earth) will be found in only 1 in 200 such systems! To put that in perspective; for each Earth-like planet found by Kepler, 199 more stars with planets exactly like our own will have been be tossed aside.

faceonRV

The other common detection technique, known as radial velocity, is marginally less wasteful. This uses the to-and-fro of the star imprinted in the colour variations (or spectra) to find the delicate gravitational tug of a planet. While this works for planets in most orbits, if they happen to circle their star in a face-on orientation, no signal will be received at all. For both cases, this means that even if no planetary signal is detected at all, we can’t definitively say there isn’t one there.

These techniques are also only sensitive to planets larger than a certain size. While the Kepler mission was able to find Earth-sized worlds, similar transit surveys from the ground will only ever be able to find large Gas Giants. Any Mars or Mercury-sized planets will be missed entirely. Radial Velocity is also limited by size, with Neptune or Super-Earth-sized worlds the current limit. These searches are both also bias towards planets close to their stars. To detect worlds at Earth distances is a much trickier prospect than those scraping the surface of their stars.

So many planets will be missed entirely. How can we talk with any certainty about the number of planets in the whole Galaxy?

DistPlanets

Well, because the exact problems with these techniques are known, astronomers can estimate how many planets we expect to find. If we know the number of stars studied and the probability of an orbit being perfectly aligned, we can use the number of planets found to estimate the number of planets around all stars.

For example, a study of gravitational lensing by planets showed that on average every star has a planet larger than 5 Earth Masses from 0.5 to 10AU. Similar studies have also been done with Kepler, finding basically the same number: More than one planet bigger than Earth from 0 to 2AU around every star. It should also be noted that these results also only cover a tiny portion of potential planets. Distant Jupiters or low-mass rocky planets were missed completely. So, as our searches become more and more sensitive to small and distant worlds, those numbers can only go up. It’s likely that on average every star in the Milky Way has its own Solar System with multiple planets.

But what about lonely, planet-less worlds? There are certain to be stars without any planetary material wandering the cosmos. For example, those dislodged from triple-star systems, as can happen due to gravitational resonance and scattering, might not hold onto any planetary material. But until we’re able to study a star in perfect detail and definitively say no planets exist, we are forced to stick with what has become the default setting: all stars have planets, and it’s just a matter of time until we find them.

Related articles

1000 Exoplanets

Exoplanet Detection, , , , ,

At around midday on Tuesday this week, on a page buried deep in the internet, a small counter ticked over to an important new value. Despite it’s obscurity, the slow and infrequent beat of this clock feels the pulse of an entire scientific community. And it’s one that is gaining vitality and momentum with every year. 1000thExoplanet

This new number was, of course, exoplanet number 1000. It was also joined by numbers  1001 through 1010, which were announced simultaneously by the WASP team. These eleven new worlds were added to the swelling ranks of alien planets that, less than 20 years ago, seemed completely beyond the grasp of science.

While it may sound like a definitive tally, the politics over who keeps track of exoplanet numbers is a disputed area. The figure of 1000 was logged by the exoplanet.eu database which includes some unpublished and contentious planets. The NASA database and US-based exoplanets.org, on the other hand, lag behind with 919 and 755 entries respectively.

But despite the arguments, the real take-home message is that exoplanetary science is an incredibly dynamic young  field. This year alone has seen another 141 new worlds discovered, with more than a dozen expected by the end of the year (Our WASP team has another 30 confirmed planets to publish in the next few months). To put it into perspective; from the first discovery of such a planet in 1994, it took 11 more years to reach 150. Helped by new technology and a ground-swell of funding into the subject, we will reach this tally in one. ExoplanetProgress

A quick analysis of the numbers shows that the number really is expanding exponentially. If we continue to discover new worlds at this rate (as fitted by the x^4 red line above), that number will pass 10,000 worlds by 2029 and 100,000 in only 40 years. It was more than 2000 years ago that Epicurus wrote “there are infinite worlds both like and unlike this world of ours inhabited by living creatures and plants and other things we see in this world” and in the space of only 20 we have proved him right.

New WASP planets published here: http://arxiv.org/abs/1310.5654 , http://arxiv.org/abs/1310.5630 and http://arxiv.org/abs/1310.5607

Related articles and blogs

Rogue Planet or Failed Star?

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It sounds like an interstellar sob story: a lonely planet expelled from it’s Solar System at a young age and forced to wander the galaxy alone. But what makes us so sure such objects are even planets, and does their discovery change how we view the universe?

ps1_lonely_planet-450

More than 2 years ago, the PanSTARRS telescope on Hawaii captured a dim red blob on its sensitive cameras. However, the importance of this dot was overlooked and the image was added to a 4000TB database of images, where the evidence of this discovery sat in wait. More than 18 months later it was rediscovered by Michael Liu and colleagues at the University of Hawaii who decided to take a closer look.

They found the point of light, now named PSO J318.5-22, to be an extremely red object only 80 light years away and floating freely through space. By studying the colours of the object they were able to determine a surface temperature of only 1160K and a mass only 6.5 times more than Jupiter . To begin nuclear fusion in the centre of a star, it needs to be larger than 13 Jupiter masses, making this object far too cold and small to be a normal star.

It is not the first ‘Rogue planet’ to have been discovered, with a further 4 objects found by similar sky surveys. These all have sizes in the region between large Gas Giant Planets (5Mjup) and small Dwarf stars (15Mjup). In all cases, including with PSO J318.5-22, these size estimates are extremely unreliable with a margin for error of up to 5Mjup either way.

startoplanet

Logic might suggest that, if a ball of gas is too small to be a star, it must be a planet. However the boundary between the smallest stars and the largest planets is a very blurred one. The astronomers involved were careful not to call their discovery a planet, instead giving it the label of “late-L dwarf”, similar to a Brown Dwarf (right). That being said, similar sized objects such as the gas giants around HR8799 have made it into the nearly 1000-strong catalogue of exoplanets. So what makes this a special case?

One reason is the loneliness of PSO J318.5-22. In 2006 the International Astronomical Union met for a now-infamous meeting to demote Pluto to the diminutive status of dwarf planet. This decision also came with a new set of definitions for what it takes for an object to be considered a planet. Not surprisingly, clause number one was: it must orbit a star.

While the recent discovery falls down on this particular point, many commentators have pointed out that PSO J318.5-22 may well have been formed around a star before being expelled. This is not as far-fetched as it might sound; many models of planet-star interactions in complicated two-star systems have shown that planets could be tossed around like billiard balls.

starforming

However, there is another option: PSO J318.5-22 could have formed in a collapsing cloud of gas and dust just like every other star in the universe. Such a scenario would completely exclude it from the definition of planet, making it more ‘Failed Star’ than ‘super-Jupiter’. Without further investigations it is impossible to know the answer.

In many ways the question of formation is unimportant: without a star to orbit, these are not planets. It may be a case of  soul-searching but, while the slow cooling of PSO J318.5-22 from warm proto-star to a lifeless ball of gas might interest a handful of stellar physicists, it is conventional planets like our own that can really challenge the understanding of our place in the universe.

Read the paper here on ArXiv