Category Archives: Exoplanet Detection

The hunt for an exo-Earth: How close are we?

This blog was first published as a guest post on Andrew Rushby’s excellent II-I- blog.

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In the 1890s Percival Lovell pointed the huge, 24-inch Alvan Clark telescope in Flagstaff, Arizona towards the planet Mars. Ever the romantic, he longed to find some sign of life on the Red Planet: To hold a mirror up to the empty sky above and find a planet that looked a little bit like home. Of course, in Lovell’s case, it was the telescope itself that gave the impression of life, imposing faint lines onto the image that he mistook for canals. But, with Mars long since relegated to the status of a dusty, hostile world, that ideal of finding such a planet still lingers. In the great loneliness of space, our species yearns to find a world like our own, maybe even a world that some other lineage of life might call home.

A hundred years after Lovell’s wayward romanticism, the real search for Earth-like planets began. A team of astronomers at the University of Geneva used precise spectroscopy to discover a Jupiter-sized world around the star 55-Peg. This was followed by a series of similar worlds; all distinctly alien with huge gas giants orbiting perishingly close to their stars. However, as techniques improved and more time & money was invested on exoplanet astronomy, that initial trickle of new worlds soon turned into a flood. By 2008 more than 300 planets had been discovered including many multi-planet systems and a handful of potentially rocky planets around low-mass stars. However, the ultimate goal of finding Earth-like planets still seemed an impossible dream.

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In 2009 the phenomenally sensitive Kepler mission launched. Here was a mission that might finally discover Earth-sized planets around Sun-like stars, detecting the faint dip in light as they passed between their star and us. Four years, 3500 planetary candidates and 200 confirmed planets later, the mission was universally declared a success. Its remarkable achievements include a handful of new terrestrial worlds, such as Kepler-61b and 62e, orbiting safely within their star’s habitable zones. However, despite lots of column inches and speculation, are these planets really the Earth 2.0s we were sold?

Even more damning is the size of these planets. Rather than being truly Earth-like, the crop of currently known ‘Habitable planets’ are all super-Earths. In the case of Kepler’s goldilocks worlds, this means they have radii between 1.6 and 2.3 times that of Earth. That may not sound too bad, but the mass of each planet scales with the volume. That means, when compression due to gravity is taken into account, for such planets to be rocky they would need masses between 8 and 30 times that of Earth. With 10ME often used as the likely limit of terrestrial planets, can we really call such planets Earth-like. In fact, a recent study of super-Earths put the maximum theoretical radius for a rocky planet as between 1.5 and 1.8RE, with most worlds above this size likely being more like Mini-Neptunes.

So it appears our crop of habitable super-Earths may not be as life-friendly as previously thought. But it is true that deep in Kepler’s 3500 candidates a true Earth-like planet may lurk. However the majority of Kepler’s candidates orbit distant, dim stars. This means the hope of confirming these worlds by other techniques, especially tiny exo-Earths, is increasingly unlikely. And with Kepler’s primary mission now ended by a technical fault, an obvious question arises: just when and how will we find a true Earth analogue?

Future exoplanet missions may well be numerous, but are they cut out to discover a true Earth-like planet? The recently launched Gaia spacecraft, for example, will discover hundreds of Gas Giants orbiting Sun-like stars using the astrometry technique, but it would need to be around a hundred times more sensitive to discover Earths. New ground-based transit surveys such as NGTS are set to be an order of magnitude better than previous such surveys, but still these will only be able to find super-Earth or Neptune-sized worlds.

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Similarly, Kepler’s successor, the Transiting Exoplanet Survey Satellite which is due to be launched in 2017, will only be able to find short-period planets with radii more than 50% larger than Earth. HARPS, the most prolific exoplanet-hunting instrument to date, is also due for an upgrade by 2017. Its protégée is a spectrometer named ESPRESSO that will be able to measure the change in velocity of a star down to a mere 10cms-1. Even this ridiculous level of accuracy is not sufficient to detect the 8cms-1 effect Earth’s mass has on the Sun.

While such worlds may well have surfaces with beautifully Earth-like temperatures, there are a number of problems with calling such worlds definitive Earth twins. For a start the majority of these potentially habitable planets (such as Kepler-62e) orbit low-mass M and late K-type stars. These are dimmer and redder than our Sun and, due to the relative distance of the habitable zone, such planets are likely to be tidally locked. The nature of such stars also makes them significantly more active, producing more atmosphere-stripping UV radiation. This means, despite appearances, ‘habitable’ planets around M-dwarfs are almost certainly less conducive to life than more sun-like stars.

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So despite billions spent on the next generation of planet-finders, they all fall short of finding that elusive second Earth. What, precisely, will it take to find this particular Holy Grail? There is some hope that the E-ELT (European-Extremely Large Telescope), with its 35m of collecting area and world-beating instruments will be able to detect exo-earths. Not only will its radial velocity measurements likely be sensitive enough to find such planets, it may also be able to directly image earth-analogues around the nearest stars. However, with observing time likely to be at a premium, the long-duration observations required to find and study exo-earths could prove difficult.

Alternatively, large space telescopes could be the answer. JWST will be able to do innovative exoplanet research including taking direct images of long-period planets and accurate atmospheric spectra of transiting super-Earths and giants. Even more remarkably, it may manage to take spectra of habitable zone super-Earths such as GJ 581d. But direct detection of true Earth-analogues remains out of reach. An even more ambitious project may be required, such as TPF or Darwin. These were a pair of proposals that could have directly imaged nearby stars to discover Earth-like planets. However, with both projects long since shelved by their respective space agencies, the future doesn’t look so bright for Earth-hunting telescopes.

After the unabashed confidence of the Kepler era, the idea that no Earth-like planet discovery is on the horizon may come as a surprisingly pessimistic conclusion. However not all hope is lost. The pace of technological advancement is quickening. Instruments such as TESS, Espresso, E-ELT and JWST are already being built. These missions may not be perfectly designed to the technical challenge of discovering truly Earth-like planets, but they will get us closer than ever before. As a civilisation we have waited hundreds of years for such a discovery; I’m sure we can hold out for a few more.

2013 in Exoplanets

The last year has been an extraordinary one for exoplanetary science. With nearly 200 new planets found, and thousands of scientific papers produced, it was tough work narrowing down such a year to a highlights reel. But, without further ado, here is my Top 10 discoveries from the past 12 months:

10. New Habitable Zone Definitions

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In early 2013 a new team took a fresh look at the calculations defining the habitable zone, that hypothetical band around every star that an Earth-like planet would have liquid water on the surface. Using modern data, they redefined the distances and suggested that earth was closer to the dangerous inner edge than previously thought. This led to other interesting habitable-zone related news that I have left out to avoid accusation of vanity.

9. Three habitable worlds around GJ667C

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Radial Velocity measurements of the star GJ667C found two more potentially habitable planets around the smallest member of this unusual triple star system. All three of these planets, c, e and f, made the Habitable Exoplanet Catalogue’s list of the most life-friendly worlds.

8. Seven-planet solar system found

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Reanalysis of the Kepler data by a variety of teams (including the Planet Hunters citizen science project) added a seventh planet to the six worlds already known to circle Kepler-90. With all seven planets circling within the orbit of Earth this not only makes the most numerous planetary system, but also the most compact.

7. ‘Family portrait’ spectra of 3 hot young Jupiters

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Studying the atmospheres of exoplanets is extremely tricky business. But a single measurement with the Hale telescope in California was able to take a peek at the atmospheres of three young planets around the star HR 8799. The team found the distinctive signature of methane in the atmospheric spectra of all three worlds as well as a tentative detection of either ammonia (NH3) or acetylene (C2H2).

6. Kepler and CoRoT die

A review of the year would not be complete without taking a moment to consider those lost.

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In May a fault with Kepler’s third reaction wheel left it seriously wounded. This problem means, after 4 years of service and 3500 planetary candidates, its primary mission is over. Despite its injury, Kepler will still be able to contribute to space science and a secondary mission is due to be chosen in early 2014.

July saw ESA’s CoRoT mission pronounced officially dead. This transit-observing spacecraft suffered a major computer fault in November after 6 years of dedicated service and was unable to be resuscitated.

5. TESS selected

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With death comes new life, and new missions launched and proposed in 2013 look certain to take up the mantle of Kepler and CoRoT.

In April, the Transiting Exoplanet Survey Satellite (TESS) was selected by NASA to launch in 2017. Unlike Kepler, it will scan the entire sky looking for exoplanetary transits, and potentially find hundreds of small rocky exoplanets around nearby stars.

4. Gaia Launched

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Blasting off on board a Soyuz rocket from Guyana in December was Gaia. The most sensitive camera ever sent into space to do astronomy, the space telescope will look for the subtle motion of stars due to planetary companions. If all goes to plan by 2020 it will have found another thousand gas giants to add the current crop of exoplanets.

3. Three habitable worlds found by Kepler

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April saw NASA announce evidence for three new planets discovered by the Kepler mission. These were not the usual crop of Jupiter-like worlds, however. The planets were some of the most Earth-like yet found. Two of these were found in the Kepler -62, with both e and f orbiting within the star’s Habitable Zone and with radii only 40% and 60% larger than Earth’s respectively. While Kepler-62 is a dim dwarf star, Kepler-69c circles a G-type star similar to our own Sun and, once again, the 1.7RE planet was found within the habitable zone.

2. 1000 exoplanets and WASP’s 100th

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In October the number of exoplanets recorded by the exoplanet.eu database ticked over 1000 after the announcement of more than a dozen planets found by the WASP transit survey. These planets included WASP-100 and 101, giving WASP a similar milestone and making the planet-hunting telescopes by far the most successful ground-based transit survey.  By the end of the year, the counter stood at 1056, with a total of 188 new planets added in 2013 alone, and it is likely that 2014 will see even more new planets discovered.

1.  An Earth-like planet is only 13ly away

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2013 saw a fundamental question about the universe finally answered: How far do we have to go before we find a planet that looks like home? Thanks to the wealth of data from Kepler, astronomers were able to definitively say: 13 light years.

By looking at the number of Earth-like worlds in the habitable zone of M-Dwarfs, the most common stars in the galaxy, the Kepler team were able to estimate that 6% of such stars will have their own Earths orbiting them. And while 13 light years is probably far too far away for a visit, proposed telescopes will be able to take a closer look, potentially even hunting for signs of life.

Gaia: Planets and Parallax

In six hours’ time, A Soyuz rocket will blast of from Guyana with the hope of delivering a €1billion Christmas present to astronomers across the world. That present will be Gaia, ESA’s flagship science mission, which hopes to revolutionise how we look at the galaxy around us by providing a 3D map of a billion stars and finding hundreds of new exoplanets.

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So what is Gaia? It is essentially the most sensitive camera ever to be pointed at the heavens. That may sound the same as most space telescopes, but its specifications mean it will be able to pinpoint the location of stars with accuracy previously only dreamed of. Using a 1.5m mirror and a Gigapixel CCD camera, it will image more than a billion stars at least 70 times over a 5 year mission to provide the most accurate catalogue of stars in the Milky Way ever seen.

It is not the sensitivity of the telescope that is extraordinary, however, but rather its angular resolution. Consider the previous such mission, Hipparcos. It was capable of resolving objects tens of thousands of times closer together than the human eye, for example even from 200km away, it’s camera was capable of spotting two lights placed only a millimetre apart. This corresponds to the order of milliarcseconds, or 1/3600000th of a degree. Gaia, on the other hand, will be able to resolve stars mere microarcseconds apart. That is equivalent to being able to read 20pt text from 30,000km above Earth, or resolving two bright lights only 170m apart at the distance of Pluto.

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Astronomical Parallax

What this amazing technological shift means is that Gaia will not only be able to compile the most accurate catalogue of star positions in history, it will also be able to map them in 3D. It may seem strange, but measuring the distance to a far-away point source like a star is nearly impossible. For nearby stars the shift of Earth’s position during the course of the year can act as a sort of cosmic depth perception, with the location of nearby stars wobbling subtlety between July and January, depending on how far away they are. It is this Parallax effect that, thanks to the incredible resolution of Gaia, will enable the distance to 1% of the stars in our galaxy to be precisely measured.

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But when this effect due to the motion of Earth is corrected for, what motion is left? It’s likely the star will be moving in some direction through the galaxy relative to our solar system. This straight-line speed is the star’s ‘proper motion’ and can be as high as 10.3 arcsecs per year. But that’s not the only thing Gaia might spot. Stars are also tugged at by the gravitational pull of all nearby objects. This is most prominently done by planets in the stars vicinity. For example, an observer 30 lightyears away would see the sun shift by nearly 500µas due to the orbit of Jupiter. That means Gaia would see the Sun perform a slow ellipse across the sky every 5 years each time Jupiter orbits.

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Planets Gaia can detect bounded in Blux lines. Upper line: Sun-like star. Lower line: M-dwarf

The biggest signals come from Gas Giant planets circling far from their stars, and Gaia will be able to search the nearest 400,000 stars for such worlds. Due to its 5-year mission, it will find these Jupiter analogues between 1 and 4AU. With any luck, more than 1000 candidates will be found; potentially doubling the current crop of exoplanets. And with Kepler dead and TESS still on the drawing board, Gaia may well become our best tool to mine the skies for new planets.

Even more interesting for exoplanet astronomers is that Gaia will find planets missed by other detection techniques. Both the transit and radial velocity methods are more sensitive to close-in planets, and have such discovered hundreds of bloated Hot Jupiters circling close to their star. Gaia, on the other hand, will be able to scan regions much further from the star. This will potentially answer the question of whether these Hot Jupiter systems are common or if other solar systems are more like our own stellar back

Another remarkable feat that Gaia will be able to achieve is pinning down the exact mass of some exoplanets. Worlds discovered by radial velocity give us an estimate of their size based on the to-and-fro motion of the star due to planets. Astrometry by Gaia will be able to give the side-to-side motion and determine in what precise inclination the planets are in. By tying down the planets orbit like this, their mass can be precisely determined.

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Gaia, if successfully launched in the next few hours, will be capable of incredible feats. First and foremost, it’s incredible parallax measurements will turn astronomy from a two-dimensional star map into a complex three dimensional system where the distances to almost every object is known precisely. And tagged on for free are another thousand potential exoplanets to add to the exponentially growing list of alien worlds! If all goes well in Guyana at 9am, a collective sigh of relief will emanate from astronomers worldwide, and it might just signal the start of a new era of astronomy.

Infographic on Gaia:

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A Planet for Every Star?

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.

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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.

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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?

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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.

1000 Exoplanets

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

Rogue Planet or Failed Star?

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?

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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.

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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.

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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

MASCARA: Planet-Hunting on a Budget

Kepler LaSilla
The European Southern Observatory, Chile

If you were told to picture a cutting-edge telescope capable of discovering new planets you would probably think of the $600million Kepler space telescope or the huge La Silla observatory high on a Chilean mountain top. A waist-high bin with a handful of digital cameras in would certainly not spring to mind. But that is exactly what Ignas Snellen and his team at Lieden University plans to use to potentially discover dozens of new exoplanets.

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NASA’s Kepler Telescope

Hidden away high on a hill at the La Palma observatory, the first MASCARA or Multi-site All-Sky CAmeRA is already being built, and will be joined by 4 other sites across the globe if all goes to plan. Five off-the-shelf digital cameras are the workhorses of the small observatories, and together are able to constantly take images of the entire night sky. By taking repeated measurements of thousands of stars over the course of more than a year, the team hopes to hunt for exoplanet transits, the dip in light when a planet crosses its host star.

Unlike other planet-hunting telescopes, the cameras remain stationary rather than tracking the stars as the Earth spins. They instead will rely on short exposures and software designed to calculate the change in light of the stars and spot an elusive transit. This approach also means only the brightest stars can be studied. While bright stars may seem like the lowest hanging fruit, previous transit surveys (eg SuperWasp) have focused on scanning thousands of dim stars in much smaller patches of the sky. These bright star planets may also be perfect for the new generations of telescopes able to probe the atmosphere of these alien worlds.

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Crude designs for MASCARA

Since the first transiting exoplanet to be discovered in 2002, ground-based surveys have discovered dozens of large Jupiter-sized worlds close to their star. MASCARA will hunt for the hottest worlds which orbit their stars every few hours, but the team also seem confident that the array of cameras will be able to pick up smaller planets too. They suggest up to 7 Neptune-sized planets and even a handful of rocky worlds could be found, however transit surveys with similar goals have struggled to overcome problems with noise and small planets have not been forthcoming.

La PalmaWhether it finds small worlds or not, Mascara is certainly a novel way of hunting for exoplanets. It’s innovative and relatively cheap design (only €50K per station) is proof that sometimes in astronomy big money isn’t the only way to get big results.