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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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