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MARCH 2009
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The hunt for other Earths The search for our sister planet begins
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Contents Features + 22 Kepler’s quest for other Earths 27 Amateur astronomy at ESPC 2008 68 Uranus: a new season of exploration Focus + 56 The five Ws of comets 60 The mystery of Comet Holmes 64 Rosetta: our key yo comets Regulars + 08 20 21 34 39 72 74 76 78 81 82 84 85 88
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Kepler’s Quest other
Earths March sees the launch of NASA’s Kepler spacecraft -the next step on the road to discovering and imaging another planet just like Earth, orbiting another star. Kulvinder Singh Chadha explores how close we are to making that momentous discovery.
+ If the current search for terrestrial planets is like using a quill pen, then Kepler will be the equivalent of the printing press. Over 300 exoplanets have been found around other stars in the last two decades. Most are hot, inhospitable gas giants close to their suns, but what really drives the search for exoplanets is the holy grail of finding another Earth. It may at first seem like an impossible dream, but with the launch of he Kepler space mission that dream moves a step closer to reality.
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The smallest exoplanets detected up to new is called COROT-Exo-7b. With a mass less than twice that of Earth, its discovery by the COROT (Convection Rotation and Planetary Transits) spacecraft was announced just this February. It obits very close to its parent star, every 20 hours, and has a surface temperature of up to1,500 degrees Celsius. The next smallest has the prosaic title MOA-2007-BLG-192-L b, which has a mass somewhere between 1.4 and three times that of the Earth. It was discovered in 2008 through a fluke of nature called microlensing–similar to gravitational lensing, but on a small scale: the chance brightening of a distant star by the gravity of an intervening star or planets are in the minority–you can count the ones we’ve found on one hand. What makes the scientists behind the Kepler mission so sure they will be able to find the dozens of Earth-sized planets that they claim? Like COROT, Kelper will look for transits rather than microlensing events around 100,000 stars, all at the same time. A transit is the dip in light of a star as an orbiting planet passes in front of it. For an Earth-sized planet, this dip in light can be as small as 0.01 percent of the total light of the star (in comparison, a Jupiter-mass gas giant will cause a dip in light if maybe just one percent of the star’s total light). In order to see this tiny variation
in brightness, Kepler will unleash a barrage of photometric techniques to help it in its search. Only two prior space missions have photometry to anywhere near this extent: Canada’s MOST [Micro variability and Oscillations of Stars] satellite and the French/ESA COROT probe. Although COROT has proven it can detect terrestrial exoplanets, neither it or Most are kitted out to embark in a survey as large as Kepler’s. Onboard Kepler is a 0.95-metre telescope, with a photometer built from 42CCDs, so it is will equipped to meet the challenge. “The mission duration is ten times larger [ than previous star-survey missions like MOST and COROT], the collecting area is nine times larger, and the field id view is a factor of 20-50 times bigger, ” says Dr David Koch of NASA’s Ames Research Center, who is Kepler’s Deputy Principal Investigator.
Patience You can’t just flit from star to star to find another Earth. We don’t know which stars possess planets or which of those have a viewing geometry that makes transits possible, and to observe several transits to confirm the existence of a planet can take many years spent staring at any given star. Most of the exoplan-
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ets discovered thus far are hence large and in short orbits (mere days in most cases) for a reason: they’re the easiest to find. Planets in short orbits would obviously transit in front if their star many times over Kepler’s three-anda–half -year primary mission duration. When looking for a planet with an annual orbit as long as ours, just one year of observations (i.e. a single transit) wouldn’t do. You would need several years of transits to compare them against one another. The period between then must be exactly the same to confirm that it is indeed a planet. Ground-based telescopes and non-specialist space missions couldn’t be requisitioned for this task for this length of time. And with ground-based telescopes there’s also the problem of Earth’s turbulent atmosphere. An instrument like Hubble also couldn’t be used as it doesn’t have a specialist photometer and its field of view is too small. A dedicated mission like Kepler is the only way to go.
Field of dreams Kepler’s field of view on the sky is a whopping 10*10 square degrees (20 Moon-diameters across). The target area was chosen to incorporate the largest number of stras, which lie between 650 and 3,200 light tears away. “To maximize our results we need a rich star field, with as many stars as we can get, and that means we want to look into the galactic plane, preferably down a spiral arm of the Galaxy rather than across a spiral arm, ” says Koch. But as well as targeting stars, there is one you have to avoid. “We want to view the sane piece of sky continuously for the entire duration of the mission,” says Koch. ”This basically means that you cannot look into the ecliptic plane because the Sun will cause a problem every year for that part of the sky. So you have to look outwards, and the question is how fat out do you have to go?” This is determined by the size of Kepler’s sunshade, which has a Sun-avoidance angle of 55degrees. ” That means we have to look above 55 degrees ecliptic latitude or below -55 degrees,” says Koch. There are only two parts of the galactic plane that are above or below those latitudes, and the one that was
selected was simply the one with the most stars. This turns out to be the Cygnus region. Not all will have visible transits– we’ll be looking at many of them at the wrong angle. Nevertheless, Koch reckons that there are 40-50 Earth-sized( or smaller) exoplanets waiting to be found in Kepler’s survey area.
Beyond Kepler On the eve of its three-and-a-half year mission (there is the opportunity to extend it to six years), can we be sure that Kepler will be successful and find the Earth-like planet outside if our Solar System? It depends what kind of star it is orbiting, says Professor Andrew Collier Cameron of St Andrew’s University. For Sun-like G-type stars, Kepler and perhaps the
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proposed Plato mission from ESA are the best bets for finding terrestrial planets. “[But] for Earth-mass planets round red dwarf stars the technique id gravitational microlensing is like to get there first,” says Cameron. “Searches with the radial velocity method are already finding super-earths (large terrestrial planets several times Earths mass) around these stars.” If Kepler is successful, then what? A transit can tell you the size of the orbit and the diameter of the planet, and the mass and density can be subsequently worked out fairly easily. But when we say we are searching for an Earthlike planet, that’s exactly what we mean. We’re looking for an azure blue world with oceans and clouds and landmasses and, possibly even life. “If I’ve got the distance of the planet and the temperature of the star, I can then calculate where the habitable zone (the region where temperatures are amenable to liquid water) for that star is,” says Koch. In order to know if an Earth-like planet were possibly life-bearing, we would need to know if its atmosphere contained oxygen, water, and other chemicals essential for life. Even without photographing the planet, we can find these out through spectroscopy. Spectroscopy has been performed on the atmospheres of ‘hot jupiters’ – gas giant orbiting very close to their suns – by NASA’s Spitzer Space Telescope and the Hubble Space Telescope, finding carbon dioxide and water vapor in their atmospheres, for example. Of course, ‘hot jupiters’ are not habitable, but it proves that these ingredients vital to life can be detected light years away. Conducting spectroscopy of smaller, terrestrial worlds will be more difficult.
Kepler Mission to see other Earths NASA’s Kepler spacecraft moved to its launch pad at Cape Canaveral Air Force Station, Florida, Thursday and will soon begin a journey to search for worlds that could potentially host life. Kepler is scheduled to blast into space aboard a Delta II rocket March 5 at 7:48 p.m. Pacific Time (10:48 p.m. Eastern Time). It is the first mission with the ability to find planets like Earth - rocky planets that orbit Sun-like stars in a warm zone where liquid water could be maintained on the surface. Liquid water is believed to be essential for the formation of life. “Kepler is a critical component in NASA’s broader efforts to ultimately find and study planets where Earth-like conditions may be present,” said Jon Morse, the Astrophysics Division director at NASA headquarters in Washington. “The planetary census Kepler takes will be very important for understanding the frequency of Earth-size planets in our galaxy and planning future missions that directly detect and characterize such worlds around nearby stars.” The mission will spend three-and-a-half years surveying more than 100,000 Sun-like stars in the Cygnus-Lyra region of our Milky Way Galaxy. It is expected to find hundreds of planets the size of Earth and larger at various distances from their stars. If Earth-size planets are common in the habitable zone, Kepler could find dozens; if those planets are rare, Kepler might find none.In the end, the mission will be the first step toward answering a question posed by the ancient Greeks: Are there other worlds like ours or are we alone?
Finding earthlike worlds The leading method of finding planets orbiting distant stars spots mostly Jupiter-sized worlds. Technology limitations make it difficult to detect smaller planets. But that is about to change.“We are at the cusp of a new era in planet searches,” says CfA astrophysicist Chih-Hao Li. “With this technology we are developing, astronomers will finally be able to find the first truly Earth-like worlds in terms of size and orbit. Planets orbiting other stars are much too faint and far away to be seen directly and photographed. If the wobble is along our line of sight, then sensitive instruments called spectrographs may be able to detect it. The size of the wobble depends on the planet’s mass and its distance from the star. The larger the mass of the planet, the bigger the star’s wobble will be, making larger planets easier to detect. At the same time, a planet in a tight, short-period orbit is easier to find than one in a wide, long-period orbit. Current technology, although very stable and sensitive, isn’t quite up to the task of finding Earths. The best instruments can only find 5-Earth-mass planets in tight, Mercury-like orbits.The new device developed by Li and his colleagues, called an astro-comb, will be able to spot Earth-mass planets in Earth-like orbits. It uses femtosecond (one millionth of one billionth of a second) pulses of laser light, linked to an atomic clock, to provide a precise standard against which light from a star can be measured. The astro-comb can make measurements accurate to one part in a trillion. This may increase the resolution of the wobble planethunting technique by about 100 times, which would allow astronomers to detect Earthsized planets. A prototype astro-comb will be tested this summer at CfA’s Mount Hopkins Observatory in Arizona. Those tests will be used to refine the design. An improved astrocomb is destined for a project being built in the Canary Islands called the New Earths Facility. Thus, the researchers expect it to be operational in 2010.
Kulvinder Singh Chadha is Astronmy Now’s Assistant Editor March 2009 | Astronomy Now | 25