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Cutting-edge-science Image courtesy of ESA

The CoRoT satellite: the search for Earth-like

Malcolm Fridlund from the (ESA) describes the search for extra- planets and explains how they can help us to understand the origin of life on Earth.

Artist’s impression of the -sized extra-solar plan- et HD 189733b, now known to have methane and water in its atmos- phere (from studies with the Hubble and Spitzer Space Telescopes). Methane is the first organic molecule to be found on an extra-solar . The discover- ies come from spectroscopic stud- ies of from the parent that has passed through the CoRoT planet’s atmosphere This article on the search On 27 December 2006, the French for can trigger space agency CNES (Centre National w1 scientific discussions on d’Etudes Spatiales), ESA and their w2 what life is, and why we partners launched the CoRoT satel- Earth and the stellar surface (a plane- are interested in studying lite to search for small Earth-like plan- tary ). the physical and chemical ets outside our (extra- All three phenomena can be studied characteristics of celestial solar planets, or exoplanets) and by measuring the changes in the light bodies. It can also be used detect ‘starquakes’. The satellite’s emission of the observed . The as a basis for philosophi- name is derived from Convection convection from the interior of a star cal and social discussions (Co), Rotation (Ro) and planetary causes the intensity of the light it about the relationship of Transits (T), and its scientific objec- emits to increase or decrease by a few humans with possible tives are to study the rotation of stars parts per million. Areas of intense alien life forms. and the convection – the upwelling of magnetic activity inhibit convection,

hot gas – from the stellar interior, and forming areas of reduced surface tem- REVIEW Marco Nicolini, Italy to detect planets that pass between perature which are visible as darker

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As we go to press

CoRoT first caught sight of a planet transiting the star And not only that: it is only 2.5 million km away from CoRoT-7, to the left of in the of its host star, or 23 times closer than is to the (the Unicorn), about 500 light-years away, , which also makes it the closest known planet to its in Spring 2008. However, confirming the planet’s host star. It is so close that it must experience extreme nature took months with large ground-based tele- conditions, which make it uninhabitable to life as we scopes, so its discovery wasn’t officially announced know it: the probable temperature on its ‘ face’ is until 3 February 2009. above 2000 degrees Celsius, but minus 200 degrees To measure the planet’s and density, astronomers Celsius on its ‘night face’. then used the High Accuracy Radial velocity Planet The calculated density is close to that of Earth, suggest- Searcher (HARPS) spectrograph attached to the 3.6 m ing that the planet’s composition is similarly rocky. telescope at the European Southern ’sw4 La Theoretical models suggest that the planet may have Silla Observatory in , performing the longest set of lava or boiling oceans on its surface. observations (70 hours) on this machine so far. On 16 The astronomers found from their dataset that CoRoT-7 September 2009, the results were finally announced. hosts another slightly further away from the The planet, known as CoRoT-7b, is about the mass of star than CoRoT-7b. Designated CoRoT-7c, it circles its Earth, which puts it among the lightest known exoplan- host star in 3 days and 17 hours and has a mass about ets. With a diameter less than twice that of Earth, it is eight times that of Earth. Unlike CoRoT-7b, this sister also the smallest exoplanet measured so far. planet does not pass between its star and Earth, so Every 20.4 hours, CoRoT-7b eclipses a small fraction astronomers cannot measure its radius and thus its den- (one part in 3000) of the light of its star for a little over sity. one hour. Orbiting its star at a speed of more than The finding brings astronomers ever closer to discover- 750 000 km/h, more than seven times faster than ing inhabitable extra-solar planets, but such planets Earth’s motion around the Sun, it is the fastest-orbiting would need to be further from their star to support life exoplanet known. as we know it.

Artist’s impression of Corot-7b Image courtesy of ESO / L. Calcada BACKGROUND

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Cutting-edge-science

Image courtesy of ESA starspots. As the star rotates, its light This plot shows the transit output changes by a very small of the first exoplanet amount, depending on the number of discovered by CoRoT: starspots on the hemisphere that has 1.000 CoRoT-Exo-1b. The transit rotated into view – so monitoring the results in a decrease in 0.995 the of the Sun- starspots tells us how fast the star is like parent star when the 0.990 rotating. Finally, when a planet in planet passes in front of it orbit around a star passes between 0.985 every 1.51 days. It is a the CoRoT satellite and the star, it can very -like giant Normalised flux be detected as a small dip occurring 0.980 planet, similar to Jupiter also in mass (as deter- periodically in the star’s light output. 0.975 mined by spectroscopic Planetary transits are used to detect -0.05 0.00 0.05 observations from the exoplanets, while the convection and ground) and 1.49 +/- Phase rotation measurements are used to 0.08 times Jupiter’s radius characterise the star around which the discovered planets orbit. CoRoT will also be used for astroseismology: detecting acoustical waves generated diameter – was designed specifically Extraterrestrial life deep inside a star that send ripples for this purpose. The only instrument Why is it important important to across its surface, known as ‘star- on board is a camera that takes one know how common Earth-like (i.e. quakes’. The exact nature of the rip- picture every 32 seconds. The on- small and rocky) planets are? Firstly, ples allows astronomers to calculate board computer then measures the because we would like to know the star’s precise mass, age and chem- light (changes) from each star, and, whether our planet is unique. ical composition. In this article, how- over time, produces a . Furthermore, finding Earth-like plan- ever, we will concentrate on the The is directed at the same ets outside our Solar System may help search for exoplanets. spot in the sky for up to 150 days at a us to understand how life arose on Measuring these phenomena time, simultaneously observing up to Earth about 3.5 billion years ago. requires a with a very 12 000 stars. The longer it remains Based on a hypothesis made more precise photometer (or light meter). pointed towards the same stars, the than 30 years ago, scientists assume Unlike the larger Hubble Space more transits it can see (see diagram that all types of ‘life’ work the same Telescope (launched in 1990), CoRoT above). as that on Earth, and that alien life – which measures only 30 cm in CoRoT can detect planets that are forms would have the same sort of close to their star – taking up to 50-75 metabolism as ours. Therefore, days to circle it (i.e. this is the length researchers base their search on what of their ‘year’) – and can find planets happened on Earth. Although the Observations in zone 2 as small as our own Earth. The shape process by which life on Earth first of the light curve (see diagram above) emerged is still not known, it is Observations in zone 1 tells us how the planet is moving, believed to be linked to the presence how the outer layers of the star of liquid water on a hard, rocky plan- behave, and also the size of the plan- etary surface. So if there are any other Orbital plane et. Once a planet has been detected by Earth-like planets, have any of them CoRoT, astronomers can observe the evolved life? star and its with Finding an extra-solar planet as Image courtesy of ESA other types of instruments on very small as Earth is difficult. How much Earth orbit large telescopes on the ground, and harder would it be to observe life learn more about it. forms at such distances? It would be CoRoT is pointed in the same direction Already, the CoRoT satellite has particularly difficult if they were just for more than 150 days at a time, before found several large planets. It is now bacteria, which were the only living the Earth’s movement around the Sun also beginning to pick up what we organisms on Earth for the first few leads to the unwanted effect of the Sun’s think are small planets. This should billion years and still outnumber rays entering the telescope. CoRoT then turns 180 degrees around its long axis enable us to find out how common other species by a million to one and points in the other direction our own type of planet is in the today in terms of individuals, and Universe. possibly also species number.

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Image courtesy of ESA / CNES The key is to find a planet with its have also produced life and, if so, atmosphere out of chemical equilibri- what happened to it. um. A planet’s atmosphere (like Ultimately, we hope to apply this almost everything else) tends towards knowledge to understanding the evo- a state of equilibrium (in which every lution of life on our own planet. chemical reaction proceeds at the same speed as its reverse process). Web references Life, however, changes its environ- w1 – To learn more about the ment: for instance, all the free oxygen European Space Agency, see: (O2) in our atmosphere has been pro- www.esa.int duced by living organisms - plants w2 – To find out more about the and other organisms take up carbon CoRoT satellite and the partners of dioxide and process it to oxygen, the mission, see: which is then released. Oxygen is so www.esa.int/science/ reactive that, if all life were to disap- Artist’s w3 – For more information about impression of pear from Earth, the free oxygen in ESTEC, see the ESA websitew1 the CoRoT satellite our atmosphere would disappear in or use the direct link: less than 4 million years (a short peri- in its orbit at 900 km altitude over http://tinyurl.com/39nw3r od given the billions of years of Earth’s poles w4 – To learn more about the Earth’s history). European Southern Observatory, A similar chemical disequilibrium see: www.eso.org occurred when life originated on Earth and bacteria produced an over- aid of telescopes like CoRoT, designed abundance of methane. What hap- specifically to find small rocky plan- Resources pened to the methane-producing bac- ets similar to Earth, we can expect – Listen to the author’s podcast about teria and their world? We don’t actu- within the next few years – to find the CoRoT project, which can be ally know, but it is believed that new other stars that are circled by planets found on the ESA websitew1 or via organisms evolved which produced very similar to our own. this direct link: http://tinyurl.com/ oxygen instead: the oxygen was poi- However, current technology is not ydoggpy sonous for the methane producers, sufficient to analyse the atmospheres Find out more about the search for and of them died out. of such smaller planets. The light we exoplanets in this article: The gas composition and other con- receive from an exoplanet is extreme- Jørgensen UG (2006) Are there ditions such as temperature and pres- ly feeble, and Earth-like planets around other sure tell you what the equilibrium apertures are needed: of all the pho- stars? Science in School 2: 11-16. should be (in the case of Earth, simi- tons radiated by an exoplanet, only a www.scienceinschool.org/2006/ lar to the atmosphere of Mars). So if few per square metre arrive issue2/exoplanet we could analyse the chemical equi- on Earth. Furthermore, our atmos- librium of an exoplanet’s atmosphere, phere contains so much oxygen and To view all the Science in School we might determine if there is life as methane that there are already many articles about space science, see: we know it, and maybe even to what ‘oxygen photons’ and ‘methane pho- www.scienceinschool.org/space stage evolution has progressed tons’ (photons with methane or oxy- (methane or oxygen producers). gen signatures, respectively). The few The atmospheres of two very large ‘oxygen photons’ and ‘methane pho- Malcolm Fridlund is a Swedish and hot exoplanets have been studied tons’ from an exoplanet would have astronomer who has worked at the with photometric measurements by to compete with all of these, making European Space Research and the Hubble and Spitzer space tele- it impossible to detect them. Technology Centrew3 (ESTEC) for scopes as well as ground-based tele- Therefore we need to go into space – more than 20 years. He has spe- scopes, and water and methane have with large telescopes – which is both cialised, scientifically, in the area of been detected on one (see image on very difficult and very expensive. exoplanets and the methods used to page 15). This is another step towards Scientists are developing the next find and study them. He is currently proper comparative planetology, com- generation of instruments to be tech- ESA’s project scientist for the CoRoT paring the planets in our Solar System nically able to carry out the necessary mission. with those in other systems. With the observations to tell us if these planets

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