sis_13_2_RZ:Layout 1 16.11.2009 11:40 Uhr Seite 15 Cutting-edge-science Image courtesy of ESA The CoRoT satellite: the search for Earth-like planets Malcolm Fridlund from the European Space Agency (ESA) describes the search for extra-solar planets and explains how they can help us to understand the origin of life on Earth. Artist’s impression of the Jupiter-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 planet. The discover- ies come from spectroscopic stud- ies of light from the parent star that has passed through the CoRoT planet’s atmosphere This article on the search On 27 December 2006, the French for exoplanets 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 transit). the physical and chemical ets outside our Solar System (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 stars. 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 www.scienceinschool.org Science in School Issue 13 : Autumn 2009 15 sis_13_2_RZ:Layout 1 16.11.2009 11:40 Uhr Seite 16 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 Orion in the constellation of its host star, or 23 times closer than Mercury is to the Monoceros (the Unicorn), about 500 light-years away, Sun, 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 ‘day face’ is until 3 February 2009. above 2000 degrees Celsius, but minus 200 degrees To measure the planet’s mass 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 Observatory’sw4 La Theoretical models suggest that the planet may have Silla Observatory in Chile, 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 exoplanet 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 16 Science in School Issue 13 : Autumn 2009 www.scienceinschool.org sis_13_2_RZ:Layout 1 16.11.2009 11:40 Uhr Seite 17 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 luminosity 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 hot Jupiter-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 light curve. Furthermore, finding Earth-like plan- ever, we will concentrate on the The spacecraft 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 space telescope 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 planetary system 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.