ASTROWATCH

Compiled by Marc Turler, INTEGRAL Science Data Centre The hunt for Earth-sized exoplanets The discoveries of three new extrasolar planets were recently announced within the space of a week. With masses around or less than 20 times that of the Earth, these new exoplanets are the lightest known so far. After Jupiter- and Saturn-like extrasolar planets, the detection of these Neptune-and Uranus-sized objects represents an important step in the quest for exoplanets similar to Earth. Following the discovery in 1995 of the first extrasolar planet, and fuelled by its enormous impact on the general public, the hunt for exoplanets has already led to the discovery of about 130 planets around nearby stars. Two groups on either side of the Atlantic are leading the race. The European group discovered the very first exoplanet around the An artist's rendition of the newly discovered Neptune-sized exoplanet, which orbits the star 51 Pegasi and was also the first to reddish dwarf star Gliese 436 every 2.6 days. (Image courtesy of NASA.) announce the discovery of a lightweight planet on 25 August this year. The American group, which has discovered about half of all Picture of the month known exoplanets, responded only one week This spectacular X-ray false-colour image of later by announcing their discovery of two the supernova remnant Cassiopeia A is the Neptune-sized planets. most detailed image ever made of the The presence of a planet orbiting a star is remains of an exploded star. It is the result of revealed by its gravitational pull making the a one-million-second (11.5 day) observation star wobble around the centre of mass of the by NASA's Chandra X-ray observatory. The system. The wobbling can be detected in a point source at the centre of the expanding sequence of radial velocity measurements gas bubble, which is 10 light-years wide, is using high-resolution spectra of the star. This presumed to be a neutron star created during method allows the orbit of the planet - in the supernova explosion some 340 years particular, the period and the distance from ago. But, unlike the pulsar in the Crab the star - as well as a lower limit for its Nebula, this neutron star does not show mass to be deduced. Statistically, the true evidence for pulsed radiation. (Credit: mass will in most cases be not much higher NASA/CXC/GSFC/U Hwang eta/.) than this limit. The most precise instrument for detecting exoplanets is currently the High Accuracy shorter revolution periods of less than three of the star. The next step will be the direct Radial Velocity Planet Searcher, HARPS. days. One is orbiting a reddish low-mass star imaging of the planets with projects such as Installed a year ago on the 3.6 m telescope of called Gliese 436 (Butler eta/. 2004), while ESA's Darwin interferometric mission or the European Southern Observatory at La the other orbiting 55 Cancri is the fourth planet NASA's Terrestrial Planet Finder. Finding the Silla, Chile, it is able to measure the wobbling found around this metal-rich star (McArthur et spectral signature of life in the atmosphere of stars with a precision of about 1 metre per al. 2004). Astronomers can only speculate on of Earth-like planets will be the ultimate goal second, or around walking speed. the true nature of these lightweight planets. of these missions. The three newly discovered exoplanets are They are most likely made of a rocky core all very close to their star. Discovered with surrounded by a small gaseous envelope. Further reading HARPS by the European group (Santos eta/. The hunt to detect smaller and smaller P Butler et al. 2004 ApJ http://arxiv.org/ 2004), the Uranus-sized planet (14.5 Earth exoplanets is far from over. HARPS is able to abs/astro-ph/0408587. masses) orbiting ji-Arae accomplishes a full find planets down to a few Earth masses, B E McArthur et al. 2004 ApJ http://arxiv.org/ revolution of the star in 9.5 days. The two paving the way for space missions such as abs/astro-ph/0408585. Neptune-sized planets (17 Earth masses) COROT, Eddington and Kepler, which aim to N C Santos etal. 2004 A&A http://arxiv.org/ discovered by the American group have even detect a possible transit of the planet in front abs/astro-ph/0408471.

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