First results from velocimetry

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In the early 1990s, several teams of astronomers embarked upon systematic searches for low-mass companions of nearby Sun-like stars. Their targets were not only exoplanets, but also brown dwarfs - embryonic stars that have insufficient mass (less than 0.08 times the mass of the Sun) to trigger the fusion reaction to 'burn' hydrogen into helium. This mass is equivalent to about 80 times the mass of Jupiter, and so brown dwarfs are a priori less difficult to find than exoplanets.

ELODIE

ELODIE

50000 51000 52000 JD-2 400 000 (d)

53000

The velocimetric curve for 51 Pegasi, derived by Swiss astronomers Michel Mayor and Didier Queloz in 1995, using the high-resolution ELODIE spectrometer at the Haute-Provence Observatory. (Upper) Radial velocity as a function of phase, adjusted to a sinusoidal curve of 4.2 days. (Lower) The departure from the curve as a function of time.

51 Pegafii

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51 Pegafii

100hMaaa " °-'ta "ji» /Cirl i P " 4.230 day.

The velocimetric curve for 51 Pegasi derived by American astronomers Geoffrey Marcy and Paul Butler in 1997, at the Lick Observatory in California, (left) Radial velocity as a function of orbital phase, (right) Velocity as a function of time (over one month beginning 12 October 1995).

The 193-cm telescope at the

Haute-Provence Observatory. The telescope was installed in 1958, and the ELODIE high-resolution spectrograph, built at the observatory, began its work in mid-1993.

The three teams deeply involved in the hunt were led by Bruce Campbell and Gordon Walker in Canada, Geoffrey Marcy and Paul Butler in the USA, and Michel Mayor and Didier Queloz at the Geneva Observatory. The Canadians in particular carried out much pioneer work in the development of the instrumentation involved in this research, and in 1994 they announced a find which was not confirmed by later investigations. In 1994 Marcy stated that after two years of research a review of a third of his target stars had yielded negative results. Meanwhile the Swiss team had begun a systematic study using a new high-resolution spectrometer, working with the 1.93-metre telescope of the Haute-Provence Observatory in southern France. In autumn 1995 they announced the first discovery of an exoplanet orbiting a 7 billion-year-old Sun-like star: 51 Pegasi. Its companion appeared to be at least half as massive as Jupiter, with an orbital period of only 4.2 days. So, here was a planet orbiting a star at a distance of only 0.05 AU (a twentieth of the distance of the Earth from the Sun)! In the weeks which followed, Marcy was able to confirm the discovery and announce the detection of two other exoplanets - one orbiting the star 47 Ursae Majoris and the other orbiting 70 Virginis, and both pursuing very elliptical orbits. With masses of at least 2.6 and 7 times that of Jupiter, these planets were at mean distances of 0.5 and 2 AU from their respective stars.

It immediately became clear that astronomers were dealing with distant stellar systems very different from our own; and this evidence was confirmed by later discoveries.

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