Published online 10 September 1998 | Nature | doi:10.1038/news980910-5


Planetary companion to red dwarf

The hunt for planetary companions around stars other than our own Sun enters a new phase with a report of a planet around a small, dim red dwarf star just 15 light years from the Sun. This is the first report of a planet round a M-class red dwarf, and it so near that direct imaging should be possible in the near future. The research raises prospects for finding extrasolar planets, as more than seven in 10 stars in the Galaxy are M-class dwarfs.

Geoffrey W. Marcy of San Francisco State University, San Francisco and colleagues discuss the details in a report in the October 1 issue of the Astrophysical Journal. Marcy gave a preliminary account at a conference in June, where it was announced that Xavier Delfosse of the Geneva Observatory, Switzerland and colleagues had discovered the same planet independently.

The star concerned is in the constellation Aquarius and is known as Gliese 876. Although very close to the Sun, it is invisible to the naked eye: it has barely a hundredth the luminosity of the Sun, and barely a third its mass.

The planetary companion orbits Gliese 876 once every 61 days in a highly elliptical orbit, with a mean distance from the star of about 0.21 AU (astronomical units, where 1 AU is the mean distance between Sun and Earth) and a mass of between twice and four times that of the planet Jupiter. For comparison, Mars orbits at 1.52 AU and Jupiter at 5.2 AU in our own Solar System: the new planet approaches its star closer than Mercury (0.3 AU) does to our Sun.

Large planets that orbit extremely close to their stars - 'epistellar Jovians' - are typical findings in the hunt for extrasolar planets. The first such planet, 51 Pegasi b, caused a stir, and there was case for considering such discoveries as possible artefacts of observation. But there are now four other epistellar Jovians known, in addition to 51 Peg B and the companion to Gliese 876 - as well as planets in more 'conventional' orbits - so the observations are increasingly convincing. Working out how large planets come to be orbiting their primaries so closely still presents a headache for theorists. The planets probably form much further from their primaries and migrate inwards later on. How they come to settle in close, stable orbits is still a matter with which theorists must wrestle.

None of these candidate extrasolar planets have been observed directly. Some, such as the planets orbiting the nearby star Lalande 21185, are inferred to exist 'astrometrically', from watching the way in which the star 'wobbles' in response to the gravitational fields of its unseen companions. Most - including 51 Peg B and the Gliese 876 companion - have been deduced 'spectroscopically', from periodic fluctuations in the stars' spectra related to the Doppler effect. These are interpreted as wobbles in the stars' motion indicative of the effects of unseen companions.

The discovery of the latest planet could change all that. Because the star is so close to the Sun, is small, with a large planet that orbits its star relatively quickly, independent, astrometric confirmation could follow soon. And at its greatest separation from its parent star, the planet might - just - be visible on its own, using the Keck or the Large Binocular Telescope, or several large telescopes working together as a near-infrared 'interferometer'. As they say - watch this space.