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Extrasolar planets

Direct detection at last

Nature volume 402, pages 732–733 (1999)Cite this article

In the past four years, astronomers have discovered at least 30 planets in orbit around nearby stars1,2,3,4,5,6,7,8,9,10,11. Their detection relies on the fact that the force of gravity that keeps a planet in its orbit has an opposite reaction on the star, causing it to wobble in its own tiny orbit about the common centre of mass. From the size of the stellar wobble, astronomers can calculate a lower limit for the planet's mass, but they can't tell whether they are seeing the effect of a lighter planet in an orbit seen edge-on, or a heavier one in the same orbit viewed differently — until now. On page 751 of this issue, Cameron et al.12 argue that they have detected light reflected from a planet orbiting a nearby star, providing a direct measure of its mass and size.

All the stellar wobbles indicating extrasolar planets have so far been found using the Doppler technique, which measures changes in the star's velocity along the line of sight. What is actually measured is a small, systematic change in wavelength (Doppler shift) of the many absorption lines that make up the star's spectrum. Jupiter, for example, causes the Sun's velocity to vary by ∼12.5 m s−1 during its 12-year orbit, whereas a planet much closer to its parent star produces a larger velocity perturbation (in the case of the planet orbiting 51 Pegasi, ± 60 m s−1). The Doppler method favours systems quite unlike our own, with heavy giant planets close to their stars producing the strongest and most easily detected gravitational tugs. Some have orbits as short as a few days, rather than decades, and many extrasolar planets are heavier than Jupiter. Although these discoveries have serious implications for theories of planet formation, the indirect detections cannot tell us anything about the radius or composition of these planets.

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  1. the Department of Astronomy and Steward Observatory, University of Arizona, Tucson, 85721, Arizona, USA

    Adam Burrows & Roger Angel

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Correspondence to Adam Burrows or Roger Angel.

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Burrows, A., Angel, R. Direct detection at last. Nature 402, 732–733 (1999). https://doi.org/10.1038/45401

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