Fast-track planet. (Astronomy).

Astronomers have found a planet that's the closest yet known to its parent star. It resides 3.5 million kilometers away from the star, less than one-sixteenth Mercury's distance from the sun. The planet takes 28 hours and 33 minutes to complete an orbit, beating the previous record holder by a half-hour (SN: 1/18/03, p. 38).

Both close-in planets were revealed by a periodic dip in starlight, suggesting that a small body is regularly passing in front of a star, as seen from Earth. This planet-hunting technique is called the transit method.

Klaus Werner of the Institute for Astronomy and Astrophysics in Tubingen, Germany, and his colleagues homed in on a star whose brightness drops by 2 percent about every 28.5 hours. The researchers then examined the spectra of this sunlike star, known as OGLE-TR-3, and found a wobble in its motion, indicating the tug of the unseen planet.

The transit method, which requires a special alignment among the planet, the star, and Earth, provides enough information to yield the planet's actual mass, but the wobble method doesn't. In an upcoming Astronomy Astrophysics, the team reports that the planet weighs half as much as Jupiter but is only one-fifth as dense. The low density suggests the extrasolar planet has ballooned in response to the heat from its star. Werner notes that a planet can't get closer to a sunlike star than half the distance the new planet is from OGLE-TR-3. At greater proximity, a planet would be torn apart by the star's gravity.