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DOLoRes-TNG data contribute to the discovery of a giant planet which survived to the expansion of its home star

An international campaign of photometric observations using, among others, the TNG telescope has provided indirect evidence of the existence of a giant planet (about three times the mass of Jupiter) around a red giant star that has completely passed through its expansion phase. The planet's presence was discovered by studying how its gravitational pull on the star altered the timing of regularly spaced pulses observed from the star.

The new planet (named V391 Pegasi b) is one of the more than 200 'exoplanets' found around stars other than the Sun. Most orbit stars in the main phase of their lives, when they are still burning hydrogen in their cores. But recently a few have been found around stars in their red giant phase, which occurs when the cores of Sun-like stars have fused all their hydrogen into helium, causing them to heat up and expand to more than 100 times their original radii.

Located about 4,500 light years away from the Solar System, V391 Pegasi b is by far the oldest exoplanet and the first to be seen orbiting a post-red-giant star. When its home star was still burning hydrogen, the newly discovered planet orbited it at about the same distance that Earth orbits the Sun (1 astronomical unit, or 1 AU). About 100 million years ago, that all changed. The star's surface area increased by a factor of about 22,000, bringing its outer atmosphere perilously close to within 0.3 AU to the planet.

The discovery of V391 Pegasi b will help astronomers to shed light on whether the Earth will survive when our Sun will begin to expand its atmosphere, 5 billion years from now.

(R. Silvotti et al., "A giant planet orbiting the 'extreme horizontal branch' star V 391 Pegasi", 2007, Nature, vol. 449, pages 147 and 189)


Artist's impression of the planet around V391 Pegasi