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Astronomy

A Model for Earth's Destiny

The first exoplanet to have survived the red giant phase of its star was reported last September by an international team of astrophysicists1 including CNRS researchers from the Astrophysics Laboratory of Toulouse-Tarbes.2
The red giant phase occurs after a star has run out of hydrogen fuel reserves and begins to expand by a factor of a few million. As it swells, a red giant usually gobbles up the unfortunate planets within its close orbit. That’s why scientists were surprised to discover an exoplanet in the close proximity of star V391 Pegasi, a former red giant.
Twenty-two scientists led by the Italian Roberto Silvotti3 stumbled upon this planet almost by accident. The team was studying sonic waves emitted by the star V391 Pegasi, when anomalies led the researchers to suspect the presence of an invisible body in its orbit. Seven years of observation later, the team announced it had found V391 Pegasi b, one of the oldest exoplanets known to man with a mass three times that of Jupiter and an estimated ground temperature of 200°C.
The presence of an exoplanet orbiting a star in the late stages of its development could give us an advance preview of what to expect when our own star, the Sun, morphs into a red giant in approximately 5 billion years. Like all stars, the Sun will one day expand, swallowing Mercury and Venus in the process. Mars, on the other hand, is too distant to meet the same demise, but the Earth sits in a sort of gray area, its future as of yet uncertain.
Although the newly discovered planet is much bigger than Earth, there are similarities with our planet that provide new insights on our planet’s distant future. Right before the red giant phase, the exoplanet
V391 Pegasi b was only  about one astronomical unit away from its star, the same distance between the Earth and the Sun today. In 5 billion years, the Earth will also be the same age as V391 Pegasi b–i.e., approximately
10 billion years old.
In the coming years, scientists plan to monitor the star’s vibrations to better understand why V391 Pegasi b survived, and lived to tell the tale.
Lucille Hagège

Notes :

1. R. Silvotti et al., “A giant planet orbiting the 'extreme horizontal branch' star V391 Pegasi,” Nature. 449: 189-91.2007.
2. Laboratoire d'astrophysique de Toulouse et Tarbes (CNRS / Université Toulouse-III, affiliated to the Midi-Pyrénées Observatory).
3. INAF, Capodimonte Observatory, Naples, Italy.

Contacts :

Stéphane Charpinet
Laboratoire d'Astrophysique de Toulouse-Tarbes.
stephane.charpinet@ast.obs-mip.fr


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