First detection of a planet that survived the red giant expansion of its parent star. This could be the destiny of our Earth.

This discovery, published today in Nature, provides a somewhat preliminary picture of what could be the destiny of our Earth once the Sun has exhausted its primary fuel. With an age of about 10 billion years, V 391 Pegasi b — as it is named — is the first planet detected around a post-red giant star and one of the oldest planets ever discovered.

There are already more than two hundred extra-solar planets known. However, the one discovered by Roberto Silvotti, researcher at the INAF-Osservatorio Astronomico di Capodimonte, and by 22 other astronomers of the international team that have signed the manuscript appearing in Nature today, is the first to have survived “the red giant phase”, one of the most dramatic phases of the evolution of its parent star. It has occurred despite an orbital distance of only 1.7 times the medium distance between the Earth and the Sun. During a “red giant phase”, the stars, after exhausting their primary fuel, hydrogen, in the core, experience an enormous expansion (with their volume increasing by a factor of a few millions) that can easily reach and engulf the inner planets.

«The same will happen to the Sun», Roberto Silvotti says. «As far as our planets are concerned, we expect Mercury and Venus to disappear in the Sun’s envelope, whereas Mars should survive. The fate of the Earth is less clear because its position is really at the limit: it appears more likely that the Earth will not survive the red giant expansion of the Sun either, but it is not for sure. All this will happen in about five billion years, when the Earth will be more or less the same age as V 391 Pegasi b, i.e. ten billion years. This makes this gaseous giant, whose mass is at least three times Jupiter’s mass, one of the oldest planets ever discovered. Very unusual is also its parent star, V 391 Pegasi (from which the planet inherits its name, with an extra «b» indicating that it is a secondary body): with a surface temperature near 30,000 degrees, it is the hottest star among those surrounded by a planetary system. So hot that researchers believe that the surface temperature of the planet just discovered could reach 200 degrees Celsius, despite its relatively large orbital distance.

“This discovery occurred almost by chance” Silvotti continues. «We did know that in principle we could find a planet as it is commonly believed that about 5% of the stars have planets, but our primary goal was different: to study the periodicity of the light emission of V 391 Pegasi, which is a variable star with a main period of about 6 minutes». While we were analysing the irregularities of V 391 Pegasi’s luminosity variations with a method called timing method, we began to suspect that the these irregularities might have been caused by a planet. The intensity maxima were reaching the telescopes on the Earth with a small advance or delay of about 5 seconds compared to what was expected. As if the star was continuously moving and forcing the light to cover a distance either smaller or larger. It took seven years of observation and computations to be able to discard other possible interpretations of the data and confirm the only reasonable explanation: the presence of a planet with the properties of V 391 Pegasi b.

As is the case for almost all (98%) the extra-solar planets known, we happen to know that V 391 Pegasi b is there although we still cannot see it directly. «Neither with the largest telescopes available today, nor with those available tomorrow», Silvotti confirms. «But the future generations of telescopes will definitely make such a thing possible.».

It will no doubt take several years before it happens. Until then, his two children will have plenty of time to gaze at the Moon with the pair of binoculars they have just been offered…