Thursday 8 December 2016
Five billion years from now, the Sun will grow into a red giant star, more than a hundred times larger than its current size. It will also experience an intense mass loss through a very strong stellar wind. The end product of its evolution, 7 billion years from now, will be a tiny white dwarf star, about the size ot the Earth, hot and extremely dense (one tea spoon of white dwarf material weights around 5 tons!).
This metamorphosis will have a dramatic impact the planets of our solar system, including the Earth. While Mercury and Venus will be engulfed in the giant star and destroyed, the fate of the Earth is still uncertain. Due to the future brightnening of the Sun, it is well established that our planet will become hostile to any form of life in a few billion years. But the Earth’s rocky core may survive the giant phase and remain in orbit around the solar white dwarf.
L2 Pup is a nearby star (208 light-years), which is surrounded by a disk made of gas and dust which was created by its mass-loss. Thanks to precise observations of the orbital motion of the gas in this disk with ALMA, the astronomers determined that it closely follows Kepler’s law of rotation. From this motion, the team could determine very accurately the present mass of the star, finding that it is two-thirds of the mass of our Sun. Knowing the mass of L2 Pup, stellar evolution models indicate that the age of L2 Pup is around 10 billion years. Five billion years ago, L2 Pup was an almost perfect twin of the present Sun, with the same mass. The missing third of solar mass has been lost by L2 Pup during its evolution. So L2 Pup gives us a prime view point on the very distant future of our Sun.
Near L2 Pup itself, the team uncovered an object one hundred times fainter, L2 Pup B. At a separation of twice the Earth-Sun distance from its parent star (300 million kilometers), its low estimated mass implies that it is very likely a planet (or a low mass "brown dwarf"). The presence of this planet orbiting in the disk of L2 Pup gives a preview of the fate of the solar system planets, 5 billion years from now. The brightness of L2 Pup B and its coincidence with a long and thin plume of dust indicate that it may be accreting material expelled from the central star.
Credit: P. Kervella (CNRS / U. de Chile / Observatoire de Paris / LESIA)
A deeper understanding of the interactions between L2 Pup and its planet will yield precious information on the final evolution of the Sun, and how it will impact the solar system planets. Whether the Earth will eventually survive the Sun or be destroyed is still uncertain, and L2 Pup may be the key to answering this question.
The central star light has been subtracted from the ALMA image to better show the companion object. The size of the star’s disk is represented to scale. The white circle in the bottom left corner represents the resolution of the image.
Credit: P. Kervella et al. (CNRS / U. de Chile / Observatoire de Paris / LESIA / ESO / ALMA)
ALMA interferometer : http://www.almaobservatory.org
 P. Kervella (CNRS UMI 3386/U. de Chile/Obs. de Paris/LESIA), W. Homan (KU Leuven, Belgium), A. M. S. Richards (U. of Manchester, UK), L. Decin (KU Leuven, Belgium), I. McDonald (U. of Manchester, UK), M. Montargès (IRAM, France), and K. Ohnaka (U. Catolica del Norte, Chile)
 ALMA is the world’s largest observatory at millimeter wavelengths. It is installed on the high-altitude plateau of Chajnantor in the Atacama desert (Chile). It made of 66 individual radio antennas used jointly to synthesize a giant virtual telescope of 16 km in diameter.