Paper appears in Nature, August 3rd 2006. (Nature, 2006, Vol. 422, P.543-545)
Results also to be presented next week at the 15th European White Dwarf Workshop in Leicester (Press release).
Using ESO's Very Large Telescope, we have discovered a rather unusual system, in which two planet-size stars, of different colours, orbit each other. One is a rather hot white dwarf, weighing a little bit more than half as much as the Sun. The other is a much cooler, 55 Jupiter-masses brown dwarf.
The two objects, separated by less than 2/3 of the radius of the Sun or only a few thousandths of the distance between the Earth and the Sun, rotate around each other in about 2 hours. The brown dwarf moves on its orbit at the amazing speed of 800 000 km/h!
The two stars were not so close in their past. Only when the solar-like star that has now become a white dwarf was a red giant, did the separation between the two objects diminish drastically. During this fleeting moment, the giant engulfed its companion. The latter, feeling a large drag similar to trying to swim in a bath full of oil, spiralled in towards the core of the giant. The envelope of the giant was finally ejected, leaving a binary system in which the companion is in a close orbit around a white dwarf.
An artists impression of the brown dwarf orbiting the white dwarf
WD0137-349. The hot white dwarf is no bigger then the Earth, while the
brown dwarf is about the size of Jupiter, although much more massive
(55 times Jupiter's mass). The pair orbit each other every 2
hours.
Click on the image to download a high resolution tif file. Credit:
European Southern Observatory (ESO).
This star confirms the surprising prediction that a small brown dwarf (a "failed" star) can crash into a red giant and survive unscathed, while the giant loses more than half its mass in a few years or less.
Computers are now getting fast enough to simulate a small object crashing into a large object with a dense core. These simulations will be used to study phenomena like the formation of black hole binaries or the formation of superclusters of galaxies. The physics of these collisions is very complex, so these simulations need to be tested against observations. The properties we have measured for WD0137-349 and its relatively simple history make it the best test for the models currently available.
In the short term, we are using the Gemini Telescopes and the Spitzer Space Telescope to get a better look at the brown dwarf. In the longer term, we are using these telescopes and others to look for "dead solar systems" in which a planet like Jupiter is orbiting a white dwarf at a large distance. This is what we think our Solar system will look like in 6-7 billion years from now.
The Sun will become a red giant in 5-6 billion years time. If it grows large enough to engulf the Earth, then the Earth will be dragged inwards and completely destroyed. But astronomers are not sure how big the Sun's red giant will be. Some simulations say the Earth will survive, just, if it manages to remain outside the reach of the red giant's atmosphere. In either case, the red giant will certainly boil off the Earth's atmosphere and seas.
The white dwarf and brown dwarf are currently separated by a distance of 2/3rds of our Sun's radius. Since the white dwarf is descended from a star like our Sun, then there's no way the brown dwarf could have been born there (otherwise it would have been born inside the Sun-like star, which is impossible). In fact, we think it was most likely born at a separation more like that of the Earth and Sun. Then, when the star became a red giant, its expanding atmosphere overtook and engulfed the brown dwarf, dragging it in. A red giant's atmosphere is much more tenuous than the Sun's (same mass, much bigger volume, hence less dense). So the brown dwarf could survive inside it.
Fewer than 1/200 white dwarfs have a brown dwarf as a companion so it was very unlikely that we would find one until a large, sensitive survey of white dwarfs was carried out. The SPY project is a large, sensitive survey for the progenitors of supernovae among white dwarfs. This survey has identified many interesting white dwarfs, including WD0137-349.
The observations we have made will have no impact on life on Earth, but the expertise being developed in the techniques and physics used to simulate the collision of a red giant with a brown dwarf (Smoothed Particle Hydrodynamics) can be applied to improving models the Earth's atmosphere and oceans.
A white dwarf is a dense object about the size of the Earth but 100,000 times heavier. They are formed when a star like the Sun dies. This will happen to the Sun in 5-6 billion years' time.
A brown dwarf is a "failed star". Star form when clouds of gas collapse and thermonuclear fusion stars in the centre. If the mass of the cloud is 13-75 Jupiter masses, the thermonuclear fusion stops when dueterium runs out. Dueterium is rare so this phase lasts less than 100 million years. Stars like the sun are hotter and denser in the core so they can burn hydrogen for billions of years. Planets never get dense or hot enough to start thermonuclear fusion.