Posted: September 11, 2008
Using ESO’s Very Large Telescope, astronomers have studied the planet-forming discs around young Sun-like stars that may ultimately yield Earthlike planets.
Astronomers studied dusty planet-forming discs which were known to have gaps in the dust (represented by the brownish colour in the image) but which still had gas present inside these gaps (represented by the white colour), suggesting that the dust has clumped together to form planetary embryos, or that a planet has already formed and is in the process of clearing the gas in the disc. Image: ESO.
Even though over 300 planets exhibiting a diverse range of characteristics are already known to orbit stars other than the Sun, great insight into the planetary-forming process can be drawn from studying the discs around young stars where planets may currently be forming.
"This is like going 4.6 billion years back in time to watch how the planets of our own Solar System formed," says Klaus Pontoppidan from Caltech, who led the research.
The research team used a technique known as 'spectro-astrometric imaging' to give them a window into the inner regions of the discs where Earthlike planets may be forming around three young solar analogues. These three discs are just a few million years old and exhibit gaps or holes in them, indicating regions where the dust has been cleared by the possible presence of young planets. Using the CRIRES instrument on the VLT, the astronomers could measure distances as small as one-tenth the Earth-Sun distance.
“Our observations only imply the presence [of planets], but because we see so sharp and can locate the gas precisely, we can rule out several other explanations for the gap in the dust disc,” says Ewine van Dishoek from Leiden Observatory. “If a planet is the explanation for the dust gap, then indeed the dust is cleared in a zone around the young planet, which depends on the mass of the planet and the mass of the star. But we do not know if larger grains (rocks, boulders, planetesimals) are still present because we cannot see them. But a gap can also be created if small grains coagulate to larger grains.”
The CRIRES instrument on the VLT allows astronomers to make use of the innovative spectro-astrometric imaging technique. Image: ESO.
The new results also confirm that molecular gas is still highly abundant in regions where dust has been swept out, either a signature of the presence of a planetary embryo, or that a planet has already formed and is in the process of clearing the gas in the disc. For one of the stars in the study, SR 21, a likely explanation for the dust gap is the presence of a massive giant planet orbiting at less than 3.5 times the distance between the Earth and the Sun, while for the second star, HD 135344B, a possible planet could be orbiting at 10 to 20 times the Earth-Sun distance. The observations of the third star, TW Hydrae, may also require the presence of one or two planets.
“This conclusion follows from the location and the motions of the gas, combined with model simulations of the interaction of a planet with a gaseous disc,” van Dishoek tells Astronomy Now.
It is expected that these kinds of observations will complement the future work of the ALMA observatory, which will be imaging planet-forming discs in greater detail and on a larger scale. “Although it will not be easy,” adds van Dishoek. “It depends on the distance of the planet from the star whether we can image it. Radial velocity measurements could indirectly confirm the presence of these planets, if they can be done accurately enough on these young stars.”
The new research clearly reveals that the discs around the three young Sunlike stars are all very different and will most likely result in very different planetary systems. "Nature certainly does not like to repeat herself," concludes Pontoppidan.