Astronomers have been able to study planet-forming discs around young
Sun-like stars in unsurpassed detail, clearly revealing the motion and
distribution of the gas in the inner parts of the disc. This result,
which possibly implies the presence of giant planets, was made possible
by the combination of a very clever method enabled by ESO’s Very Large
Planets could be home to other forms of life, so the study of
exoplanets ranks very high in contemporary astronomy. More than 300
planets are already known to orbit stars other than the Sun, and these
new worlds show an amazing diversity in their characteristics. But
astronomers don’t just look at systems where planets have already
formed — they can also get great insights by 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.
Pontoppidan and colleagues have analysed three young analogues of
our Sun that are each surrounded by a disc of gas and dust from which
planets could form. These three discs are just a few million years old
and were known to have gaps or holes in them, indicating regions where
the dust has been cleared and the possible presence of young planets.
The new results not only confirm that gas is present in the gaps in
the dust, but also enable astronomers to measure how the gas is
distributed in the disc and how the disc is oriented. In regions where
the dust appears to have been cleared out, molecular gas is still
highly abundant. This can either mean 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.
For one of the stars, SR 21, a likely explanation 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.
"Our observations with the CRIRES instrument on ESO’s Very Large
Telescope clearly reveal that the discs around these three young,
Sun-like stars are all very different and will most likely result in
very different planetary systems," concludes Pontoppidan. "Nature
certainly does not like to repeat herself."
"These kinds of observations complement the future work of the ALMA
observatory, which will be imaging these discs in great detail and on a
larger scale," adds Ewine van Dishoeck, from Leiden Observatory, who
works with Pontoppidan.
To study the gaps in dust discs that are the size of the Solar
System around stars that are located up to 400 light-years away is a
daunting challenge that requires a clever solution and the best
"Traditional imaging cannot hope to see details on the scale of
planetary distances for objects located so far away," explains van
Dishoeck. "Interferometry can do better but won’t allow us to follow
the motion of the gas."
Astronomers used a technique known as ‘spectro-astrometric imaging’
to give them a window into the inner regions of the discs where
Earth-like planets may be forming. They were able not only to measure
distances as small as one-tenth the Earth-Sun distance, but to measure
the velocity of the gas at the same time.
"The particular configuration of the instrument and the use of
adaptive optics allows astronomers to carry out observations with this
technique in a very user-friendly way: as a consequence,
spectro-astrometric imaging with CRIRES can now be routinely
performed," says team member Alain Smette, from ESO.