Imaging planets around other stars is challenging enough because their light is overwhelmed by the proximity of the parent star. But what about comets? We may not be able to see them directly, but minute variations in light can mark their passage across the stellar disk. Nearly 500 comets have been detected around the star Beta Pictoris using these methods. New work led by Flavien Kiefer (IAP/CNRS/UPMC) analyzes this cometary hoard to give us a look at what is happening in a young planetary system.
Using the HARPS instrument at the European Southern Observatory’s site at La Silla in Chile, Kiefer and team have compiled what the ESO is calling ‘the most complete census of comets around another star ever created.’
Beta Pictoris is becoming an old friend, a young star some 63 light years from the Sun that is no older than 20 million years. The star is surrounded by a disk of material that has been the subject of intense study as we watch the interaction between gas, dust and the asteroids and comets that continue to produce them. Cometary ices evaporate as the comets approach the star, producing the familiar cometary tails we associate with the objects. Usefully, light passing through the released gas and dust can be analyzed to tell us about cometary composition.
Image: An artist’s impression of exocomets around the star Beta Pictoris. Credit: ESO.
Working with data from over 1000 observations obtained between 2003 and 2011, Kiefer’s team produced measurements of the size and speed of the gas clouds and was able to deduce orbital properties for a subset of the comets studied. What emerges is the presence of two distinct families of exocomets, one of them old and showing orbits highly influenced by the massive planet Beta Pictoris b, which was thought to orbit at a distance of one billion kilometers from the star. That number may now be reduced, for the eccentricity and orientation of the comets indicate they are in orbital resonance with an object about 700 million kilometers from the star.
The other comet family is newer and more active, with comets that are all on similar orbits, an indication of a common origin. The likelihood, the researchers say, is that this newer family of exocomets results from the breakdown of a larger object whose remains are now in an orbit that grazes the star. Says Kiefer: “For the first time a statistical study has determined the physics and orbits for a large number of exocomets. This work provides a remarkable look at the mechanisms that were at work in the Solar System just after its formation 4.5 billion years ago.”
The paper is Kiefer et al., “Two families of exocomets in the β Pictoris system,” Nature 514 (23 October 2014). Abstract available.
An Awakening Comet of Our Own
Meanwhile, a good deal closer to home, the comet 67P/Churyumov-Gerasimenko, under close scrutiny by the Rosetta spacecraft and its OSIRIS imaging system, is showing increasing signs of life. Still more than 450 million kilometers from the Sun, the comet is producing jets of dust along much of its surface, whereas in past months the dust was confined to the ‘neck’ region that connects the two lobes. OSIRIS principal investigator Holger Sierks (MPS) notes that the jets are now appearing on the ‘body’ and ‘head’ of the comet.
Image: Two views of the same region on the “neck” of comet 67P/Churyumov-Gerasimenko. The right image was taken with an exposure time of less than a second and shows details on the comet’s surface. The left image was overexposed (exposure time of 18.45 seconds) so that surface structures are obscured. At the same time, however, jets arising from the comet’s surface become visible. The images were obtained by the wide-angle camera of OSIRIS, Rosetta’s scientific imaging system, on 20 October, 2014 from a distance of 7.2 kilometers from the surface. ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
We should see much more activity as the comet reaches 300 million kilometers from the Sun and closer. Interestingly, the soon to be renamed ‘Site J’ at the ‘head’ of the comet, designated as the landing site, remains comparatively quiet. Rosetta’s Philae lander will make a landing attempt in November, and we’ll have the opportunity to see an awakening comet close up. Both lander and orbiter are to remain in operation until the comet’s closest approach to the Sun in August of 2015. Keep an eye on the European Space Agency’s Rosetta page for updates.