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A New Look at ‘Exocomets’

Moving groups are collections of stars that share a common origin, useful to us because we can study a group of stars that are all close to each other in age. Among these, the Beta Pictoris moving group is turning out to be quite productive for the study of planet formation. These are young stars, aged in the tens of millions of years (Beta Pictoris itself is between 20 and 26 million years old). Within the moving group, we’ve detected planets around 51 Eridani and Beta Pictoris, while infalling, star-grazing objects have been found around Beta Pictoris.

Evidence of comet activity around another of these stars was discussed at the American Astronomical Society meeting in Texas. The star HD 172555, 23 million years old and about 95 light years from Earth, shows the presence of the vaporized remnants of cometary nuclei, marking the third extrasolar system where such activity has been traced. All the stars involved are under 40 million years old, giving us a glimpse of the kind of activity that happens during the era when young terrestrial planets have begun to emerge in their systems.


Image: This illustration shows several comets speeding across a vast protoplanetary disk of gas and dust and heading straight for the youthful, central star. The comets will eventually plunge into the star and vaporize. The comets are too small to photograph, but their gaseous spectral “fingerprints” on the star’s light were detected by NASA’s Hubble Space Telescope. The gravitational influence of a suspected Jupiter-sized planet in the foreground may have catapulted the comets into the star. This star, called HD 172555, represents the third extrasolar system where astronomers have detected doomed, wayward comets. The star resides 95 light-years from Earth. Credit: NASA, ESA, and A. Feild and G. Bacon (STScI).

Carol Grady (Eureka Scientific/NASA GSFC) led the study reported on at the AAS. Her thoughts:

“Seeing these sun-grazing comets in our solar system and in three extrasolar systems means that this activity may be common in young star systems. This activity at its peak represents a star’s active teenage years. Watching these events gives us insight into what probably went on in the early days of our solar system, when comets were pelting the inner solar system bodies, including Earth. In fact, these star-grazing comets may make life possible, because they carry water and other life-forming elements, such as carbon, to terrestrial planets.”

The deflection of comets by the gravitational influence of a massive gas giant in an emerging planetary system is a vivid picture, one clarified by Grady and team’s work with the Hubble Space Telescope Imaging Spectrograph (STIS) and the Cosmic Origins Spectrograph (COS) in 2015. The team’s spectrographic analysis, using Hubble data collected from two observing runs separated by six days, detected carbon gas and silicon in the light of HD 172555 moving across the face of the star at a speed of 160 kilometers per second.

This work follows up a French study that first found exocomets transiting the same star in archival data from the HARPS spectrograph. That work detected signs of calcium. Grady and team have extended the analysis with a spectrographic analysis in ultraviolet light. They believe they are seeing gaseous debris left behind as comets disintegrated, vaporized materials that contain large chunks of the original comet. Helpfully, the disk around HD 172555 is seen almost edge-on from Earth, offering Hubble a clear view of the highly dispersed activity.

“As transiting features go, this vaporized material is easy to see because it contains very large structures,” Grady said. “This is in marked contrast to trying to find a small, transiting exoplanet, where you’re looking for tiny dips in the star’s light.”

To confirm that they are seeing the disintegration of icy comets as opposed to rocky asteroids, Grady’s researchers hope to use the STIS again to search for oxygen and hydrogen, a composition that would add further weight to these conclusions.


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  • andy January 12, 2017, 15:58

    HD 172555 was previously claimed to be experiencing the aftermath of a giant hypervelocity impact between two planets. Do the exocomet results still support this idea?

  • Albert A Jackson January 12, 2017, 23:13

    That is very interesting.
    I was at that meeting and missed that!
    Big meetings get to be can’t see the trees for the forest.
    (The IAC in Guadalajara Mexico had a program book so big it would have taken the knowledge of the Krell to read it!)