Hubble spots exocomets plunging into young star

Washington

NASA's Hubble Space Telescope has discovered comets plunging into a 23-million-years old star located 95 light-years from Earth.

The exocomets were not directly seen around the star, but their presence was inferred by detecting gas that is likely the vaporised remnants of their icy nuclei.

The star, HD 172555 represents the third extrasolar system where astronomers have detected doomed, wayward comets. All of the systems are young, under 40 million years old.

The presence of these doomed comets provides circumstantial evidence for "gravitational stirring" by an unseen Jupiter-size planet, where comets deflected by its gravity are catapulted into the star.

These events also provide new insights into the past and present activity of comets in our solar system. It is a mechanism where infalling comets could have transported water to Earth and the other inner planets of our solar system.

Astronomers have found similar plunges in our own solar system. Sun-grazing comets routinely fall into our sun.

"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," said Carol Grady from NASA's Goddard Spaceflight Centre in the US.

"These star-grazing comets may make life possible, because they carry water and other life-forming elements, such as carbon, to terrestrial planets," said Grady.

The star is part of the Beta Pictoris Moving Group, a collection of stars born from the same stellar nursery.

It is the second group member found to harbour such comets. Beta Pictoris, the group's namesake, also is feasting on exocomets travelling too close.

A young gas-giant planet has been observed in that star's vast debris disk.

The stellar group is important to study because it is the closest collection of young stars to Earth.

At least 37.5 per cent of the more massive stars in the Beta Pictoris Moving Group either have a directly imaged planet, such as 51 Eridani b in the 51 Eridani system, or infalling star-grazing bodies, or, in the case of Beta Pictoris, both types of objects.

The grouping is at about the age that it should be building terrestrial planets, Grady said.

French astronomers first discovered exocomets transiting HD 172555 in archival data gathered between 2004 and 2011 by the European Southern Observatory's HARPS (High Accuracy Radial velocity Planet Searcher) planet-finding spectrograph.

Hubble detected silicon and carbon gas in the starlight.

The gas was moving at about 360,000 miles per hour across the face of the star.

The most likely explanation for the speedy gas is that Hubble is seeing material from comet-like objects that broke apart after streaking across the face of the star.

Hubble gleaned this information because the HD 172555 debris disk surrounding the star is slightly inclined to Hubble's line of sight, giving the telescope a clear view of comet activity. — PTI