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Hubble Space Telescope Detects Comet Shower in HD 172555

This illustration shows several comets speeding across a vast protoplanetary disk of gas and dust and heading straight for the youthful, central star. These "kamikaze" 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. Credits: NASA, ESA, A. Feild and G. Bacon (STScI)

Recent interstellar forecast for a nearby star indicates a comet shower. This was revealed by NASA’s Hubble Space Telescope.

Comets plunging onto the star HD 172555 have been discovered by NASA’s Hubble Space Telescope. The star HD 172555, residing 95 light-years from Earth, is a youthful 23 million years old.

The exocomets - comets outside our solar system - were not directly observed around the star. However, their existence was inferred by detecting gas that is potentially the vaporized remnants of their icy nuclei.

Doomed, wayward comets have been detected by astronomers in the third extrasolar system, which is represented by HD 172555. All of the systems are young, under 40 million years old.

The existence of these doomed comets offers circumstantial evidence for “gravitational stirring” by an unseen Jupiter-size planet, where comets deflected by its gravity are catapulted into the star. New insights into the present and past activity of comets in the solar system are provided by these events. It is a mechanism in which water could have been transported to Earth and the other inner planets of the solar system by infalling comets.

Similar plunges have been discovered in the solar system by astronomers. Sun-grazing comets regularly fall into the 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. 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.

Carol Grady, NASA's Goddard Spaceflight Center

Grady presented her team’s results on Jan. 6 at the winter meeting of the American Astronomical Society in Grapevine, Texas.

The star HD 172555 is part of the Beta Pictoris Moving Group, which refers to a collection of stars born from the same stellar nursery. This star is the second group member that harbors such comets. Beta Pictoris, the group’s namesake, is also feasting on exocomets traveling extremely close. A young gas-giant planet has been identified in the vast debris disk of the star.

It is vital to analyze the stellar group as it is the closest collection of young stars to Earth. Almost 37.5% of the majorly 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. Grady stated that the grouping is at about the age that it should be building terrestrial planets.

A group of French astronomers initially discovered exocomets transiting HD 172555 in archival data collected between 2004 and 2011 by the European Southern Observatory’s HARPS (High Accuracy Radial velocity Planet Searcher) planet-finding spectrograph.

A spectrograph separates light into its component colors, permitting astronomers to identify the chemical makeup of an object. The HARPS spectrograph identified the chemical fingerprints of calcium imprinted in the starlight, and this indeed is proof that comet-like objects were falling into the star.

In 2015, Grady’s team, as a follow-up to that discovery, used Hubble’s Space Telescope Imaging Spectrograph (STIS) and the Cosmic Origins Spectrograph (COS) to carry out a spectrographic analysis in ultraviolet light, which enables Hubble to detect the signature of specific elements. Two observations, separated by six days, were made by Hubble.

Silicon and carbon gas were detected by Hubble in the starlight. The gas was traveling at about 360,000 miles per hour across the face of the star. The cause for this speedy gas is due to the fact that Hubble is seeing material from comet-like objects that broke apart after streaking across the face of the star.

The gaseous remains from the disintegrating comets are greatly dispersed in front of the star.

As transiting features go, this vaporized material is easy to see because it contains very large structures. 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.

Carol Grady, NASA's Goddard Spaceflight Center

Hubble obtained this detail because the HD 172555 debris disk surrounding the star is a bit inclined to Hubble’s line of sight, enabling the telescope to get a clear view of comet activity.

Grady’s team plans to use STIS again in follow-up observations to look for hydrogen and oxygen, which indeed would confirm the identity of the disintegrating objects as comets.

Hubble shows that these star-grazers look and move like comets, but until we determine their composition, we cannot confirm they are comets. We need additional data to establish whether our star-grazers are icy like comets or more rocky like asteroids.

Carol Grady, NASA's Goddard Spaceflight Center

The Hubble Space Telescope is a project of global cooperation between the European Space Agency and NASA. The telescope is managed by NASA Goddard. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, carries out Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington, D.C.

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