Exocomets Plunging into Star (Artist's Illustration)
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, and A. Feild and G. Bacon (STScI). Hi-res image
Interstellar forecast for a nearby star: Raining comets! NASA's
Hubble Space Telescope has discovered comets plunging into the star HD
172555, which is a youthful 23 million years old and resides 95
light-years from Earth.
The exocomets — comets outside our solar system — were not directly
seen around the star, but their presence was inferred by detecting gas
that is likely the vaporized remnants of their icy nuclei.
HD 172555 represents the third extrasolar system where astronomers
have detected doomed, wayward comets. All of these 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 the massive object's 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's 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
study leader Carol Grady of Eureka Scientific Inc., in Oakland,
California, and NASA's Goddard Space Flight Center in Greenbelt,
Maryland. "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."
Grady will present her team's results Jan. 6 at the winter meeting of the American Astronomical Society in Grapevine, Texas.
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 harbor 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 Beta Pictoris Moving Group is important to study because it is
the closest collection of young stars to Earth. At least 37.5 percent of
the more massive stars in the 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 around the age where it should be building
terrestrial planets, Grady said.
A team of 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) spectrograph. A spectrograph divides light into its component
colors, allowing astronomers to detect an object's chemical makeup. The
HARPS spectrograph detected the chemical fingerprints of calcium
imprinted in the starlight, evidence that comet-like objects were
falling into the star.
As a follow-up to that discovery, Grady's team used Hubble's Space
Telescope Imaging Spectrograph (STIS) and the Cosmic Origins
Spectrograph (COS) in 2015 to conduct a spectrographic analysis in
ultraviolet light, which allows Hubble to identify the signature of
certain elements. Hubble made two observations, separated by six days.
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 star's disk.
The gaseous debris from the disintegrating comets is vastly dispersed
in front of the star. "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."
Hubble gleaned this information because the HD 172555 debris disk
surrounding the star is viewed close to edge-on through the disk, giving
the telescope a clear view of comet activity.
Grady's team hopes to use STIS again in follow-up observations to
look for oxygen and hydrogen, which 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," Grady said. "We need additional data to establish whether our
star-grazers are icy like comets or more rocky like asteroids."
Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu
Carol Grady
Eureka Scientific Inc., Oakland, California,
and Goddard Space Flight Center, Greenbelt, Maryland
301-286-3748
carol.a.grady@nasa.gov
Contact
Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu
Carol Grady
Eureka Scientific Inc., Oakland, California,
and Goddard Space Flight Center, Greenbelt, Maryland
301-286-3748
carol.a.grady@nasa.gov
Source: HubbleSite/Newscenter