Credit: Danielle Futselaar
AUSTIN — Astronomers from The University of Texas at Austin, working with scientists from other institutions and amateur astronomers, have spotted the dusty tails of six exocomets — comets outside our solar system — orbiting a faint star 800 light years from Earth.
These cosmic balls of ice and dust, which were about the size of
Halley’s comet and traveled about 100,000 miles per hour before they
ultimately vaporized, are some of the smallest objects yet found outside
our own solar system.
The discovery by Andrew Vanderburg, NASA Sagan Fellow at UT Austin,
and the team marks the first time that an object as small as a comet has
been inferred using transit photometry, a technique by which
astronomers observe a star’s light for telltale dips in intensity. Such
dips signal potential transits, or crossings of planets or other objects
in front of a star, which momentarily block a small fraction of its
light.
“It’s just thrilling to find these comets,” Vanderburg says. “No one
has ever seen anything quite like these transits before. These are some
of the first glimpses at the population of comets outside our own solar
system.”
The researchers were able to pick out the comet’s tail, or trail of
gas and dust, which blocked about one-tenth of 1 percent of the star’s
light as the comet streaked by.
“It’s amazing that something several orders of magnitude smaller than
the Earth can be detected just by the fact that it’s emitting a lot of
debris,” says Saul Rappaport, professor emeritus of physics in MIT’s
Kavli Institute for Astrophysics and Space Research. “It’s pretty
impressive to be able to see something so small, so far away.”
The team have published their results this week in the Monthly Notices of the Royal Astronomical Society. Rappaport
is the paper’s lead author. Other authors include Vanderburg, Adam
Kraus, and Aaron Rizzuto of The University of Texas at Austin;
astronomers from NASA Ames Research Center and Northeastern University;
and amateur astronomers including Thomas Jacobs of Bellevue, Washington.
The detection was made using data from NASA’s Kepler Space Telescope,
a stellar observatory that was launched into space in 2009. For four
years, the spacecraft monitored about 200,000 stars for dips in
starlight caused by transiting exoplanets.
To date, the mission has identified and confirmed more than 2,400
exoplanets, mostly orbiting anonymous stars in the constellation
Cygnus, with the help of automated algorithms that quickly sift through
Kepler’s data, looking for characteristic dips in starlight.
The smallest exoplanets detected thus far measure about one-third the
size of the Earth. Comets, in comparison, span just several football
fields, or a small city at their largest, making them incredibly
difficult to spot.
However, on March 18, Jacobs, an amateur astronomer who has made it
his hobby to comb through Kepler’s data, was able to pick out several
curious light patterns amid the noise.
Jacobs, who works as an employment consultant for people with
intellectual disabilities by day, is a member of the Planet Hunters — a
citizen scientist project first established by Yale University to enlist
amateur astronomers in the search for exoplanets. Members were given
access to Kepler’s data (which are now public) in hopes that they might
spot something of interest that a computer might miss.
In January, Jacobs set out to scan the entire four years of Kepler’s
data taken during the main mission, comprising over 200,000 stars, each
with individual light curves, or graphs of light intensity tracked over
time. Jacobs spent five months sifting by eye through the data, often
before and after his day job, and through the weekends.
“Looking for objects of interest in the Kepler data requires
patience, persistence, and perseverance,” Jacobs says. “For me it is a
form of treasure hunting, knowing that there is an interesting event
waiting to be discovered. It is all about exploration and being on the
hunt where few have traveled before.”
“Something we’ve seen before”
Jacobs’ goal was to look for anything out of the ordinary that
computer algorithms may have passed over. In particular, he was
searching for single transits — dips in starlight that happen only once,
meaning they are not periodic like planets orbiting a star multiple
times.
In his search, he spotted three such single transits around KIC
3542116, a faint star located 800 light years from Earth (the other
three transits were found later by the team). He flagged the events and
alerted Rappaport and Vanderburg, with whom he had collaborated in the
past to interpret his findings.
“We sat on this for a month, because we didn’t know what it was —
planet transits don’t look like this,” Rappaport recalls. “Then it
occurred to me that, ‘Hey, these look like something we’ve seen
before.’”
In a typical planetary transit, the resulting light curve resembles a
“U,” with a sharp dip, then an equally sharp rise, as a result of a
planet first blocking a little, then a lot, then a little of the light
as it moves across the star. However, the light curves that Jacobs
identified appeared asymmetric, with a sharp dip, followed by a more
gradual rise.
Rappaport realized that the asymmetry in the light curves resembled
disintegrating planets, with long trails of debris that would continue
to block a bit of light as the planet moves away from the star. However,
such disintegrating planets orbit their star, transiting repeatedly. In
contrast, Jacobs had observed no such periodic pattern in the transits
he identified.
“We thought, the only kind of body that could do the same thing and
not repeat is one that probably gets destroyed in the end,” Rappaport
says.
In other words, instead of orbiting around and around the star, the
objects must have transited, then ultimately flown too close to the
star, and vaporized.
“The only thing that fits the bill, and has a small enough mass to get destroyed, is a comet,” Rappaport says.
The researchers calculated that each comet blocked about one-tenth of
1 percent of the star’s light. To do this for several months before
disappearing, the comet likely disintegrated entirely, creating a dust
trail thick enough to block out that amount of starlight.
Vanderburg says the fact that these six exocomets appear to have
transited very close to their star in the past four years raises some
intriguing questions, the answers to which could reveal some truths
about our own solar system.
“Why are there so many comets in the inner parts of these solar
systems?” Vanderburg says. “Is this an extreme bombardment era in these
systems? That was a really important part of our own solar system
formation and may have brought water to Earth. Maybe studying exocomets
and figuring out why they are found around this type of star … could
give us some insight into how bombardment happens in other solar
systems.”
The researchers say that in the future, the MIT-led mission TESS
(Transiting Exoplanet Survey Satellite) will continue the type of
research done by Kepler.
Apart from contributing to the fields of astrophysics and astronomy,
Rappaport says, the new detection speaks to the perseverance and
discernment of citizen scientists.
“I could name 10 types of things these people have found in the
Kepler data that algorithms could not find, because of the
pattern-recognition capability in the human eye,” Rappaport says. “You
could now write a computer algorithm to find this kind of comet shape.
But they were missed in earlier searches. They were deep enough but
didn’t have the right shape that was programmed into algorithms. I think
it’s fair to say this would never have been found by any algorithm.”
This research made use of data collected by the Kepler mission,
funded by the NASA Science Mission directorate. This work was performed
in part under contract with the California Institute of Technology/Jet
Propulsion Laboratory funded by NASA through the Sagan Fellowship
Program executed by the NASA Exoplanet Science Institute. Original
release text courtesy of MIT News.
Media Contact:
Rebecca Johnson
Communications Mgr. McDonald Observatory
The University of Texas at Austin
512-475-6763
Science Contact:
Dr. Andrew Vanderburg, NASA Sagan Fellow
The University of Texas at Austin
Department of Astronomy
512-471-6493
Source: McDonald Observatory/News
