Oct. 4, 2006
Erica Hupp/Dwayne Brown
Headquarters, Washington
202-358-1237/1726
Ray Villard
Space Telescope Science Institute, Baltimore
410-338-4514
RELEASE: 06-326
HUBBLE FINDS EXTRASOLAR PLANETS FAR ACROSS GALAXY
NASA's Hubble Space Telescope has discovered 16 extrasolar planet
candidates orbiting a variety of distant stars in the central region
of our Milky Way galaxy.
The planet bonanza was uncovered during a Hubble survey called the
Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS).
Hubble looked farther than has ever successfully been searched before
for extrasolar planets. Hubble peered at 180,000 stars in the crowded
central bulge of our galaxy 26,000 light-years away. That is
one-quarter the diameter of the Milky Way's spiral disk. The results
will appear in the Oct. 5 issue of the journal Nature.
This tally is consistent with the number of planets expected to be
uncovered from such a distant survey, based on previous exoplanet
detections made in our local solar neighborhood. Hubble's narrow view
covered a swath of sky no bigger in angular size than two percent the
area of the full moon. When extrapolated to the entire galaxy,
Hubble's data provides strong evidence for the existence of
approximately six billion Jupiter-sized planets in the Milky Way.
Five of the newly discovered planets represent a new extreme type of
planet not found in any nearby searches. Dubbed Ultra-Short-Period
Planets (USPPs), these worlds whirl around their stars in less than
one Earth day.
"Discovering the very short-period planets was a big surprise," said
team leader Kailash Sahu of the Space Telescope Science Institute,
Baltimore. "Our discovery also gives very strong evidence that
planets are as abundant in other parts of the galaxy as they are in
our solar neighborhood."
Hubble could not directly view the 16 newly found planet candidates.
Astronomers used Hubble's Advanced Camera for Surveys to search for
planets by measuring the slight dimming of a star due to the passage
of a planet in front of it, an event called a transit. The planet
would have to be about the size of Jupiter to block enough starlight,
about one to 10 percent, to be measurable by Hubble.
The planets are called candidates, because astronomers could only
obtain follow-up mass measurements for two of them due to the
distance and faintness of these systems. Following an exhaustive
analysis, the team ruled out alternative explanations such as a
grazing transit by a stellar companion that could mimic the predicted
signature of a true planet. The finding could more than double the
number of planets spied with the transit technique to date.
There is a tendency for the planet candidates to revolve around stars
more abundant in elements heavier than hydrogen and helium, such as
carbon. This supports theories that stars rich in heavy elements have
the necessary ingredients to form planets.
The planet candidate with the shortest orbital period, named
SWEEPS-10, swings around its star in 10 hours. Located only 740,000
miles from its star, the planet is among the hottest ever detected.
It has an estimated temperature of approximately 3,000 degrees
Fahrenheit.
"This star-hugging planet must be at least 1.6 times the mass of
Jupiter, otherwise the star's gravitational muscle would pull it
apart," said SWEEPS team member Mario Livio. "The star's low
temperature allows the planet to survive so near to the star."
"Ultra-Short-Period Planets seem to occur preferentially around normal
red dwarf stars that are smaller and cooler than our sun," Sahu
explained. "The apparent absence of USPPs around sun-like stars in
our local neighborhood indicates that they might have evaporated away
when they migrated too close to a hotter star."
There is an alternative reason why Jupiter-like planets around cooler
stars may migrate in closer to the star than such planets around
hotter stars. The circumstellar disk of gas and dust out of which
they formed extends in closer to a cooler star. Since the discovery
of the first "hot Jupiter" around another star in 1995, astronomers
have realized this unusual type of massive planet must have spiraled
in close to its parent star from a more distant location where it
must have formed. The inner edge of a circumstellar disk halts the
migration.
Planetary transits occur only when the planet's orbit is viewed nearly
edge-on. However, only about 10 percent of hot Jupiters have edge-on
orbits that allow the planet to be observed transiting a star. To be
successful, transit surveys must view a large number of stars at
once. The SWEEPS transit survey covered a rich field of stars in the
Sagittarius Window.
The term "window" implies a clear view into the galactic center, but
much of the galactic plane is obscured by dust. Hubble monitored
180,000 stars for periodic, brief dimming in a star's brightness. The
star field was observed over a continuous seven-day period Feb.
23-29, 2004.
To ensure the dimming was caused by an object orbiting a star, the
team used Hubble to detect from two to 15 consecutive transits for
each of the16 planet candidates. Two stars in the field are bright
enough that the SWEEPS team could make an independent confirmation of
a planet's presence by spectroscopically measuring a slight wobble in
the star's motion due to the gravitational pull of an unseen
companion. They used the European Southern Observatory's Very Large
Telescope, located on Mount Paranal in Chile, to measure a slight
wobble in the star.
One of the planetary candidates has a mass below the detection limit
of 3.8 Jupiter masses. The other candidate is 9.7 Jupiter masses,
which is below the minimum mass of 13 Jupiter masses for a brown
dwarf. A brown dwarf is an object that forms like a star but does not
have enough mass to shine by nuclear fusion.
Since the stars are so faint and the field of view is so densely
packed with stars, measuring the slight wobble in the star's motion
using spectroscopy to confirm most of the planet candidates is not
feasible. Future telescopes such as NASA's James Webb Space Telescope
will provide the needed sensitivity to confirm most of the planet
candidates.
The Hubble SWEEPS program is an important proof-of-concept for NASA's
future Kepler Mission, scheduled for launch in 2008. The Kepler
observatory will continuously monitor a region of the Milky Way
galaxy to detect transiting planets around mostly distant stars.
Kepler will be sensitive enough to detect possibly hundreds of
Earth-size planet candidates in or near the habitable zone, the
distance from a star where liquid water could feasibly exist on a
planet's surface.
The Hubble Space Telescope is a project of international cooperation
between NASA and the European Space Agency. The Space Telescope
Science Institute conducts Hubble science operations. The institute
is operated for NASA by the Association of Universities for Research
in Astronomy, Inc., Washington.
For images and more information about this research. visit:
www.nasa.gov/hubble -end-
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