Peering more than 26,000 light years away, UCLA and NASA
scientists using the Hubble Space Telescope have discovered 16 new
planet candidates, leading them to conclude there are probably
billions of planets spread throughout the galaxy.
The discovery also marks the opening of a new category of
planets, “Ultra-Short-Period Planets.” Five of these
candidates complete an orbit in less than one Earth day, according
to a NASA press release.
The small piece of the sky examined by Hubble was only about a
10th of the size of the moon as seen from Earth, yet the telescope
saw over 180,000 stars as it looked toward the central bulge of the
galaxy, said Dr. Michael Rich, a UCLA professor of astronomy and
member of the Sagittarius Window Eclipsing Extrasolar Planet Search
team.
“We monitored stars near the galactic center continuously
for seven days,” said team leader Dr. Kailash Sahu of the
Space Telescope Science Institute, Baltimore. “If (a planet)
orbits around the star, and happens to come in front of the star,
then it blocks a small amount of light. … We detected 16 such
planet candidates that pass in front of a star.”
To confirm that the dimming was caused by an object orbiting a
star rather than something simply passing by it, the team used
Hubble to detect between two and 15 consecutive transits, or
instances of the planet passing in front of the star, for each of
the 16 planet candidates, NASA said.
Examining the center of the Milky Way, the scientists found that
planets are spread out uniformly throughout the galaxy.
In the solar neighborhood, 6 percent of the stars have planets
around them, and the new information from Hubble confirmed that
roughly 5 percent to 6 percent of stars in the galactic center have
planets as well.
“The frequency of planets in the solar neighborhood is
similar even at such a large distance as the galactic bulge,”
Sahu said.
“That gives us some confidence in extrapolating our solar
neighborhood results to the entire galaxy … so now we can say,
yes, indeed, there are billions of planets in the galaxy,” he
said.
Rich said some planets scientists have recently discovered are
called “hot Jupiters,” gas giants that orbit very close
to their stars. Since these planets bear some similarity to planets
in our own solar system, Rich said it is reasonable to infer that
we might someday find Earth-like planets orbiting other stars as
well.
But he added that it would be difficult to detect Earth-like
planets using the methods scientists employed for this study, which
currently detects objects roughly the same size as Jupiter.
“While (Earth-like planets) do block out some of the
light, it would be too small for Hubble to measure,” Rich
said.
NASA’s future Kepler Mission will continuously monitor
about 100,000 stars in the Milky Way for four or five years to
detect transiting planets, Sahu said. 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, NASA
said.