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A
tantalizing group of alien planets that may include the smallest, most Earth-sized
world yet seen has been discovered around a star like our sun, NASA announced
Thursday.
Observations
from the Kepler
space observatory confirmed two Saturn-sized planets that orbit a star
about 2,300 light-years from Earth. They also revealed a candidate for a
possible planet roughly the size of Earth within the same system.A
Astronomers
have not yet confirmed the potential Earth-like planet, but early analysis
suggests it has a radius just 1.5 times that of Earth. The Earth’s radius is
about 3,962 miles (6,378 km).
The
system’s two larger planets - called Kepler-9b and Kepler-9c - were found to
have similar diameters, masses and densities as Saturn. The findings represent
the first candidate system of multiple
planets found by Kepler to be confirmed
as such. [The
Strangest Alien Planets]
However,
the planets are so close to their parent star that their orbits would fit
inside the orbit of Mercury in our own solar system, astronomers said. The
Earth-sized world, if confirmed, would be so close to its star that it would
look nothing like Earth. It would be hot and likely not habitable.
A
study led by Matthew Holman, associate director of the Theoretical Astrophysics
Division at the Harvard-Smithsonian Center for Astrophysics in Cambridge,
Mass., examined the newly confirmed planetary system. The results are detailed in
the Aug. 26 online edition of the journal Science.
The
Kepler planets are the second group of alien worlds to be announced this week.
On
Tuesday, astronomers with the European Southern Observatory announced the discovery
of up to seven planets (though two remain to be confirmed) circling a star
127 light-years from Earth. That find also included a potential Earth-like
planet, one that is 1.4 times the mass of Earth.
Another
Earth?
Astronomers
do not yet have a mass estimate for the possible Earth-like world seen by the
Kepler observatory, Holman said.
For
the time being, the potential third planet about Earth’s size remains only a
candidate, but if confirmed, it would be the "smallest known planet to date," he added.
Kepler’s
observations suggest the planet has a radius that is about 1.5 times the radius
of Earth, and an orbital period of about 1.6 Earth days, which is much shorter
than Kepler-9b and 9c.
It
takes the larger planet Kepler 9b about 19.2 days to complete one orbit. The
other Saturn-sized world makes one orbit every 38.9 days.
More
measurements need to be taken, however, before the possible Earth-sized world
can be confirmed as a planet.
"If
we knew its mass, we would know if it’s actually a planet," Holman told
SPACE.com. "We also need to prove that the body is orbiting the same star,
and not some astrophysical false positive. We want to make sure that we’ve
exhausted all the other possibilities and have convincing evidence that it is a
planet."
Still,
the ability to detect such a small candidate demonstrates the sensitivity of
Kepler’s instruments, and shows that the mission is on track, he said.
"This
shows that we’re able to find not only long-period systems, but small planets
as well," Holman said. "One of the messages of this work is that
Kepler is making progress toward its goal of finding systems of planets that
are similar to our own solar system.
Too
hot to handle
But,
in terms of habitability, the Kepler-9 system may not be the best place to
search for life forms of any kind.
"It’s
very unlikely that these planets are habitable," Holman said.
Holman
estimated the temperatures of the two large planets by measuring the amount of
light they reflect, which gives a rough idea of how much the planets absorb the
parent star’s light. Given these assumptions, the Saturn-sized planets are
probably 740 degrees Kelvin (872 degrees Fahrenheit) and 540 degrees Kelvin
(512 degrees Fahrenheit), he said.
"Those
temperatures are far beyond the boiling point of water, so these are not habitable
planets," Holman said. "This would not be the first place to look for
habitability."
The
Kepler-9 system
Holman
and his colleagues analyzed seven months of Kepler data, and were able to
determine the size of the planets by measuring the amount of light they block
when passing in front of the parent star. To estimate their masses, however,
the researchers had to observe the gravitational interaction between the two
exoplanets.
"What
you get from Kepler is just the size - you don’t get the mass," Holman said.
But in the case of the Kepler-9 system, there is more than one planet
transiting the star. We can see the deviation from the times at which the
planets transit and measure how much the planets are gravitationally
interacting, which is due to their mass."
The
Kepler space observatory stares at a patch of the Milky Way that contains more
than 156,000 stars, in the constellations Cygnus and Lyra. The spacecraft
monitors the stars for subtle changes in their brightness, which could indicate
the presence of alien planets passing in front of them as seen from Earth.
By
measuring the tiny decreases in the brightness of stars when planets cross - or
transit - in front of them, astronomers can determine the size of the planet.
In
June, Kepler mission scientists announced that more than 700
candidate exoplanets had been found, including five systems that appear to
have more than one transiting planet.
Researchers
run these candidates through a set of procedures to verify that the signals are
indeed from planets, and are not false positives created by binary stars.
"What
is really important here is timing signatures," Holman said. "They
are characteristic of the mass of the planets, so we can say with confidence
that these are planets and not stars."
Studying
the orbits
The
two Saturn-sized planets in the Kepler-9 system do not have precisely circular
orbits, said Holman, but they are not highly elliptical, either. From detailed
modeling and by analyzing the timing variations, the researchers were also able
to discern the planets’ orbital plane.
"We
can verify by the timing variations that the planets occupy a very similar
orbital plane," Holman said. "We expect that they have similar
co-planarity to our solar system - probably co-planar to 10 degrees or so. But,
their orbits are pretty different."
Furthermore,
it is possible that there are other planets that transit the same parent star
in the system that have yet to be found.
"It’s
possible that there are other planets there that don’t actually transit the
star," Holman said. "If a planet has a more inclined orbit, and
doesn’t cross the face of the star from our line of sight, we would only detect
it from the timing variations."
Holman
and his colleagues are hoping to expand upon their study by using Kepler and
other data to develop a census of the planets in the system.
- Original Story: Newfound Alien Planets May Include Smallest One Yet
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