A twin star system that is home to two planets. The planets have two suns like the fictional planet Tatooine in the "Star Wars" universe. |
Astronomers have for the first time discovered two alien planets
whirling around a pair of stars: a complete solar system with twin suns
just like Luke Skywalker's fictional home world Tatooine.
Most stars like our sun are not singletons, but rather come in pairs
that orbit each other. Scientists had found planets in these binary
systems, so-called circumbinary planets with two suns like Tatooine in the "Star Wars" universe.
To find more circumbinary planets, astronomers analyzed data from NASA's prolific Kepler space telescope,
which has detected more than 2,300 potential alien worlds since its
March 2009 launch. Kepler had to date detected four systems with
circumbinary planets — Kepler-16, 34, 35 and 38.
The scientists have now announced the detection of Kepler-47, the first
system seen with multiple worlds encircling a pair of stars. The star
and its planets, called Kepler-47b and Kepler-47c, dwell about 5,000
light-years away, in the constellation Cygnus, the Swan.
"Kepler-47 shows us that binary stars can have close-in planetary
systems, just like the ones we see in single stars," study lead author
Jerome Orosz at San Diego State University told SPACE.com. "Most of the
stars in the galaxy are in binary or higher-order multiple systems, so
the fact that planetary systems can exist in these types of systems is
important. If we were restricted to looking for planets around single
stars, we would be missing most of the stars in the galaxy."
Spotting "Tatooine" solar system
The planets are much too far away to see with the naked eye. Rather, they were discovered by the drop in brightness they cause when they cross in front of, or transit, their host stars.
This dimming is tiny, only 0.08 percent for planet Kepler-47b and 0.2
percent for planet Kepler-47c. By comparison, Venus blocked about 0.1
percent of the sun's surface during its recent transit. Data from Kepler
enabled researchers to deduce the relative sizes of the objects and
orbits. They also relied on follow-up observations performed by
telescopes at the McDonald Observatory in West Texas.
One of the stars is much like our sun, and the other is about a third
its size and 175 times fainter. The inner and outer planets are
respectively 3 and 4.6 times Earth's diameter — the smaller planet is
the smallest circumbinary planet seen yet.
The inner world completes an orbit every 49.5 days, while the outer one
takes 303.2 days, giving it the largest known orbit for any transiting
exoplanet. The stars themselves whirl around each other every 7.5 days.
The scientists published their findings online Aug. 28 in the journal
Science. They will also detail their results Aug. 29 at the General
Assembly of the International Astronomical Union in Beijing.
Planet in the habitable zone?
Intriguingly, the outer planet lurks in the system's habitable zone,
where a rocky planet like Earth is the right temperature to have liquid
water on its surface.
"We've learned that circumbinary planets can be like the planets in our own solar system, but with two suns," said study co-author Joshua Carter at the Harvard-Smithsonian Center for Astrophysics.
Although the outer planet is probably a gas giant slightly larger than
Uranus and therefore not suitable for life as we know it, these findings
do show that circumbinary planets can and do exist in habitable zones.
"The thing I find most exciting is the potential for habitability in a
circumbinary system," said study co-author William Welsh at San Diego
State University. "Kepler-47c is not likely to harbor life, but if it
had large moons, those would be very interesting worlds."
Circumbinary planets might undergo relatively wild climate swings.
On Earth, the sun is a relatively stable source of light, with the
solar insolation or energy we receive only varying by 0.1 percent or so.
"As a result, we don't need to worry about what the sun is doing, at
least over time-scales of years to decades," Orosz said. "For a planet
around a binary system, there can be changes in the insolation of
several percent on the time-scale of days to weeks. In addition, if the
planet's rotation axis is tilted, then that also has an effect.
Therefore the seasons are rapid and complicated."
"Also, let's say for the sake of discussion that the planet's rotation
period is 24 hours, just like the Earth," he added. "Because you have
two suns instead of one, you can have more than 12 hours of daylight,
depending on the positions of the stars at sunrise or sunset."
In addition, since all the circumbinary planets the Kepler space telescope
has discovered have orbits closely aligned with the orbits their stars
have with each other, "you would see your suns eclipse each other on a
regular basis," Orosz said. "In the case of Kepler-47, when the
secondary star passes in front of the primary, the total light drops by
15 percent. This would happen every 7.5 days or so."More exotic alien solar systems possible
The new discovery show that planetary systems can form and survive even in the chaotic environment around binary stars.
The researchers expect that the planets in Kepler-47 originated much
farther out than their present orbits, at locations where the conditions
for the formation of giant planets are more favorable. They eventually
then migrated inward due to interactions with the disk of gas and dust
that also encircled the stars.
"We think these planets and most other planets formed from a residual
disk of debris left over from the star-formation process," Orosz said.
"It was not at all obvious that this disk could survive near a newly
formed binary star, given the orbital motions of the two stars. However,
it now appears that apart from minor differences in the orbital
spacings, planetary systems around binary stars can be similar to those
around single stars."
In the future, the researchers want to look for smaller and smaller alien planets around binary stars.
"We are at the limit of what simple visual searches can do, so we need
better software to help us automate the process," Orosz said. "Given
more time and data, I think we can find more circumbinary planetary
systems in the Kepler data."
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