Imagine living on a planet with seasons so erratic you would hardly
know whether to wear Bermuda shorts or a heavy overcoat. That is the
situation on a weird, wobbly world found by NASA's planet-hunting Kepler
space telescope.
The planet, designated Kepler-413b, precesses, or wobbles, wildly on
its spin axis, much like a child's top. The tilt of the planet's spin
axis can vary by as much as 30 degrees over 11 years, leading to rapid
and erratic changes in seasons. In contrast, Earth's rotational
precession is 23.5 degrees over 26,000 years. Researchers are amazed
that this far-off planet is precessing on a human timescale.
Kepler 413-b is located 2,300 light-years away in the constellation
Cygnus. It circles a close pair of orange and red dwarf stars every 66
days. The planet's orbit around the binary stars appears to wobble, too,
because the plane of its orbit is tilted 2.5 degrees with respect to
the plane of the star pair's orbit. As seen from Earth, the wobbling
orbit moves up and down continuously.
Kepler finds planets by noticing the dimming of a star or stars when a
planet transits, or travels in front of them. Normally, planets transit
like clockwork. Astronomers using Kepler discovered the wobbling when
they found an unusual pattern of transiting for Kepler-413b.
"Looking at the Kepler data over the course of 1,500 days, we saw
three transits in the first 180 days -- one transit every 66 days --
then we had 800 days with no transits at all. After that, we saw five
more transits in a row," said Veselin Kostov, the principal investigator
on the observation. Kostov is affiliated with the Space Telescope
Science Institute and Johns Hopkins University in Baltimore, Md. The
next transit visible from Earth's point of view is not predicted to
occur until 2020. This is because the orbit moves up and down, a result
of the wobbling, in such a great degree that it sometimes does not
transit the stars as viewed from Earth.
Astronomers are still trying to explain why this planet is out of
alignment with its stars. There could be other planetary bodies in the
system that tilted the orbit. Or, it could be that a third star nearby
that is a visual companion may actually be gravitationally bound to the
system and exerting an influence.
"Presumably there are planets out there like this one that we're not
seeing because we're in the unfavorable period," said Peter McCullough, a
team member with the Space Telescope Science Institute and Johns
Hopkins University. "And that's one of the things that Veselin is
researching: Is there a silent majority of things that we're not
seeing?"
Even with its changing seasons, Kepler-413b is too warm for life as
we know it. Because it orbits so close to the stars, its temperatures
are too high for liquid water to exist, making it inhabitable. It also
is a super Neptune -- a giant gas planet with a mass about 65 times that
of Earth -- so there is no surface on which to stand.
NASA's Ames Research Center at Moffett Field, Calif., is responsible
for the Kepler mission concept, ground system development, mission
operations and science data analysis. NASA's Jet Propulsion Laboratory
in Pasadena, Calif., managed Kepler mission development. Ball Aerospace
& Technologies Corp. in Boulder, Colo., developed the Kepler flight
system and supports mission operations with the Laboratory for
Atmospheric and Space Physics at the University of Colorado in Boulder.
The Space Telescope Science Institute in Baltimore archives, hosts and
distributes Kepler science data. Kepler is NASA's 10th Discovery mission
and was funded by the agency's Science Mission Directorate.
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