NASA's Kepler spacecraft, once the world's first census-taker for planets orbiting other stars, has detected the first extra-solar planet of its encore career as an astrophysics observatory.
The planet, dubbed HIP 116454b, orbits a star 180 light-years away in the constellation Pisces. The "Super-Earth" has about 12 times Earth's mass and is about 20,000 miles across. It orbits a mere 8.4 million miles from its host star, which is smaller and cooler than the sun. Initial estimates of the planet's density suggest it either is a water world or a mini-Neptune with an extended atmosphere, according to the team announcing the discovery.
It's a milestone for the spacecraft, whose initial assignment was to stare at a patch of the sky near the constellation Cygnus and monitor some 170,000 stars for regular, subtle variations in light that would signal an eclipsing planet. A major portion of its new assignment is to continue the hunt for planet candidates, including Earth-mass planets, but much closer to Earth. These would become targets for follow-up studies by the James Webb Space Telescope, currently slated for launch in October 2018.
“Like a phoenix rising from the ashes, Kepler has been reborn and is continuing to make discoveries. Even better, the planet it found is ripe for follow-up studies,” said Andrew Vanderburg, a researcher at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. Mr. Vanderburg is a doctoral student at Harvard University and the lead author of a paper that has been accepted for publication in the Astrophysical Journal describing the results.
But extra-solar planets are not the only targets the repurposed craft has on its agenda. The primary goal for Kepler version 1.0 was to survey the stars in its field of view for extra-solar planets in order to estimate the number of Earth-mass planets orbiting sun-like stars at earth-like distances. Version 2.0 has Kepler serving as a unique multi-purpose orbiting observatory – one that can stare relentlessly at one patch of the sky for months.
"There are a lot of things you can do with this repurposed Kepler mission" because it now is scanning a swath of the sky all along the ecliptic – the path the sun and planets appear to take as they move against the backdrop of stars dotting the celestial sphere, says Brad Schaefer, an astrophysicist at Louisiana State University in Baton Rouge.
Kepler's original mission aimed to answer a profound question about the number of potential Earths in the galaxy, Dr. Schaefer says. But it now "has so many targets, it can pick and choose," allowing it to address a much broader set of astronomical questions, he says.
Repurposing came about after Kepler lost the use of two of its four reaction wheels, devices critical to sustaining the precision pointing the craft needed to conduct its extra-solar-planet census. Kepler needed that precision because it detected planets by measuring subtle changes in starlight imparted by a eclipsing, or transiting, planet. This precision also helped researchers tell the difference between a true transiting planet and other phenomenon on the host star or in its neighborhood that could be mistaken for a planet's transit.
Other than the problem with the reaction wheels, Kepler was healthy. But the degraded pointing left it with an uncertain future. That changed when engineers found a way to use radiation pressure from sunlight, in concert with the remaining reaction wheels, to restore some of the craft's pointing precision. The improvement was good enough for a range of potential observations involving objects or events at distances ranging from inside the solar system to beyond the galaxy.
For instance, Schaefer and his wife, Martha Schaefer, a planetary geologist at LSU, have been working for years to figure out what's up with Neptune's moon Nereid – "in several ways the weirdest of all the moons in the solar system," the two noted in their proposal for observing time with Kepler.
Nereid, one of 14 moons Neptune hosts, orbits the planet once every 360 days and has the most oblong orbit of any moon in the solar system, perhaps an inner moon that was nearly ejected from the system. Disparate observations the duo have taken using ground-based telescopes between 1987 and 2008 suggests that the moon brightens at irregular intervals, ranging from hours to weeks.
Researchers have offered several explanations for the episodic brightening, but no one has been able to observe Nereid nonstop over several cycles of brightening and dimming. Kepler is aptly suited for that.
Their observing run is underway during an observing cycle that began Nov. 14 and will end Feb. 3, 2015.
With observations coming in every minute for one 20-day stretch during this time, "Kepler will be able to do what my wife and I have spent 400 nights on ground-based telescopes doing," Schaefer says.
Theirs is one of 116 projects Kepler is feeding during the current observing run. These involve topics ranging from to hunting for extra-solar planets to studying the behavior of various types of flaring stars and using red giant stars to help uncover the history of the galaxy.
Since Neptune is passing through the craft's field of view during this run, it's a popular object.
Another team is hunting for Neptune quakes, which would show up as variations in light the planet reflects from the sun. Observations of Jupiter and Saturn have yielded evidence for planet quakes. If Kepler can detect these on Neptune, the signals, in principle, could allow researchers to uncover fresh details about the structure of the planet's interior from a mere 2.8 billion miles away.
Yet another team aims to hunt for rings outside the boundary of Neptune's six known rings – something that is speculative and might not rise to a level of importance that would earn time on ground-based telescopes. Rings around the outer planets yields clues about how the planets formed and may yield insights on the structure of a significant number of Neptune-class planets Kepler uncovered during its first incarnation, according to the team of Hungarian researchers that proposed the project.
"It's an inventive idea," Schaefer says. "The odds of having a far outer ring like that are probably pretty low. But you don't know until you look."