The Kepler space telescope, launched in 2009, normally focuses on planets around distant stars, and it does that very well. Occasionally the spacecraft turns its attention to objects in our own solar system. And every once in a while, there’s an observation that does service to both kinds of science. Recent Kepler observations of Neptune, for example, could tell astronomers more about brown dwarfs—objects that are typically bigger than planets, but not large enough to become stars.
Neptune has been visited by only one spacecraft, Voyager 2 in 1989. The giant gas planet also is observed regularly with two instruments: the Hubble Space Telescope orbiting Earth, and the Keck telescope in Hawaii. Kepler gave astronomers a new viewing opportunity in 2014 when it began its K2 mission, looking at targets near the ecliptic—the plane of the solar system that encompasses the planets’ orbits.
The spacecraft had spent its first few years gazing at one spot in the constellation Cygnus, looking for signs of planets orbiting other stars. But after Kepler lost two gyroscopes and couldn’t point precisely, the project came up with the K2 mission to look at selected targets around the sky. “Planets are naturally going to appear in the K2 field of view,” says Kepler team member Jason Rowe, an astronomer at the University of Montreal. When NASA was still considering whether to fund the K2 extension, he co-authored a white paper suggesting that astronomers could use the spacecraft to look at Neptune.
Rowe’s proposal for K2 was approved, and Kepler spent five months—between December 2014 and March 2015—gazing at a patch of the sky that happened to include Neptune. The planet moved quickly across the field of view because it’s so much closer than the distant stars, which presented an analysis challenge.
“Neptune was moving across hundreds of pixels, which made the data reduction and overall potential quality of the data a bit worse,” Rowe says. It took several months to sort it all out, but next time, he says, the data analysis will go faster.
He and his team were particularly interested in Neptune’s clouds. They used reflected light seen by Kepler and compared them to pictures taken by Hubble as part of its its regular observing program as well as targeted observations.
Neptune shares some properties with brown dwarfs, and Kepler’s observations of the planet could help scientists interpret weather patterns on brown dwarfs when telescopes become powerful enough to see them, Rowe says. The research was published in the journal Earth and Planetary Astrophysics.
The astronomers also were hoping to see convective patterns in Neptune’s upper atmosphere to learn about its interior, just as they do with the sun. No definitive pattern was spotted on Neptune, but Rowe says he hopes to try again with Kepler soon. The scientists also wondered if it’s possible for Kepler’s sensitive light meter to detect variations in the sun’s light by looking at the light reflected from Neptune. Rowe calls this “a difficult exercise to do,” but says it could potentially allow astronomers to resolve features on the surfaces of distant stars by looking at their planets. Some “systematic differences” were spotted, but Rowe said it’s not clear yet if those came from the sun. More observations are needed.
Rowe is now turning his attention to new Uranus observations from Kepler. Luckily, they were made just before the spacecraft temporarily went into emergency mode in April. With the spacecraft recovered, Rowe said he looks forward to getting new data from yet another nearby planet.