Kepler's planet-hunting days may not be over just yet.
A technical failure in May left the NASA spacecraft, launched in 2009 to search for exoplanets, or planets outside our solar system, permanently crippled. But it could be repurposed to spot planets by studying the gravitationally warped light of distant stars, scientists say. (Related: "Kepler Spacecraft Disabled; 'Exciting Discoveries' Still to Come.")
"Kepler was not designed for this at all. This is a reuse, a repurposing, of Kepler," said Keith Horne, an astronomer at the University of St. Andrews in the U.K. and a co-author of a new paper outlining the alternative Kepler mission.
Horne and his colleague Andy Gould of Ohio State University say that by using a technique called gravitational microlensing, Kepler could spot cool, massive planets that are a different breed from the "hot Jupiters" the spacecraft has been so good at finding in the past.
Moreover, there's a small chance that by using their method, Kepler could confirm the existence of one of the most bizarre planetary systems ever dreamed up by astronomers: worlds that circle black holes.
"This is a very exotic type of planet, and we would have to be very lucky to pick up an object like that," Horne said. "But in principle, Kepler would be capable of that."
Hard Times for Kepler
Kepler was initially designed to find exoplanets by scanning for the faint dimming of starlight that happens when a planet passes in front of, or "transits," its parent star, as seen from Earth.
Kepler has used this so-called transit technique to great effect: During four years of operation, it discovered 134 exoplanets and more than 3,000 planet candidates.
The transit technique requires very high precision, however, and Kepler relied on four gyroscopic reaction wheels to keep it accurately pointed at a patch of sky in the direction of the constellations Cygnus and Lyra.
But in July 2012, and then again in May 2013, Kepler lost the functionality of first one and then two reaction wheels. Despite repeated attempts by NASA to reactivate them, the wheels remain unresponsive.
In August, NASA announced it was ending attempts to restore the reaction wheels and declared an end to Kepler's original mission. At the same time, the space agency also began soliciting ideas from scientists for alternative "two-wheel" experiments for Kepler. (Also see "Q&A: The Future of the Kepler Spacecraft.")
One of those ideas, submitted in a white paper by Gould and Horne, proposes that Kepler's instruments be repurposed to find alien worlds using an alternative method.
Instead of trying to catch planets as they pass in front of their stars, Kepler would try to find alien worlds by looking for "microlensing events"—instances in which a planet subtly warps passing starlight by its gravity.
To understand how microlensing works, imagine a situation in which a star is positioned behind another star as viewed from Earth's line of sight.
As light from the background star travels past the foreground star on its way to us, it will get distorted, or bent, by the intervening star's gravity.
"The gravitational field of the intervening star acts like a lens," Horne explained. "It will magnify the light of the background star for a few weeks, leading to increases and decreases in brightness."
By measuring the duration of that stellar flicker, astronomers can calculate the approximate mass of the lens star. If the star harbors a planet at just the right location, the planet's gravitational field will also deflect the background star's light.
"You'll have a much briefer magnification effect," Horne explained. "The light that's already magnified by the gravity of the lens star for a few weeks will also get magnified by the planet for a few days."
Astronomers are already using ground-based telescopes to search for exoplanets using gravitational microlensing, but Kepler would bring a valuable component currently lacking in ground observations: depth.
"Kepler is circling the sun in an orbit that trails behind the Earth by quite a big distance," Horne explained, "so by looking at the same events from a different perspective, we can tell how far away the lens star is, and we can use that information to determine what the mass of the star and the mass of the star's planet is."
A Pathfinder Mission
Horne and Gould have modest expectations for how many planets Kepler might find with gravitational microlensing. "We expect to get two to four planets from this," Gould said. (See "Most Earthlike Planets Found Yet: A 'Breakthrough.'")
But the pair hope that a successful demonstration of the technique by Kepler could pave the way for other gravitational microlensing missions.
"We're proposing this as a pathfinder mission," Gould said.
Future satellites dedicated to gravitational microlensing observations would be cheaper, he added, because they would not require the high level of precision Kepler was originally designed for.
Turning Kepler into a gravitational microlensing mission would also allow astronomers to glimpse a class of planets that have been elusive until now.
"Kepler previously was very sensitive to planets that are very close to their stars, so hot planets primarily," Horne explained. "But microlensing is more sensitive to planets farther out [from their stars], so colder planets."
These cooler planets are located outside of the so-called habitable zone—the region around a star where the temperature is warm enough to sustain liquid water, and thus life as we know it. (Related: "Explosion in Number of Potentially Habitable Worlds.")
"By measuring the abundance of planets in this region, we'll be getting information about how abundant Earth-size planets are outside of the habitable zone to complement Kepler's measurements inside the habitable zone," Horne said.
Almost as an aside, Horne and Gould note that if they are very lucky, their technique could allow Kepler to spot another class of exotic planets: worlds circling black holes.
The Impossible Planet
In the TV series Dr. Who, the main character once found himself on a dark planet orbiting a black hole, an improbable situation that was reflected in the episode's title: "The Impossible Planet."
But Brad Hansen, an astronomer at UCLA who has probably given more serious thought to black hole planets than anyone, doesn't think such worlds are impossible—just very improbable.
In theory at least, there are several pathways by which a planet could come to orbit a black hole, he says.
Perhaps the simplest way is for a wandering black hole to gravitationally capture a planet as it travels past a star. The odds of this happening might be higher in a globular cluster, Hansen said, where stars are more closely packed.
Alternatively, a free-floating, or "rogue," planet traveling through the Milky Way could stray close enough to a black hole that it slows down and gets gravitationally roped into an orbit. (Related: "'Nomad' Planets More Common Than Thought, May Orbit Black Holes.")
Another possibility is for two neutron stars—the dead, super-dense cores of exploded stars—to merge and form a black hole. The merger could leave behind enough rubble to kickstart planet formation, leading to the creation of what some astronomers call "second-chance planets."
"Mergers of things tend to leave behind disks, and disks often form planets," Hansen said.
Finally, a planet already in orbit around a massive, dying star could survive when the star explodes in a supernova and becomes a black hole.
"The chance for planets to survive supernova explosions are not zero, but it's very low," said Penn State astronomer Alex Wolszczan.
In 1992, Wolszczan helped discover the first exoplanets ever, a pair of planets orbiting a dead, spinning star known as a pulsar—a scenario that most astronomers at the time also considered improbable.
Nevertheless, Wolszczan thinks black hole planets top even his discovery in the weirdness department. "Nothing beats planets around a black hole," he said. "That would be the most bizarre scenario I can imagine."
While the chances of Kepler finding a black hole are small, the signal from one should be easy to distinguish from those of normal stars.
That's because most of the lens stars that scientists expect Kepler to observe are red dwarfs. These diminutive, reddish stars are the most common stars in the Milky Way and have masses equal to about half of our sun.
Black holes are much more massive. The intermediate-mass black holes Kepler might stumble upon have average masses equal to about 20 suns.
If Kepler did spot a black hole, "its light curve ... would tell us that it's a 20 solar mass object. If that were a star, it would be so bright that we would see it," Horne explained.
To Kepler's eyes, then, a black hole would look like a giant star that was inexplicably dark. If the black hole hosted a planet at the right distance, its mass would also warp the passing starlight and could be picked up by Kepler as well.
The team says that while they would be overjoyed if Kepler discovered a black hole planet using their technique, they aren't holding their breaths.
"Kepler could find a black hole planet. I just don't think it's likely," Gould said.
Kepler project scientist Steve Howell called Gould and Horne's plan for the spacecraft "well thought out," but noted it's only one of several under consideration.
In total, 42 white papers outlining alternative Kepler missions have been submitted to the Kepler science team, Howell said. Not all of the ideas involve using Kepler to look for exoplanets; some propose using the spacecraft to study galaxies, comets, and stars.
The submissions are currently being assessed for their scientific and technical merits by two separate panels of scientists. "By the first of November, we need to have a plan put together about what mission we believe will be the best science we can do with Kepler as a repurposed telescope," Howell said.
The goal is to have a plan that is "cost effective but still produces high impact science," he added.
That plan will then be forwarded to NASA headquarters, which will decide whether it should be included in the space agency's upcoming "senior review" next spring of operating space missions.
If the alternative Kepler plan is deemed worthy of funding, the spacecraft's first two-wheel experiments could begin as early as next summer.
Howell said that while gravitational microlensing is a proven technique, implementing Gould and Horne's ideas using Kepler will not be easy.
"It's something that would be challenging to do, but that's okay," Howell said. "We like challenging things."
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