Last month, NASA declared its Kepler mission to hunt exoplanets at an end when one of the space telescope’s reaction wheels failed. Unable to keep itself pointed in the right direction, it could no longer carry on its hunt for planets beyond the Solar System. That seemed like the end of things, but Keith Horne of the University of St Andrews and Andrew Gould of Ohio State University disagree. They claim that Kepler could still hunt for exoplanets using gravity microlensing to detect how stars with planets distort space.
Before its malfunction, Kepler found over 3,000 possible exoplanets with 132 confirmed. The reaction wheel that failed was one of four used by the spacecraft to keep it pointing steady. One of the wheels had failed previously and the loss of a second left the space telescope unable to maintain the precise control needed for hunting exoplanets.
This precise control was needed because Kepler sought exoplanets by measuring how the light from a star would dip as a planet passed in front of it. Kepler needed a high degree of stability and very precise control to record exoplanet transits. The most likely candidates for exoplanets will be close to their stars and so move very quickly across them, meaning that Kepler must remain precisely aligned or its a classic case of “blink and you've missed it.”
The loss of the reaction wheel removed this capability and Kepler is currently in a rest state to conserve fuel for its attitude rockets and to keep its solar panels pointed at the Sun.
Horne and Gould’s idea is based on the fact that not only is there more than one way to skin a cat, there’s also more than one way to hunt exoplanets. They propose that Kepler could still be used to detect what is called “gravity microlensing.”
Gravity lenses are a product of Einstein's General relativity theory. The idea is that the mass of star is great enough that it produces enough gravity (or distorts space enough) to bend light like a lens, which means that it will alter how things like other stars or galaxies behind the star will look. If the star is massive enough, the effect is obvious, but even a small star’s lensing can be measured.
Since gravity lensing is most obvious with black holes and galaxies, this smaller scale phenomenon is called microlensing. If a planet is orbiting the star, the planet’s mass contributes to the microlensing effect and, as it moves, the distortion caused can be measured and from those measurements many properties of the planet can be deduced.
According to Horne and Gould, gravity microlensing doesn't require the same level of precision as transiting. Transits happen very quickly and are only a small fraction of the exoplanet’s year, so Kepler had to survey many stars for long periods to catch that fleeting event. Microlensing events are about ten times larger than transit events and last for weeks or even months instead of the hours or minutes for transits and the microlensing distortion is detectable across the entire event.
Horne and Gould calculate that from its position in space, Kepler is on a par with ground-based observatories and it can also study all microlensing events in its field of view simultaneously. What they see Kepler doing is working in conjunction with these observatories to provide a very long baseline to detect what is called “microlens parallax,” which acts as a sort of stereo vision for detecting effects that would otherwise be too faint to measure. It’s a bit like trying to figure out how far away something is by closing one eye and then the other to see if it shifts.
If Kepler could be repurposed, Horne and Gould say that it would be used for looking for a different kind of exoplanet. Transits work best in detecting planets close to their stars, but microlensing works best at detecting planets further away, so Kepler would be seeking cool Earth-mass planets beyond what is called the “snowline” where any water present would freeze. This may not be of much direct use in detecting inhabitable planets, but would provide valuable data on planetary systems.
Once found, the microlensing measurements would help in determining a number of properties, such as the planet’s mass and distance, how it sits in relation to its star, and its galactic position. In addition, it could also detect planets orbiting brown dwarfs or black holes as well as brown dwarf binary systems.
Using Kepler for gravity microlens study is still just an idea. If NASA does show any interest, there remains the question of whether or not it’s technically feasible, how much it would cost, whether building a new spacecraft would be cheaper, and how to rewrite Kepler’s software for the new mission.