It’s one thing to observe the periodic dimming of a star’s light, as NASA’s Kepler space telescope has done for thousands of planetary “candidates” since its launch in 2009. The front of a star, unlike any number of false positives such as a binary star companion, requires intensive follow-up work with ground-based instruments, usually a measurement of radial velocity to determine the object’s mass.
To ease this workload, planetary astronomers have come up with a few different statistical techniques, but none have been fully automated until now. Timothy Morton, a researcher at Princeton University, has developed software that can, in minutes, assess the orbital period and other data collected by Kepler to assign a statistical probability that the “candidates” of planets are or are not planets. When tested on previously confirmed exoplanets and false positives, the new technique worked almost flawlessly.
Described in the Astrophysical Journal, this new method allowed scientists to quickly review Kepler’s July 2015 planetary candidate catalog, which identified 4,302 potential planets. Of these, the new technique found that 1,284 were planets with a confidence level of 99 percent or higher. Another 1,327 are probable planets, but did not reach the 99 percent confidence threshold.
The new number of nearly 1,300 confirmed exoplanets is more than double the existing 984 exoplanets previously found by Kepler and other observatories over the course of two decades. It also more than doubles the number of known Earth-sized and super-Earth-sized exoplanets. Finally, it brings to 21 the number of planets less than twice the size of Earth discovered in the habitable zones of their stars. In those habitable zones, water may exist in liquid form on the surface.
These additions to Kepler’s planetary count move the mission’s scientists closer to their goal of quantifying the number of stars in our galaxy that harbor Earth-sized worlds in the habitable zone around their parent stars. “These are the hardest planets to find,” Natalie Batalha, co-author of the paper and the Kepler mission scientist at NASA’s Ames Research Center, said during a conference call Tuesday.
However, there is enough data to make some educated guesses. The Kepler mission has observed about 150,000 stars. During its first four years of flight, the spacecraft was tuned primarily to stars smaller and fainter than the Sun, mostly M-class stars. Batalha said Kepler has found that about 24 percent of these lower-mass, reddish stars harbor potentially habitable worlds the size of Earth (smaller than 1.6 times Earth’s radius). Based on the number of M-class stars in the galaxy, that alone represents about 10 billion potentially habitable, Earth-like worlds.
Since the original mission, Kepler has entered a new phase of data collection, known as the K2 mission, in which the spacecraft is observing more K- and G-class stars. These stars are as warm as or nearly as warm as the Sun. Initial analysis of this K2 data suggests that about a quarter of these larger stars may also have Earth-sized worlds in their habitable zones. Taken together, these main-sequence stars make up about 70 percent of the estimated 100 billion stars in the Milky Way. “So by my calculation, there are tens of billions of potentially habitable planets in the galaxy,” Batalha said.
Kepler will continue making observations for now, but scientists say they hope to complete cataloging of the data by October 2017. worlds closer to Earth, which would make follow-up observations easier.
That process will begin as early as next year, with the launch of the Transiting Exoplanet Survey Satellite, which will observe about 200,000 stars near the sun to find potentially habitable planets. This instrument uses the same technique as Kepler, looking for faint light from those stars.
By finding planets closer, the TESS spacecraft would identify targets for subsequent instruments, such as the James Webb Space Telescope and possibly larger ground-based telescopes coming online in the 2020s, such as the Giant Magellan Telescope. These instruments will have the ability to study starlight passing through the atmospheres of nearby exoplanets, yielding some information about their composition. Scientists will look for water, methane, carbon dioxide and other gases that may indicate the presence of biological activity.
Future missions like the WFIRST telescope and successor instruments may allow astronomers to actually image promising planets, first Neptune-sized and eventually all the way to Earth-sized worlds by eclipsing the light from their parent stars. But these telescopes are probably one to two decades in the future.
For now, astronomers will have to content themselves with nearly 1,300 new worlds and the prospect of our galaxy teeming with planets likely much like our own pale blue dot.