Kepler’s Planet-Hunting Days May Be Over, But Discoveries Are Still Expected

Artist impression of NASA’s Kepler Space Telescope. Image Credit: NASA

Following months of analysis and testing, the Kepler Space Telescope team has ended its attempts to restore the spacecraft to full working order. However, the spacecraft still could be used for other scientific research.

In the meantime, scientists will be poring over the data Kepler gathered while it was operating as designed. William Borucki, Kepler science principal investigator at NASA’s Ames Research Center in Moffett Field, Calif., anticipates it will take the Kepler team 3 to 4 years to find the prime target: an Earth-sized planet in an Earth-like orbit around a Sun-like star.

“We’ve already found planets smaller than Earth, smaller than Mars, even the size of our Moon,” said Borucki during a press conference. The goal now is to find a rocky planet in a 365-day orbit around its Sun, and that takes years to both detect and then confirm because the data needs to show the planet orbiting its star not just once but several times.

Even though Kepler can no longer be used to hunt for planets, Borucki says he’s extremely satisfied with the mission. “Before we started, it was sort of like I was standing in the desert,” he said. “Now it’s like I’m standing in the bottom of the ocean, covered with an ocean full of data of all these possible planets.”

Kepler completed its prime mission in November 2012 and began its four-year extended mission at that time. However, the spacecraft needs three functioning wheels to continue its search for Earth-sized exoplanets. Two of Kepler’s four gyroscope-like reaction wheels, which are used to precisely point the spacecraft, have now failed. The first was lost in July 2012, and the second in May. Engineers’ efforts to restore at least one of the wheels have been unsuccessful.

An artistic view of the system seen from Kepler-62f. One of Kepler’s main goals was to find planets orbiting in the habitable zone of Sun-like stars. Image Credit: Danielle Futselaar/SETI Institute

On Aug. 8, engineers conducted a system-level performance test to evaluate Kepler’s current capabilities. They determined wheel 2, which failed last year, can no longer provide the precision pointing necessary for science data collection. The spacecraft was returned to its point rest state, which is a stable configuration where Kepler uses thrusters to control its pointing with minimal fuel use.

One of the main goals of Kepler was to find planets outside our solar system orbiting stars like our Sun in the habitable zone — the range of distances from a star where the surface temperature of a planet might be suitable for liquid water, and therefore also suitable for life.

“At the beginning of our mission, no one knew if Earth-size planets were abundant in the galaxy. If they were rare, we might be alone,” said Borucki. “Now at the completion of Kepler observations, the data holds the answer to the question that inspired the mission: Are Earths in the habitable zone of stars like our Sun common or rare?”

From the data collected in the first half of its mission, Kepler has confirmed 135 exoplanets and identified over 3,500 candidates. The team continues to analyze all four years of collected data, expecting hundreds, if not thousands, of new discoveries including the long-awaited Earth-size planets in the habitable zone of Sun-like stars. Though the spacecraft will no longer operate with its precision pointing, scientists expect Kepler’s most interesting discoveries are still to come.

NASA’s Kepler spacecraft discovered numerous planetary systems containing between two and six transiting planets. Click on the image above for an animation representing some of these discoveries (Harvard University, 2011). Credit: Daniel Fabrycky

There are several different possible missions in Kepler’s future, including another exoplanet hunt. There has been a call for scientists to submit white papers suggesting more ideas, and these papers are due in a few weeks. The Kepler project team will then need to figure out which among the many ideas would be the most practical given the spacecraft’s current capabilities.

Depending on the outcome, which is expected to be completed later this year, NASA will assess the scientific priority of a two-wheel Kepler mission. Such an assessment may include prioritization relative to other NASA astrophysics missions competing for operational funding at the NASA Senior Review board early next year.

There is already another exoplanet mission in the works: The Transiting Exoplanet Survey Satellite (TESS). This Explorer-class mission is expected to launch sometime in 2017 or 2018, and like Kepler it will look for planetary transits. All of Kepler’s planets are orbiting stars that are relatively far from Earth, while TESS will find planets orbiting the brightest and closest stars in the sky.