Search for Habitable Extrasolar Planets Continues Despite Kepler's Misfortune

June 25, 2013

After the last month's issue on one of its stabilising wheels, the Kepler space telescope's search for estrasolar planets may have come to an end. Inspite of that, Kepler's legacy is vast and the search for habitable exoplanets continues with new missions.

Artist impression of the Kepler space telescope.

Artist impression of the Kepler space telescope.

On May 15, 2013, NASA scientists found the Kepler space telescope in "safe mode", caused by a broken stabilising wheel. At the launch in 2009, Kepler had four reaction wheels which help stabilise the telecope in space. The first has broken in July 2012 and with the second wheel now also broken, the telescope can't point precisely at its targets with only two wheels remaining, three needed for it to work like it was meant to.

This does not have to be the end of Kepler's mission though. Keith Horne of the University of St. Andrews in the UK and Andrew Gould of the Ohio State University in the USA suggest that the telescope could be used to search for planets using gravitational microlensing. The focus would shift to planets outside of the habitable zone around the star, where liquid water could not exist. This would be important for better understanding habitable zones around stars, even though it would be a step away from Kepler's original mission.

As mentioned, Kepler was launched in March 2009 and designed to last three and a half years. It has discovered 132 confirmed exoplanets and detected another 3216 candidates awaiting confirmation from ground-based telescopes. Kepler was searching a Earth-sized planet in habitable zone around a star that was similar to our Sun. Most of the planets it discovered are larger than Earth, but the mission is far from being a failure. With over 3200 candidates waiting to be confirmed, astronomers are guaranteed to be kept busy for years to come. The Kepler mission has increased the number of known extrasolar planets and planet candidates by factor of five and has revolutionized the study of exoplanets by discovering systems with longer orbital periods and smaller planet radii than any mission before it.

Recent data collected by the telescope suggest that M-dwarf stars, with masses about one half of the solar mass and surface temperatures less than 4000K, are twelve times more common than solar-type stars. This is significant because new research suggests that these stars might have planets around them able to support life, despite previous skepticism.

It has been considered that the chances of finding habitable planets around such stars were slim because of two reasons. Firstly, because they are smaller, cooler and less luminous, M-dwarf stars would have habitable zones closer to them, its gravity causing the planet to be tidally locked to the star like the Moon is to the Earth, always facing the star with the same side. That would make the planets surface either too hot or too cold. Secondly, small stars tend to flare, possibly having an effect on planet's atmosphere.

In the light of the new discoveries, M-dwarfs became much more interesting because planets around them are close to their stars and have shorter orbital periods, making it easier for astronomers to detect them. This, in combination with the number of M-dwarf stars, makes the focus on exoplanets around small stars a logical move. Courtney Dressing and David Charbonneau of Harvard-Smithsonian Center for Astrophysics reported that they have indentified 64 dwarf stars with 95 candidate planets, drawing statistical conclusions that one in six M-dwarfs should have an Earth-sized planet orbiting it inside its habitable zone. They believe that the nearest planet in a habitable zone lies within fifteen light-years from us.

Even if the latest development means the end of the Kepler telescope mission, the search for habitable planets does not end. On October 19, 2012, the European Space Agency announced it would launch Cheops (Characterising ExOPlanets Satellite) space telescope in 2017, which should take a look at bright stars in our neighbourhood. NASA is also planing to launch the Tess (Transiting Exoplanet Survey Satellite), also in 2017, which should survey about 2 million nearby stars.


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