On May 10, NASA announced that the Kepler mission had discovered and confirmed 1,284 new extra-solar planets (exoplanets). This is the largest trawl of exoplanets ever announced at one time, and takes the total of known exoplanets to over 3,000. It is sometimes hard to remember that the first exoplanets were only being discovered in the mid-1990s; we have come a long way since then.
Kepler discovers planets using the ‘transit technique’. This involves staring at a particular patch of the sky and looking for a dimming of particular stars. If the dimming of a particular star happens on a regular basis, it is almost certainly due to our seeing that star’s planetary system edge-one. It is a safe bet that repeated and regular dimming is caused by a planet passing across the disk of the star. This is similar to the effect Mercury would have had on the Sun during the recent Transit of Mercury (see my blogpost here about that event).
Kepler was launched in March 2009 and put into an Earth-trailing orbit. In July 2012 one of the four reaction wheels used for pointing the telescope stopped working. In May 2013 a second one failed, and in August 2013 NASA announced that they had given up trying to fix the two failed reaction wheels and Kepler ceased operation. It used the reaction wheels to keep it pointing at the same patch of the sky, a nearly square patch which covered parts of the constellations Cygnus, Lyra and Draco. This field is shown in the diagram below.
Although the first exoplanets were discovered using the parallax technique (see my blog here for details of that method), Kepler has led to a huge increase in the number of known exoplanets. In fact, since its launch in 2009, Kepler has slowly become the dominant instrument for detecting new exoplanets. It took a few years for it to do this, as so much data were acquired during its mission that it has taken several years for the results to start coming out.
Incredibly, in addition to this announcement of 1,284 more confirmed exoplanets, Kepler has found a further 1,327 which are more than likely to be exoplanets but require more study to be confirmed. Of the nearly 5,000 planet exoplanets found to date, more than 3,200 have been verified and 2,325 of these have been discovered by Kepler. Based on their size, nearly 550 of the newly announced 1,284 exoplanets could be rocky planets like the Earth. Nine of these 500 orbit their star in the habitable zone, the zone around a star where we believe it is possible for liquid water to exist. This means that we have discovered a total of 21 exoplanets in the habitable zone.