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In October 2013 a team of researchers at the UK’s leading exoplanet detection program, WASP (the Wide Angle Search for Planets), found 11 new exoplanets - sending the tally over a thousand. SuperWASP uses two robotic observatories (one based on La Palma in the Canary Islands, the other in South Africa), operating continuously, to examine both hemispheres of the sky.

What’s an exoplanet?

An exoplanet, or extrasolar planet, is a planet orbiting a star outside of our solar system. Most of the ones discovered so far are in the Milky Way, but we think some are in star systems outside our galaxy and are therefore extragalactic planets.

When was the first one found?

The first exoplanets were found in 1992, orbiting the pulsar PSR B1257+12 in the constellation of Virgo, 1000 light years away from our Sun.  In 1995 Didier Queloz and Michel Mayor discovered the first exoplanet orbiting a main-sequence star – 51 Pegasi B.

How are exoplanets discovered?

Most exoplanets can’t be seen directly, as they are hidden by the bright light coming from their star. The Very Large Telescope, part of the ESO Paranal Observatory in Chile, took the first photograph of an extrasolar planet in 2004.

The first photograph of an exoplanet. Taking such a photo is very challenging, which is why this image isn’t as visually stunning as you might expect.
(Credit: ESO)

51 Pegasi B was discovered because its gravitational interaction with the star it orbits (51 Pegasi) causes tiny wobbles in the star’s orbit. Professor Queloz (then a PhD student at the University of Geneva) made the discovery using the radial velocity method, looking for Doppler shifts in the spectrum of light from the star. About half of the extrasolar planets discovered have been found with this technique.

The transit method detects an exoplanet as it crosses (‘transits’) in front of its star. When it does so, it makes the star look slightly less bright. Although this method has discovered a lot of exoplanets, it can produce a lot of false positives, and another method of detection is usually used for confirmation. NASA launched the Kepler space observatory in March 2009, to look for Earth-like exoplanets within the Milky Way using the transit method. Kepler has now stopped functioning correctly, although it may be that its planet-hunting mission can be continued in a different way.

Transit Timing Variation (TTV) can be used when there are multiple planets orbiting the same star, because each one causes slight variations in the orbits of the others. In binary star systems, timing eclipses can reveal a planet that orbits both stars; although this can be used to discover new planets, it’s often used to confirm the existence of ones spotted using a different method.

These are the main methods of detection, but there are lots of other ways to spot an exoplanet, which look for different things and are likely to detect different types of planet. It’s an exciting area of astrophysics, and teams of researchers around the world are looking for exoplanets, or investigating the ones that have already been located.

Is there a list of the exoplanets we’ve found?

An artist’s impression of Tau Bootis, one of the first confirmed exoplanets
(Credit: STFC/David A Hardy)

If you’re looking for a list then the Extrasolar Planets Encyclopaedia is a good place to start, but there are several different directories of exoplanets, and they contain slightly different numbers. One of the reasons for that is simply the use of different criteria for when the directory includes a listing, but there are also differences in opinion on what is (or isn’t) an exoplanet. A large proportion of the exoplanets we have discovered are large, similar in size to Jupiter or Saturn. This is a sampling bias, as it’s easier for us to find these larger planets, and in some cases it is difficult to decide whether what has been found is an exoplanet or a brown dwarf (a failed star).

The Kepler space mission has identified thousands more potential planets, and we expect that each star has at least one exoplanet, which would mean 100-400 billion exoplanets in the galaxy. The HARPS-N instrument has been designed to investigate the potential planets that Kepler has identified.

There have also been detections of what may be exocomets – comets outside of our solar system – and they may be common in the Milky Way as well.

Why are we looking for exoplanets?

Learning more about our Universe is an exciting goal by itself, but scientists are particularly interested in finding exoplanets that orbit inside their star’s habitable zone (also known as the  Goldilocks Zone), where it is possible for liquid water to exist on the surface of the planet.

"If a planet’s orbit is too close to its star, then it’s too hot. If it’s too far away, then it’s too cold. The Goldilocks Zone is where the star’s energy keeps the planet at just the right temperature for there to be liquid water on the surface."

Some of the scientists looking for exoplanets are also looking for signs of extraterrestrial life. On 7 January 2013, astronomers at the Kepler space observatory announced their discovery of Kepler-69c, an Earth-like planet orbiting a start similar to our Sun; it may be a good home for alien life.

Even if it isn’t, then there are plenty of other places to look. Earlier this year it was estimated that there are at least 17 billion planets in the Milky Way that are of a similar size to Earth. Alpha Centauri Bb is awaiting confirmation as an exoplanet, but if it is then it’s the closest one to us.

What’s in store?

NASA is planning to launch TESS (Transiting Exoplanet Survey Satellite) in 2017, to scan the entire sky for exoplanets. ESA are planning their own launch in 2017, of CHEOPS (Characterizing Exoplanets Satellite) , to look for signs of life on known planetary systems. And part of the mission for the James Webb Space Telescope (due to be launched in 2018) is also to search for life on extrasolar planets. The exoplanet revolution is only just beginning.

Would you like to help?

Two teams of researchers have just reported that they have found a seventh planet orbiting dwarf star KIC 442793, making it the most crowded system we’ve seen so far. One of the teams included volunteers using Kepler data via the Planet Hunters website – part of the Zooniverse citizen science project. If you fancy finding an exoplanet of your own, sign up today!

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