When I was a kid, the universe had nine planets and I knew each one by name. Today, there are more than 900 confirmed planets, with a new study just published in the Astrophysical Journal (July 10) estimating there are more than 60 billion habitable-zone planets in the Milky Way Galaxy alone.
Early astronomers found the first planets in our solar system using naked-eye observations. Today, astronomers use telescopes, cameras, spectrographs and computers to detect planets orbiting stars up to 20,000 light years away.
The first extrasolar planet was found in 1995, when a giant planet was detected orbiting a star 50 light years from Earth. This planet – and most since – was found using the Doppler method. All stars are in motion, and their velocity, either towards or away from us, can be determined using a high-resolution spectrograph, which measures the composition of light emitted by a star. “If a star has a variable velocity, or ‘velocity wobble’, then a likely explanation is that it has an invisible planet orbiting it,” says John Hearnshaw, professor of astronomy at the University of Canterbury.
The Doppler method is best for detecting large gaseous planets, like Jupiter, orbiting very close to their star. Hearnshaw, with a team of New Zealand and Japanese colleagues, has detected planets using another technique – gravitational microlensing. Using a telescope at Tekapo’s Mt John Observatory, the Microlensing Observations in Astrophysics project has been involved in detecting 13 extrasolar planets.
“We typically look at stars in the centre of the Milky Way, about 20,000 light years away,” says Hearnshaw. “Just occasionally, if a star of intermediate distance passes through the line of sight to the more distant star, the light rays can be bent by the gravity of what we call the lensing star. The effect is to make the more distant star brighter – maybe for a few weeks.”
But if the lens star is being orbited by a planet, says Hearnshaw, “you get really interesting aberrations in the lensing … the brightness doesn’t go up and down in a symmetrical curve, it shows sudden spikes or dips caused by the planet”. By measuring the size and duration of the spikes, and estimating the mass of the lensing star, astronomers can also estimate the planet’s mass, distance from its parent star and orbital period.
But the “best prospect for finding truly habitable Earth-mass planets”, says Hearnshaw, is the transit technique. If a star is periodically dimmed, that could indicate the passage of a planet across its face.
Denis Sullivan, professor of physics at Victoria University, was involved in the first detection of an extrasolar planet using the transit technique. Working at Mauna Kea Observatories in Hawaii in 1999, he and son Tiri confirmed a planetary transit predicted by a team of astronomers from Harvard University. Nasa’s Kepler space observatory, launched in 2009 in search of Earth-like planets in the habitable zone, has so far used the transit method to find 132 confirmed planets in 76 different star systems. More than 3000 other possible planets are now awaiting confirmation by the Doppler method.
“It’s fun to have lived part of my life through this development,” says Sullivan. “It’s now clear the formation of planetary systems is just part of the formation of stars, with rocky, Earth-sized planets typically formed in the disk debris around a star.”
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The Aoraki Mackenzie Starlight Festival at Lake Tekapo, October 11-13, celebrates the creation of the Southern Hemisphere’s first International Dark Sky Reserve. Events include stargazing, art exhibitions, talks from astronomers and a Nasa astronaut, a Symphony under the Stars concert and a children’s writing competition.