Australasian Science Magazine

By David Reneke

News from the space and astronomy communities around the world.

A joint effort of citizen scientists and professional astronomers has led to the first reported case of a planet orbiting twin suns that in turn is orbited by a second distant pair of stars.

Aided by volunteers using the planethunters.org website, a Yale-led international team of astronomers identified and confirmed the discovery of the phenomenon, which is called a “circumbinary planet” in a four-star system.

Only six planets are known to orbit two stars, and none of these are orbited by distant stellar companions. “Circumbinary planets are the extremes of planet formation,” said Meg Schwamb of Yale. “The discovery of these systems is forcing us to go back to the drawing board to understand how such planets can assemble and evolve in these dynamically challenging environments.”

Dubbed PH1, the planet was first identified by citizen scientists participating in Planet Hunters, a Yale-led program that enlists the public to review astronomical data from NASA’s Kepler spacecraft for signs of planets. It is the project’s first confirmed planet.

The volunteers, Kian Jek of San Francisco and Robert Gagliano of Cottonwood, Arizona, spotted faint dips in light caused by the planet as it passed in front of its parent stars. This “transit” method is a common technique for finding extrasolar planets.

Schwamb led the team of professional astronomers who confirmed the discovery. They also characterised the planet, PH1, as a gas giant with a radius about 6.2 times that of Earth, making it a bit bigger than Neptune.

Planet Hunters, launched in 2010, is a symbiotic project pairing the discovery power of the people with follow-up by a team of astronomers. This unique system might have been entirely missed if not for the sharp eyes of the public.

“The thousands of people who are involved with Planet Hunters are performing a valuable service,” said co author Jerome Orosz, who is now associate professor of astronomy at San Diego State University.

PH1 revolves around its host stars roughly every 138 days. Beyond the planet’s orbit, roughly 1000 times the distance between Earth and the Sun, is a second pair of stars orbiting the planetary system.

Even seasoned astronomers are astonished at how we can detect, let alone glean so much information, about another planet thousands of light years away just by studying the light from its parent star. Welcome to 21st century astronomy!

Planet Found in Nearest Star System to Earth

European astronomers have discovered a planet with about the mass of the Earth orbiting a star in the Alpha Centauri system, the nearest to Earth. It is also the lightest exoplanet ever discovered around a star like the Sun. The planet was detected using the HARPS instrument on the 3.6-metre telescope at the European Southern Observatory’s La Silla Observatory in Chile.

Alpha Centauri is one of the brightest stars in the southern skies. Only 4.3 light-years away, it is the nearest stellar system to our solar system. It is actually a triple star system consisting of two stars similar to the Sun orbiting close to each other, designated Alpha Centauri A and B, and a more distant and faint red component known as Proxima Centauri.

Since the 19th century astronomers have speculated about planets orbiting these bodies, the closest possible abodes for life beyond the solar system, but searches of increasing precision had revealed nothing. That is, until now.

“Our observations extended over more than 4 years using the HARPS instrument and have revealed a tiny, but real, signal from a planet orbiting Alpha Centauri B every 3.2 days,” says Xavier Dumusque of Geneva Observatory, Switzerland. “It’s an extra­ordinary discovery and it has pushed our technique to the limit!”

A whimsical comment from a colleague suggested they may have found the Robinson family had they looked a little closer.

The European team detected the planet by picking up the tiny wobbles in the motion of the star Alpha Centauri B created by the gravitational pull of the orbiting planet. The effect is minute, causing the star to move back and forth by no more than 51 cm/s or 1.8 km/h – about the speed of a baby crawling. This is the highest precision ever achieved using this method.

The newly discovered planet, with a mass of a little more than that of the Earth, is orbiting about six million kilometres away from Alpha Centauri B, much closer than Mercury is to the Sun in the solar system.

This is the first planet with a mass similar to Earth ever found around a star like the Sun, but it may well be just one planet in a system of several. The challenge that astronomers now face is to locate an exact duplicate Earth orbiting in the habitable zone around another star.