Astrobiology Top 10: Double Discovery: Super-Earth and Ocean World

Gliese 581 e (foreground) is only about twice the mass of our Earth. The Gliese 581 planetary system now has four known planets, with masses of about 1.9 (planet e, left in the foreground), 16 (planet b, nearest to the star), 5 (planet c, centre), and 7 Earth-masses (planet d, with the bluish colour).
Credit: ESO

The discovery of the lightest exoplanet ever found, less than twice the mass of the Earth, has electrified a week-long meeting on astronomy and space science in Europe. The stunning finding was made by a team headed by Michel Mayor of the Geneva Observatory. The icing on the cake is a related discovery that a previously discovered “super-Earth” orbiting the same star appears to reside in the habitable zone.

Mayor made the very first discovery of an exoplanet, a Jupiter-sized world that orbits the star 51 Pegasi, in 1994. Among his many planet discoveries since then at ESO’s La Silla Observatory in Chile, Mayor has made a specialty of observing the star Gliese 581. Located 20.5 light-years away in the constellation Libra (“the Scales”), Gliese 581 is a red dwarf star with only one-third of the mass of our sun.

Two years ago, Mayor discovered a planet the size of Neptune and two super-Earths orbiting this star. The newly discovered planet, named Gliese 581 e, is now the fourth known planet in this solar system and the lightest, weighing in at only 1.94 Earth masses. It flies round the star at dizzying speed, taking just 3.15 days to complete an orbit. “The surprise for me was to discover a planet with by far the lowest mass seen to date,” says Mayor.

Artist’s impression of the five-Earth mass planet, Gliese 581 c, found in the habitable zone around the red dwarf Gliese 581.
Credit: European Southern Observatory.

This new planet orbits so close to the star that its water would have boiled away long ago. It is therefore not in the habitable zone – the region of a solar system where water can stay liquid on the surface of a rocky planet, and, consequently, where scientists expect life can occur. In our solar system, the habitable zone is roughly between the orbits of Venus and Mars (with Earth sitting not quite in the middle).

In finding the new planet, Mayor has been able to more accurately determine the orbit for the outermost planet, Gliese 581 d. One of the super-Earths in the solar system, this planet is closer to the host star than was thought when it was discovered in 2007. And that provided the second great surprise. “It is the only (Earth-like) exoplanet found inside the habitable water zone of the parent star,” says Mayor.

Gliese 581 d is 7 Earth-masses, and team member Stephane Udry says the planet is probably too massive to be made only of rocky material. “We can speculate that it is an icy planet that has migrated closer to the star,” he says. At the European meeting, Mayor added the latest news indicated, “No icebergs, but there may be an ocean at the surface, meaning this is a new class of ocean planet.”

To detect exoplanets, Mayor’s team studies a star’s radial velocity, in which the tiny tugs exerted by orbiting exoplanets produce a complex wobble in the star. This wobble can be analyzed to learn about properties of the planets in the solar system. The velocity of a star with multiple planets has to be followed for several years to discover the different properties of its orbiting planets, and this requires instrumentation that is extremely stable from year to year — one of the big challenges in detecting exoplanets through the radial velocity technique.

The habitable zones for different types of stars, with our solar system as an example. As a planet is pulled in towards its star, it can be pulled away from the habitable zone.

The team’s observing program began back in 2004 with a sample of 400 sun-like stars. Mayor is now scooping up small exoplanets that have been missed by a rival search technique (called transit photometry) which involves measuring the tiny fall in a star’s magnitude when an exoplanet passes between the star and the Earth. Both techniques, transit photometry and radial velocity, are strongly biased to catch giant planets with the mass of Jupiter or more, as well as smaller planets that orbit very close to their star. But to find small planets orbiting within a star’s habitable zone, Mayor’s approach now seems to have the edge.

The European Southern Observatory in Chile.
Credit: ESO

The team has found that one-third (30%) of exoplanet systems found to date include small bodies. “We have discovered a new category of small exoplanets,” says Mayor. “Within a couple of years we will drive down our lower limit of detection to the mass of the Earth. The next challenge after that is to detect a twin of the Earth in the habitable zone of a solar-type star.”

The next stage for Mayor’s team is to migrate the detection technology from the current 3.6-meter telescope to ESO’s 8-meter Very Large Telescope in order to improve the precision of observations. After that, Mayor looks forward to using the European Extremely Large Telescope (E-ELT), a 42-meter eye-on-the-sky that is planned to be operating by 2018. Currently in the later stages of design, this facility will be capable of directly imaging larger exoplanets, and possibly will be able to search their atmospheres for biosignatures. E-ELT will answer fundamental questions on the formation and evolution of exoplanets, bringing us one step closer to answering the question: are we alone?