Super-Earths: How Much Are They Like Earth?

With yesterday’s announcement about finding a batch of so-called “super-Earths” – rocky alien worlds a few times more massive than our own – as well as another announcement back in May that 45 relatively low mass planets had been found, it’s obvious astronomers are constantly improving on their techniques to find new worlds. While the vast majority of the almost 300 previously discovered exoplanets are Jupiter-like gas giants, the new discoveries of large numbers of small planets – and especially that at least three of them orbit one star — suggests that they are abundant in our galaxy, and may outnumber Jupiter-sized giants by 3 to 1. But how much like Earth are these alien worlds?

Super-Earths are planets that have than ten times or less the mass of Earth. The three planets around the star HD 40307 have masses of 4.2, 6.7, and 9.4 times the mass of the Earth. They orbit their star with periods of 4.3, 9.6, and 20.4 days, respectively. That’s a short orbital period, meaning they are very close to the star. Since they are close to the star, astronomers believe its likely they are terrestrial, rocky-type planets rather than gas giants like Jupiter and Saturn. But also, being so close to the star means they are very warm – perhaps 1000 degrees Celsius. This would not be a pleasant or probable environment for life as we know it to take a foothold. But we don’t know for sure, and since we are curious creatures, we want to know more about these planets.

The observatory that made the discovery of the 3 planets around HD40307, as well as the 45 planets that were announced back in May is the High Accuracy Radial Velocity Planet Searcher (HARPS) survey based at the European Southern Observatory in La Silla, Chile. Astronomers spotted them by recording how each planet’s gravitational tug makes its parent star wobble.

But now astronomers know these planets are there, they can try other methods of studying the planets to glean some detailed information about what these planets are like. For years, astronomers have been waiting for a super-Earth to be found with an orbit that “transits” its parent star: in other words, it passes directly in front of the star as viewed from Earth. When exoplanets have short orbital periods, the likelihood of being able to observing transits increases. These new planets fit that category.

Being able to observe transits would give astronomers data to help figure out many of the planet’s characteristics, from measuring its radius to deducing its internal structure to “seeing” its atmosphere.

Getting information about the planet’s atmosphere would be especially exciting. By watching for changes in a star’s spectrum as it filters a fraction of the star’s light during a transit, the presence of methane and water vapor in the gaseous atmosphere could be revealed.

A few satellites are capable of watching for a transit, among them the Canadian MOST satellite. Another is the recycled Deep Impact spacecraft that is hosting the EPOCh (Extrasolar Planet Observation and Characterization) mission. So far, 4 new planets have been found with this spacecraft, using the transit method, and the goal of the mission is to find an exoplanet smaller than Earth. Also, EPOCh hopes to be able to identify features on an exoplanet, such as continents and oceans. Exciting prospect, indeed.

It’s only a matter of time until astronomers will be able to tell us how Earth-like these newly found Super Earths are.

Sources: New Scientist, Bad Astronomy, EPOCh