When is an "Earth-like" planet not Earth-like? It's a question that the search for life beyond the Solar System revolves around, and a team of scientist are using data from the Sloan Digital Sky Survey (SDSS) to shed some light on the subject. By looking at the composition of exosolar planets, they conclude that some Earth-like candidates, including "garnet planets" may not be as habitable as once thought.

Casual listening to space scientists often suggest that the galaxy is teeming with planets like Earth, that hold the promise of life. However, when astronomers use the term "Earth-like" they mean an exoplanet that is small enough to be a rocky orb rather than a gas giant, and that has a surface temperature that could support liquid water.

It's a good start, but if you're looking for a planet that's actually habitable, there are all sorts of other variables that need to be considered. These include how old it is, whether it has a magnetic field, if it actually has water, and so on. One important thing that the team from the University of Arizona and other institutions looked at was the chemical composition of Earth candidates.

Starting with near-infrared spectral analysis using the Apache Point Observatory Galactic Evolution Experiment spectrograph in New Mexico for the SDSS, the scientists looked at the elements present in stars with exoplanets orbiting about them, on the assumption that the stellar elements would be reflected in the composition of the planets.

The team then cross-checked the SDSS results against the Earth-like candidate planets discovered by NASA's Kepler Space Telescope. In the end, the team zeroed in on the stars Kepler 102, which is dimmer than the Sun and has five known planets, and Kepler 407, which is more Sun-like in mass and has two planets, one about three times the mass of the Earth.

Using the latest in models of planet formation and composition, the team noticed that the Earth-like planet orbiting Kepler 102, which they dubbed "Olive," was more like the Earth in composition and, like our world, was rich in olivine in its crust. Meanwhile, Kepler 407's planet, called "Janet," was richer in garnet.

This meant that Janet was not only an immense birthstone, but it was also a poor candidate for life. This is because garnet is a stiffer mineral than olivine and a crust thick in garnet won't readily flow. That means Janet can't sustain plate tectonics over much of its lifespan, and plate tectonics have played a major role in making the Earth habitable.

Without the constant recycling of the crust into the mantle to be remelted and then belched out of volcanoes, the atmosphere would not be one that could one day to sustain life. Worse, without strong plate tectonics, the Earth's magnetic field might not have been strong enough to keep the solar winds from stripping the atmosphere away entirely.

According to the team, the next step is to expand the study to all the stars with small planets in the SDSS, to gain a broader understanding of exoplanet compositions.

The research was presented at the American Astronomical Society (AAS) meeting in Grapevine, Texas.