The "test kitchen" of Earth has given us a detailed recipe, but it wasn't clear whether other planetary systems would follow the same formula. Now, astronomers have found evidence that the recipe for Earth also applies to terrestrial exoplanets orbiting distant stars. David A. Aguilar (CfA)

The same ingredients and mixing process that went to make Earth could go to build exoplanets around distant stars, says research presented at a meeting of the American Astronomical Society.

The right ingredients mixed to the right amount, heated to the right moment and baked for a few million years, cooled to the right temperature when a crust forms, seasoned with water and organic compounds, with another wait for some million years is all you may require to make an Earth-like planet.

"Our solar system is not as unique as we might have thought," says lead author Courtney Dressing of the Harvard-Smithsonian Center for Astrophysics (CfA). "It looks like rocky exoplanets use the same basic ingredients."

Using the HARPS-North instrument on the 3.6-metre Telescopio Nazionale Galileo in the Canary Islands (HARPS stands for High-Accuracy Radial velocity Planet Searcher) the team accurately measured the masses of small, Earth-sized worlds.

To find a truly Earth-like world, the focus should be on planets less than 1.6 times the size of Earth, because those are the rocky worlds.

These were shown in earlier work to possibly hold oceans for billions of years and thus offering a potential for harbouring life.

While the focus earlier was on planets less than two times the diameter of Earth, the present work targeted Kepler-93b, a planet 1.5 times the size of Earth in a tight, 4.7-day orbit around its star. The mass was measured to be 4.02 times Earth, establishing a rocky composition.

In comparing all ten known exoplanets with a diameter less than 2.7 times Earth's they found that the five planets with diameters smaller than 1.6 times Earth showed a close relationship between mass and size.

The larger and more massive exoplanets had much lower densities indicating they may include a large fraction of water or other volatiles, hydrogen and/or helium.

However, not all planets less than six times the mass of Earth are rocky. But the chances are high that they share an Earth-like composition.