Astronomers find exoplanets that followed the same rocky 'recipe' as Earth

Monday, January 5, 2015, 4:05 PM - A blend of 8 key elements, mixed together and then baked in the 'Goldilocks Zone' of a star over millions of years, with an added dash of water and organic molecules as it cools. Based on what we know if the Earth, that's what it takes to make a habitable world, but a team of astronomers has now confirmed that this 'recipe' also applies to planets around other stars too.

Knowing exactly what a distant exoplanet is like isn't easy. Although astronomers are able to detect the presence of these alien worlds through various means, observing them with the right instruments - either in orbit or here on Earth - is needed in order to figure out some of the important details that will tell us if it's a rocky world, like Earth, a heavy 'water world', or even an 'ice giant' like Neptune.

One of the instruments that astronomers use for this is HARPS - the High-Accuracy Radial velocity Planet Searcher - which has been in use for over a decade at La Silla Observatory in Chile. To find planets, HARPS takes very careful measurements of the light given off by a star, looking for a slight 'wobble' in the spectrum of that light, which could be caused by the gravity of a planet as it orbits the star. Since 2004, using this 'radial velocity' method of finding planets, HARPS has discovered 130 alien worlds, and one of the added benefits of using this instrument is that, if you know the mass of the star you're observing with it, the amount of wobble can tell you exactly how massive the planet is.

Now, a team of astronomers led by the Harvard-Smithsonian Center for Astrophysics (CfA) has used HARPS-North, a version of HARPS installed on the Telescopio Nazionale Galileo, in the Canary Islands, to examine several of the known extrasolar planets. Narrowing down the size and mass of these planets, they presented their findings at the latest meeting of the American Astronomical Society, going on this week in Seattle, Washington. What they found is that planets a little over 1.5 times the diameter of Earth apparently followed the same 'recipe' during their formation.

"Our solar system is not as unique as we might have thought," lead author Courtney Dressing, a graduate student in the Harvard University Astronomy Department said in a Harvard-Smithsonian Center for Astrophysics (CfA) press release. "It looks like rocky exoplanets use the same basic ingredients."

What is this recipe? The researchers conveniently wrote it down for us:

The recipe supplied by the 'test kitchen' of Earth. Credit: Li Zeng (CfA)

One of the exoplanets studied in particular, Kepler 93b (detailed in the video below), was chosen for the very accurate measurements of its size, and the fact that it takes only 4.7 days to go around its star once (thus giving a great number of measurements over a shorter period of time).

Based on the precise measurements from the Kepler and Spitzer telescopes, Kepler-93b was found to be around 1.5 times the size of Earth, and HARPS-N determined that it is just over 4 times Earth's mass. According to the research paper, that's just how much more massive Earth would be if it was scaled up to the size of Kepler-93b!

Four other exoplanets - Kepler-78b, Kepler-10b, Kepler-36b and COROT-7b - also had this same size to mass ratio, ranging up to about 1.6 times the size of Earth and just under six times Earth's mass, implying the same kind of rocky composition.

None of these planets (or the others in the study) are potentially-habitable worlds, unfortunately. Just like Kepler-93b, they are all scorching hot, as they all have extremely close orbits around their stars, ranging from just over 8 hours long to just shy of two weeks.

While not habitable, though, through this new research they are helping to point astronomers in the right direction of finding habitable, Earth-like worlds.

"To find a truly Earth-like world, we should focus on planets less than 1.6 times the size of Earth, because those are the rocky worlds," Dressing said in the CfA press release.

As for which planets this may point us towards, we may have already discovered some of them. The Habitable Exoplanets Catalog, maintained by the Planetary Habitability Laboratory, at the University of Puerto Rico at Arecibo, lists several by their 'Earth-similarity Index' - how Earth-like they are in size, mass and the amount of radiation they receive from their star. The planets found on this list, which conform to the size and mass of the worlds studied by Dressing and her colleagues, may be quite Earth-like indeed.