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The Recipe for Making an Earth-Like Planet

You’re going to need a bigger bowl.

by Becky Ferreira
Jan 5 2015, 3:15pm

​This planet is served with a sprig of rosemary. Image: David A. Aguilar (CfA).

​It's a very exciting time to be an exoplanet researcher, as evidenced by the glut of new findings being presented today at the 225th meeting of the American Astronomical Society. From characterizing extrasolar oceans to estimating stellar ages, the AAS is kicking off the new year with some star-studded research.

Perhaps the most tantalizing of the studies, however, whittles down the exact ingredients necessary to whip up a habitable planet. So if you have been itching for an especially ambitious baking project, behold the complete recipe as determined by Courtney Dressing, a graduate student at the Harvard-Smithsonian Center for Astrophysics.

Ingredients:
1 cup magnesium
1 cup silicon
2 cups iron
2 cups oxygen
1/2 teaspoon aluminum
1/2 teaspoon nickel
1/2 teaspoon calcium
1/4 teaspoon sulfur
dash of water delivered by asteroids

Blend well in a large bowl, shape into a round ball with your hands and place it neatly in a habitable zone area around a young star. Do not over mix. Heat until mixture becomes a white hot glowing ball. Bake for a few million years. Cool until color changes from white to yellow to red and a golden-brown crust forms. It should not give off light anymore. Season with a dash of water and organic compounds. It will shrink a bit as steam escapes and clouds and oceans form. Stand back and wait a few more million years to see what happens. If you are lucky, a thin frosting of life may appear on the surface of your new world.

Turns out planet-making is pretty straightforward, as long as you have a big-ass blender and the longevity to wait the process out for millions of years.

Regardless, this playful recipe is based on Dressing's real investigations into the masses of Earth-like worlds. Along with fellow Harvard astronomer David Charbonneau, a veteran exoplanet-hunter, Dressing employed the High-Accuracy Radial velocity Planet Searcher for the Northern hemisphere (HARPS-N) spectrograph to measure the masses of small planets. Isolating an exoplanet's mass can, in turn, help astronomers figure out the composition and density of these worlds.

For example, Dressing and Charbonneau recently used HARPS-N to deduce the mass and composition of the "Super-Earth" exoplanet Kepler-93b. The pair found that the planet was 4.02 times the mass of Earth, and that it has a rocky composition similar to our own planet.

"The exciting news is that when we look at only those planets with well-measured masses, and hence densities, all planets smaller than six Earth masses look like scaled up versions of the Earth's composition," Charbonneau told me. "Just take the recipe for Earth and multiply it by six!"

"Furthermore, there are no such planets more massive than [six Earth masses]," he added. "Apparently, nature doesn't like to make rocky planets more massive than this."

He did note that some planets appear to have both low masses and low densities, which might make them more akin to Neptune—i.e. sporting a rocky, watery core enveloped in hydrogen gas. "But those planets don't have very precisely measured masses, so they are not well constrained," Charbonneau said.

Size comparisons of Earth and Venus to exoplanets. Image: NASA/Ames/JPL-Caltech

To follow up on these findings, Dressing and Charbonneau aim to utilize space telescopes as springboards to studying new Earth-like worlds. "We plan to continue to observe small planets detected by Kepler, K2, and, in the future, TESS [slated for launch in 2017] so that we can increase the sample of small planets with well-measured masses and radii," Dressing told me.

The team will also search for rocky planets that orbit at greater distances from their stars. "The next step with HARPS-N is, I think, to find Earth-sized planets that are farther from their stars and see if they, too, follow the Earth composition formula," Charbonneau told me. "All the planets we have studied, i.e. the announcement today, are very close to their stars and hence hot, so it is possible they had atmospheres but lost them. Studying colder planets is more difficult, but clearly the next challenge for our HARPS-N instrument."

Until recently, exoplanet research was largely dominated by gas giants on the scale of Jupiter and Saturn, for the obvious reason that large planets are easier to spot. But instruments like HARPS-N are laying the groundwork for precise observations of small terrestrial planets as well.

So while making an Earth-like world from scratch as per Dressing's recipe may be just a tad unrealistic, there's no doubt that scientists are building impressive tools to characterize the ones that already exist.