After carefully measuring the masses of small, rocky exoplanets, astronomers have come to the conclusion that our solar system may not be unique and, by extension, those exoplanets surveyed are likely composed of similar materials as Earth.
Using the High-Accuracy Radial velocity Planet Searcher (HARPS) North instrument on the 3.6-meter Telescopio Nazionale Galileo in the Canary Islands, the researchers, led by Courtney Dressing of the Harvard-Smithsonian Center for Astrophysics (CfA), focused on the slight wobble small exoplanets exert on their host stars.
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As an exoplanet orbits its star, the tiny gravitational pull can tug the star slightly off-center. By measuring this "wobble" (through the analysis of the shifting of wavelength of the detected starlight), astronomers are able to determine the mass of that exoplanet. This mode of exoplanet detection is known as the "radial velocity" method. By combining these mass measurements with observations made by NASA's Kepler Space Telescope, which uses the "transit method" to detect exoplanets and determine their physical size, the density of that exoplanet can be calculated. With this valuable information, scientists can begin to understand what that world is made of.
In a new study presented today (Jan. 5) at the 225th American Astronomical Society meeting in Seattle, Wash., and accepted for publication in the The Astrophysical Journal, Dressing's team discussed their findings for small worlds that have a lot more in common with Earth than we thought.
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"Our strategy for using HARPS-North over the past year has been to focus on planets less than two times the diameter of Earth and to study a few planets really well," said astronomer David Charbonneau, also of the CfA and head of the HARPS-North Science Team.
One key discovery focused on Kepler-93b, an exoplanet identified by Kepler. It is known to be 1.5 times the size of Earth, but it has an extreme 4.7-day orbit around its host star. Before Dressing's team began measurements of the distant world, very little was known about its mass and composition. But HARPS-North was able to precisely measure its mass to 4.02 times the mass of Earth. From this information alone, the astronomers could say that this world was a rocky exoplanet.
Then, the astronomers measured the densities of "all ten known exoplanets with a diameter less than 2.7 times Earth's," according to a CfA press release, and found that five of those worlds with diameters less than 1.6 times that of Earth showed a very tight relationship between mass and size. Most interestingly, both Earth and Venus, when added to the analysis, fitted neatly into the same correlation.
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As we know that Earth has a rocky composition with an iron core (and Venus is thought to possess similar characteristics), perhaps these planets, orbiting different stars, also have the same composition. There are a few exoplanetary exceptions, but for the most part, small worlds closely orbiting their stars have some very terrestrial characteristics.
"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," said Dressing.
As a fun aside, the CfA astronomers created a recipe for how you could "bake" your very own Earth-like model world:
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.
Sounds like the kind of world I'd want to bake...