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| How do you make an Earth-like planet? 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. |
How do you make an Earth-like planet? 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.
"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."
Dressing presented the research today at a meeting of the American Astronomical Society.
The key to the discovery was the HARPS-North instrument on the
3.6-meter Telescopio Nazionale Galileo in the Canary Islands. (HARPS
stands for High-Accuracy Radial velocity Planet Searcher.) It is
designed to accurately measure the masses of small, Earth-sized worlds.
Those measurements are crucial to determine densities and therefore
compositions.
"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," explains Harvard astronomer David Charbonneau
(CfA), who currently heads up the HARPS-North Science Team.
Most recently the team targeted Kepler-93b, a planet 1.5 times the
size of Earth in a tight, 4.7-day orbit around its star. The mass and
composition of this world were uncertain. HARPS-North nailed the mass at
4.02 times Earth, meaning that the planet has a rocky composition.
The researchers then compared all ten known exoplanets with a
diameter less than 2.7 times Earth's that had accurately measured
masses. They found that the five planets with diameters smaller than 1.6
times Earth showed a tight relationship between mass and size.
Moreover, Venus and Earth fit onto the same line, suggesting that all
these worlds have similar rock-iron compositions.
As for the larger and more massive exoplanets, their densities proved
to be significantly lower, meaning that they include a large fraction
of water or other volatiles, hydrogen and/or helium. They also showed
more diverse compositions rather than fitting into a single group like
the smaller terrestrial worlds.
The team also noted that not all planets less than six times the mass
of Earth are rocky. Some low-mass worlds with very low densities are
known (such as the planets in the Kepler-11 system). But for typical
close-in small planets, the chances are high that they share an
Earth-like composition.
"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,"
recommends Dressing.
Making Other Earths
Makes one small model planet
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.
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