Sunday, January 11, 2015
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 in a press conference 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.
The
paper presenting this work has been accepted for publication in The
Astrophysical Journal.
Headquartered
in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a
joint collaboration between the Smithsonian Astrophysical Observatory and the
Harvard College Observatory. CfA scientists, organized into six research
divisions, study the origin, evolution and ultimate fate of the universe.
As a fun aside, the CfA astronomers created a recipe for how you could "bake" your very own Earth-like model world:
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.
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