Astronomers
have yet to find an Earth-size planet beyond our solar system, but that hasn't
stopped them from modeling what these worlds might look like.
A new
catalog of 14 types of such planets, some fantastical, could help planet
hunters spot what has until now remained fictional.
The
computer models provide specs for 14 planet
types, varying according to mass, diameter, composition and where the
worlds could be found in our galaxy. Some are made mostly of pure water ice,
carbon, iron, silicate, carbon monoxide or silicon carbide, while others are
mixtures of these various compounds.
"We're
thinking seriously about the different kinds of roughly Earth-size planets that
might be out there, like George Lucas, but for real," said Marc Kuchner of
NASA's Goddard Space Flight Center in Greenbelt, Md.
The work
will be detailed in the Oct. 20 issue of the Astrophysical Journal.
Planet
specs
Rather than
assuming faraway
worlds around other stars would be big or small versions of planets in our
solar system, the astronomers considered all possible types of planets given
what they know about the composition of protoplanetary disks, the wells of raw
material that form around young stars.
So far,
most of the more
than 250 planets discovered beyond our solar system are huge, Jovian-like
worlds. So scientists know something about the range of possibilities with
giant planets, but they know next to nothing about possible Earth-like and
potentially solid or watery planets except what has been learned from our solar
system.
"We
have learned that extrasolar giant planets often differ tremendously from the
worlds in our solar system, so we let our imaginations run wild and tried to
cover all the bases with our models of smaller planets," Kuchner said.
By modeling
how gravity would compress a planet of a certain composition and mass, they
predicted each planet's diameter, finding that no matter their composition, the
planets followed a similar relationship between mass
and diameter.
"All
materials compress in a similar way because of the structure of solids,"
said study team member Sara Seager of MIT. "If you squeeze a rock, nothing
much happens until you reach some critical pressure, then it crushes. Planets
behave the same way, but they react at different pressures depending on the
composition."
The team found
a pure water planet weighing the equivalent of one Earth will span about 9,500
miles across (15,289 kilometers), while an iron planet of the same mass will be
compressed to a diameter of just 3,000 miles (4,828 kilometers). Earth, made up
mostly of silicates, is 7,926 miles (12,755 kilometers) across at its equator.
Where in
the world ...
The team
also provided rough guidelines for possible hideouts of the planet types.
"We can make educated guesses about where these different kinds of planets
might be found," Kuchner said.
For
example, carbon planets and carbon-monoxide planets might take up residence
around evolved stars such as white dwarfs and pulsars, or they might form in
carbon-rich debris disks like the one around the young star Beta Pictoris.
With a slew
of planet-hunting projects and missions launched or in the queue, the
researchers hope when astronomers start finding Earth-size
planets, these models will yield insights into planet compositions based on
size and mass information.
Snags in
the model, of course, have already emerged. While the models will work well in
distinguishing between a pure water planet and one composed of iron, they could
mix up silicate planets with carbon planets, say the researchers. That's
because silicate and carbon planets have very similar masses at a given
diameter.
Observatories
yet to launch, such as NASA's James Webb Space Telescope or Terrestrial Planet
Finder, could provide finer details to help decipher the chemical compositions.
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