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These super-Jupiter planets do
not appear to grow in size as their
mass increases. For instance,
Corot-3b is a super-Jupiter that
is 22 times as heavy as Jupiter
but more or less the same size, due
to its gravity holding its gaseous
contents together. Astronomers
have calculated that the density
of Corot-3b is greater than that
of gold and even osmium, the
densest element on Earth.
Brown dwarfs and rogues
When a super-Jupiter reaches
60 Jupiter masses, it is no longer
regarded as a planet, but as a
brown dwarf. A brown dwarf
is essentially a failed star—a
ball of gas that is too small to
burn brightly through nuclear
fusion. The brown dwarf and
its star are seen as a binary star
system, not a planetary one.
Some super-Jupiters and small
brown dwarfs have broken free of
their star to become free-floating
rogue planets. One, named MOA-
2011-BLG-262, is thought to have
a satellite, and could be the first
exoplanet found with an exomoon.
Another class of planet are
called the super-Earths. These have
a mass 10 times that of Earth but
less than that of an ice giant like
Neptune. Super-Earths are not
rocky but made from gas and ice:
alternative names for them are
mini-Neptunes or gas dwarfs.Living planets
Earth’s solar system has terrestrial
planets (planets with a rocky
surface), of which Earth is the
largest. So far, exoplanet searches
have struggled to find many
terrestrial planets, because they
are generally small and beyond
the sensitivity of the planet
detectors. The first confirmed
terrestrial exoplanet was Kepler-
10b, which is three times the mass
of Earth and is so close to its starEXOPLANETS
that it orbits once an Earth day
and has a surface temperature
that would melt iron. Life seems
highly unlikely there, but the
hunt continues for rocky planets
that might be more hospitable.
Astrobiologists—scientists who
search for alien life—focus on the
particular conditions that all life
needs. When choosing likely places
to look, they assume that alien life-
forms will require liquid water and
carbon-based chemicals, just like
life on Earth. Living planets would
also need an atmosphere to shield
the surface from damaging cosmic
rays and to act as a blanket that
retains some of the planet’s heat
during the night.
The region around a star where
the temperatures would allow
planets to have liquid water, carbon
chemistry, and an atmosphere,
is known as its habitable zone,
also called the “Goldilocks zone”—Kepler 10b in the Draco constellation
is shown transiting its star in an
artist’s impression. Its extremely
hot surface temperature and dizzying
orbit mean life there is improbable.
Red dwarfs with rocky
planets could be ubiquitous
in the universe.
Phil Muirhead
Professor of Astronomy
Boston University