Astronomers
have discovered hundreds of Jupiter-like planets in our galaxy. However, a
handful of the planets found orbiting distant stars are more Earth-sized. This
gives hope to astrobiologists, who think we are more likely to find life on
rocky planets with liquid water.
The rocky planets found so far
are actually more massive than our own. Dimitar Sasselov, professor of
astronomy at Harvard University, coined the term "Super-Earths" to reflect
their mass rather than any superior qualities.
But Sasselov says that these
planets – which range from about 2 to 10 Earth masses – could be superior to
the Earth when it comes to sustaining life.
On Shaky Ground
It is said that 99 percent of
all species that ever lived have gone extinct. Earth, it seems, is a tough
place to call home. Our planet has gone through Ice Ages and global warming
trends, it has been hit by comets and asteroids (leading, in one case, to a
mass extinction that felled the mighty dinosaurs), and the amount of oxygen in
the atmosphere has risen and fallen over time. Our planet is always in a state
of flux, and life must adapt to these changes or die.
The shifting of tectonic
plates is another example of Earth's restless nature. Continents bang together,
forming mountains, only to be later torn apart. Islands grow from underwater
volcanoes, and elements are liberated from rocks when they are melted beneath
the crust.
While all this geologic
activity makes us literally stand on shaky ground, scientists have come to
believe that tectonics is one of the key features of our planet which makes
life possible. If not for tectonics, carbon needed by life would stay locked
within rocks.
The fear today is that too
much carbon dioxide in the atmosphere will lead to global warming. Yet too
little carbon dioxide in the atmosphere would make Earth a much colder place,
and the photosynthetic plants and algae that rely on CO2 would perish. The
demise of these oxygen-producing organisms would leave us all gasping for
breath.
According to Sasselov, Earth's
mass helps keeps tectonics in action. The more massive a planet, the hotter its
interior. Tectonic plates slide on a layer of molten rock beneath the crust
called the mantle. Convective currents within the mantle push the plates around.
For smaller planets like Mars, the interior is not hot enough to drive
tectonics.
Super Earths, with a
larger and hotter interior, would have a thinner planetary crust placed under
more stress. This probably would result in faster tectonics, as well as more
earthquakes, volcanism, and other geologic upheavals. In fact, Sasselov says
the plate tectonics on Super Earths may be so rapid that mountains and ocean
trenches wouldn't have much time to develop before the surface was again
recycled.
Venus, only slightly less
massive than Earth, has had a great deal of volcanic activity, but it does not
appear to have tectonics. This may be because low mass planets need water to
lubricate the process, and Venus lost its water long ago through evaporation.
Sasselov says Earth has just enough water for tectonics to work. Tectonics on
Super Earths might be so efficient that water isn't even needed.
On the other hand, it's
possible that a SuperEarth could be entirely covered by water. Sasselov says
that in the case of such an ocean world, most of the water will be in an exotic
state known as iceVII -- a very compressed, hard ice with a melting point above
212 degrees Fahrenheit (100 degrees Celsius).
Whether made of rock or ice,
Sasselov says Super Earths will be only 1 or 2 times the actual size of Earth
because they become densely compressed as they gain mass. This higher density
will result in greater gravity. Sasselov says the most massive Super Earth
would have about 3 times the gravity of Earth. Tests of human resistance to
vertical G-force, where the blood is pulled down to the legs, have found the
typical person can tolerate up to 5 Gs before losing consciousness. So while
you might feel much heavier walking on a Super Earth, the extra gravity
wouldn't be beyond what human explorers could endure. Of course, any life that
evolved on a Super Earth would be adapted to the greater gravity, just as a
human feels comfortable on the 1 G surface of Earth.
Habitable Hot Spots
This greater gravity means a
Super Earth can easily hold onto an atmosphere, so it would not end up with a
tenuous atmosphere like Mars. But the role of a planet's atmosphere in creating
prime conditions for life can be tricky. Venus has a surface temperature of
nearly 900 F (480 C) due to the thick greenhouse atmosphere that doesn't let
heat escape.
One of the biggest influences
on a planet's climate is the star it orbits. Earth has a circular orbit 150
million kilometers away from the Sun, a yellow dwarf star. This helps keep
conditions warm enough so that our oceans don't freeze over, but cool enough so
that we don't lose all our water through evaporation.
The Super
Earths discovered so far orbit a variety of stars. The first Earth-like
extrasolar planets ever found orbit a pulsar, a rotating neutron star that
emits high energy radiation. The other Super Earths orbit stars that are
smaller and cooler than our Sun.
Most of the known Super Earths
are very
close to their stars, closer than the planet Mercury is to the Sun. Even
though these stars don't burn as brightly as our Sun, the planets are so close
they are like burnt cinders flickering close to a fire.
One such hot Super Earth is
CoRoT 7-b (named for the CoRoT telescope that was used to locate the planet).
CoRoT 7-b orbits the orange dwarf star TYC 4799-1733-1 once every 20 hours. This planet is nearly 5 times the
mass of Earth, but is less than twice as big. This solar system has another hot
Super Earth, CoRoT-7c, which is 8 times as
massive as Earth and circles the star in 3 days and 17 hours.
For astrobiologists hoping to
find alien life, two Super Earths orbiting the star Gliese 581 have potential.
Gliese 581, a mce_style="color: black;"> style='color:black'>red
dwarf star, is cooler than our Sun. Based on their orbit around this star, planets
Gliese 581-c and Gliese 581-d are thought to have habitable
conditions, although some think planet "c" might have a runaway greenhouse
atmosphere like Venus.
Another aspect affecting the
potential for life is the presence of a companion moon. Earth's Moon helps
balance our planet's rotation on its axis. Sasselov notes that a Super Earth's
extra mass would give it a very stable rotation, so a moon would not be needed
to help keep the planet in line.
Superior Alien Civilizations
Missions like the Kepler space
telescope, launched just this year, could help astronomers find many Earth-like
planets in the years to come. Sasselov estimates there could be a hundred
million habitable Super Earth planets just in our Milky Way galaxy. He predicts
we'll find 50 to 100 Super Earth planets in the next 5 years.
The existence of so many Super
Earths could explain the "Fermi Paradox" of why aliens have not contacted us.
If our lower mass planet does not have the ideal conditions for life, alien
explorers would be less likely to look to us, choosing instead to target the
many Super Earths in the galaxy.
"Earth is a marginal planet when it comes to conditions we would like to see for complex life to sustain itself," Sasselov notes. "In the family of Earth-like planets, the sweet spot for complex chemistry and biochemistry to emerge and sustain itself lies in planets larger than the Earth."
If aliens on Super Earths ever
decided to investigate Earth to see if such a tiny world could harbor life,
they would have a harder time sending rockets into space because of the higher
gravity on their planet. "This could be another answer to the Fermi Paradox,"
says Sasselov, "but it's not an insurmountable problem." It could even be that
because of their deeper gravity well, aliens living on Super Earths would have
to develop a technology superior to our chemical rockets in order to explore
the universe.
Sasselov's own suggestion for the Fermi paradox relates to the age and evolution of planets in the universe. When the universe was young, only hydrogen and helium were available. Generations of stellar evolution were needed to produce the heavier elements, such as silica and iron, which build rocky planets. Even though the universe is approximately 14 billion years old, our solar system only formed about 4.6 billion years ago. (Astronomers did find a 12.7-billion-year-old Jupiter-mass object in our galaxy, but Sasselov thinks this so-called ancient planet could instead be the remnant of a red or brown dwarf star that was stripped of some of its mass.)
If other rocky worlds with life are about as young as we are, then perhaps the lack of visiting spaceships indicates there are no ancient, highly advanced alien civilizations out there.
"Most life emerges on Super-Earths with habitable potential, but Super-Earths started forming in the galaxy only relatively recently, and few technical civilizations have managed to emerge since," Sasselov says.
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