Research presented at a recent astronomical conference is being hailed
as ushering in a new era in the search for Earth-like planets by
showing that they are more numerous than previously thought and that
scientists can now analyze their atmospheres for elements that might be
conducive to life.
“This conference was very well timed. People came with new results.
It clicked together. There was a lot of excitement,” said Professor of
Astronomy Dimitar Sasselov, who heads Harvard’s Origins of Life
Initiative and who co-chaired the conference’s Scientific Organizing
Committee. “What happened this spring was a tipping point in the
field.”
The International Astronomical Union (IAU) symposium, held in May at
the American Academy of Arts and Sciences in Cambridge, was sponsored
by the Harvard Origins of Life Initiative, the California Institute of
Technology’s Michelson Science Center, the Massachusetts Institute of
Technology, and the IAU. Its focus was “Transiting Planets,” or the
technique of discovering planets by measuring changes in the light of
distant suns when a planet passes in front of them.
“The age of the discovery of Earth-like planets started last week,”
Sasselov said shortly after the conference concluded. “We can say this
is the moment where we started the exploration of planets like Earth.”
Astronomers using a variety of techniques have discovered more than
300 planets circling other stars since 1995, when a Swiss team
announced finding the first Jupiter-mass planet orbiting a sun-like
star, but few of them bear any resemblance to rocky planets like Earth.
Because planets are far smaller and dimmer than the star they circle,
most techniques rely on detecting not the planet itself, but its
effects on its star, such as changes in the star’s light or wobbles in
the star’s rotation due to a planet’s gravitational tug as it circles.
Consequently, most of the planets found so far have been large gas
giants such as our own solar system’s Jupiter, Saturn, or Neptune,
thought to be incapable of sustaining life.
That has been changing since the 2004 announcement of the discovery
of the first “super-Earth,” a potentially rocky planet 14 times larger
than Earth circling a star in the southern constellation Altar, and
with the development of new instruments that astronomers believe will
be able to find planets close to Earth’s size.
At the conference, Christophe Lovis, a scientist at the University of
Geneva who is collaborating with the Harvard Origins of Life
Initiative, announced findings that small, rocky worlds are not only
present in the universe, they’re common, outnumbering the large gas
giants by as much as a 3-to-1 ratio.
“This finding was not expected and very welcome,” Sasselov said. “It
means planets like Earth are abundant and we can study them.”
Sasselov said rocky planets up to five times Earth’s size should be
detectable with the new generation of instruments coming on line such
as the Harvard Origins of Life Initiative’s spectrometer equipped with
the new laser astro-comb, developed at the Harvard-Smithsonian Center
for Astrophysics. The spectrometer which will be deployed in the Canary
Islands for exoplanet research sometime in 2010.
“Five times larger than Earth is actually pretty good from the point
of view of geochemistry and biochemistry,” Sasselov said. “Ultimately,
we want to go down to sister Earths, as people call it. It’s my
personal belief that super-Earths are as hospitable to life as Earths,
but we need to compare them. People want to know if there are planets
just like ours out there.”
The second major finding to emerge from the conference shows that
researchers can get an idea of conditions on any planets that they do
find, Sasselov said. Presented by Harvard astronomer David Charbonneau, the results presented the first
compilation of the atmospheric spectrum of a planet orbiting another
star.
The spectrum, put together for the atmosphere of a gas giant 60
light-years away, uses the light emitted or absorbed by the planet to
detect what molecules are present in the atmosphere, in this case,
methane, potassium, sodium, water vapor, and small particulate haze,
among others. Though researchers have been able to detect single
elements that make up the atmosphere of planets since 2001, this is the
first time the complete makeup of the atmosphere of an extrasolar
planet has been determined.
“We can actually do this; it is amazing,” Sasselov said. “We can
look at a planet 60 light-years away and tell you what’s in the
atmosphere. This is really a big deal.”
Knowing a planet’s atmospheric makeup can help astronomers determine whether the conditions for life are present.
“What really keeps me up at night is the potential to apply the
techniques we’ve developed to study the atmospheres of gas giant
exoplanets to the soon-to-be discovered Earth-like exoplanets,”
Charbonneau said. “We could conduct a search for the presence of
specific molecules indicating biological activity on the planet’s
surface.”
