From the ground up
Whether the
drive behind searches for other planetary systems is to determine how our own
system developed, or to find out whether or not we are alone in the universe,
the search for planets outside Earth’s solar system proceeds at a breakneck
pace. After Swiss astronomers Michel Mayor and Didier Queloz found the first
planet around a sun-like star in 1995, the hunt began in earnest. A team led
by Geoff Marcy of the University of California at Berkeley and Paul Butler of
the Carnegie Institution of Washington followed up, finding a wealth of planets
hosted by faraway stars.
Earth-based telescopes such as the Very
Large Array in Socorro, N.M., have been important in the search for signs of
extrasolar planets. The 27 antennas can be arranged in various configurations,
and the combination of their data streams allows them to act as one giant telescope.
Image courtesy of NRAO/AUI.
“The paradigm we have for forming solar systems is that you have a cloud
of gas” from which planets will form, says Karl Stapelfeldt of NASA’s
Jet Propulsion Lab in Pasadena, Calif. The cloud initially condenses “like
snowflake particles get together in clouds on Earth,” before becoming like
snowballs, he says, “in a shooting gallery of ever-more-violent collisions.”
Stars at various ages have provided different examples of how it works: Disks
of gas and dust that coalesce around stars can give rise to large gas planets
like the ones we have in the outer limits of our solar system. They may also
form smaller rocky planets — Earth, Venus and Mars in our local system
— closer to a star and in a zone that we, at least, consider perfectly
habitable.
The current catalog of extrasolar planets shows that large gas planets seem
to form reasonably often, says Jonathan Lunine of the Lunar and Planetary Lab
at the University of Arizona, Tucson, and they do not necessarily fill “the
space where terrestrial planets should be.” From everything seen so far,
Lunine says, “there are probably lots of Earths out there waiting to be
discovered.” Yet for now, the only way to get to those Earths is to find
larger planets while working to improve the technology to see smaller ones.
Teams including Marcy, Butler and their co-workers determined that they could
deduce the presence of large planets by searching for stars that seem to “wobble.”
The teams have used mostly ground-based telescope arrays to scour stars for
the telltale tremble that indicates a large mass is pushing and pulling at a
star while orbiting around it. That means that most planets observed so far
have been on the order of a Jupiter — more than 300 times the mass of Earth
and bigger — but sometimes smaller, and often much closer to their stars
than Earth is to the sun.
One of the planets “photographed” in the last year was first detected
by the Very Large Telescope in northern Chile, in April 2004; a year later,
scientists confirmed from the Hubble Space Telescope that the sighting was an
actual recording of a planet’s light. At five times the size of Jupiter,
the planet appears as a red dot next to its faint brown dwarf star, which it
orbits at a distance that is 55 times farther than that between Earth and the
sun.
But of late, several newly found extrasolar planets, scientists calculate, are
the size of Neptune (about 17 times the mass of Earth, but much smaller than
the large planets found in the late 1990s). And last month, Marcy, Butler and
their team announced that they had found evidence for the smallest planet yet,
about six times the size of Earth, using the Keck Telescope. New Earth-based
instruments, including a new spectrometer on the Very Large Telescope, are “the
real winners” for indirect detection techniques on the ground, says Jill
Tarter, the director of the Center for SETI Research in Mountain View, Calif.,
which conducts surveys for technology signals from other planets as evidence
of life.
Ground-based observatories can go after young low-mass stars. And if they have
young planets, “the planets are still contracting, and still giving off
light,” Tarter says. Other options include looking at M stars, known as
red dwarfs, which are half the size of the sun and much less luminous. “The
precision is getting so good that we’re getting down to Neptune-mass planets,”
Tarter says, “but we won’t get to Earths that way.”
To find the smaller planets, scientists will have to get off Earth. From the
ground, says Sara Seager, an astronomer at the Carnegie Institution of Washington,
telescopes have to look through Earth’s atmosphere, which varies and also
could obscure similar signals from Earth-like planets. Seager also notes that
“stars are up to 10 orders of magnitude brighter than a planet,” she
says, “swamping” any planet’s light from the ground-view, and
even from the space-view of the Hubble Space Telescope.
The only way to get detailed information of and from planets elsewhere —
the “holy grail” of photons recorded directly from a planet —
is to send observatories far past Earth’s atmosphere, Seager and others
say. A series of missions have launch dates spanning the next two decades, all
culminating in a mission that many scientists hope will work best: the Terrestrial
Planet Finder.
Off the ground
The Terrestrial
Planet Finder is exactly what its name says: Its mission is to find Earth-like
planets, potentially with water and at around the same distance from a star
as Earth is from the sun — the so-called habitable zone that researchers
hypothesized in the 1960s. In its simplest incarnation, that zone means that
too close to a star, a planet bakes off its water, but too far away, it will
freeze. Neither condition is “just right” (like Goldilocks’ porridge)
to accommodate life, which many scientists say needs liquid water.
The Terrestrial Planet Finder program plans to launch two observatories within
the next decade: the visible-light coronagraph in 2014 and an infrared interferometer,
shown here, around 2020. Image courtesy of NASA/JPL.
To find small rocky planets in just the right spot, the Terrestrial Planet Finder
program aims to launch two observatories in 2014 and 2019, says James Kasting
of the University of Pennsylvania in University Park, to study 30 to 50 stars
in detail. Each will use a different imaging method.
The first to go will be TPF-C, which will carry a space-based telescope built
to record visible light from planets while blocking out the light from their
host stars (using a coronagraph, blocking out everything but the star’s
corona, hence the “C”). The second instrument, TPF-I, will look in
the thermal infrared wavelengths, using an interferometer (hence the “I”)
— as series of devices that act as one instrument.
“Planets are cool objects compared to stars,” which give off both
visible and near infrared wavelengths, Kasting says, “whereas planets give
off in the thermal infrared.” (Another thermal infrared-detecting mission,
known as DARWIN, should launch in 2015, by the European Space Agency.)
The 15 to 20 years it may take to get the interferometry mission “off the
ground” is in part due to funding issues, but also because of the time
it will take to develop the technology, says Lunine of the Lunar and Planetary
Lab. Plus, NASA’s new administrator, Michael Griffin, recently presented
a new budget to Congress that would require pushing back the launch of both
the Terrestrial Planet Finder and another planned mission, SIM PlanetQuest,
intended to search for planets until the Terrestrial Planet Finder gets space-borne.
SIM PlanetQuest, first called the Space Interferometry Mission and scheduled
to launch in 2011, will catalog some stars that the Terrestrial Planet Finder
will study. Seager says that like earlier methods, SIM basically will look at
the motions of stars, again searching for gravitational wobbles. Another space-based
telescope called Kepler (scheduled for 2008) will search for transits, a temporary
blocking of light as a planet passes between its star and Earth, or behind the
star so that its own light is blocked.
Astronomers recently observed another previously identified planet, in results
published in the June Astrophysical Journal, measuring how much the star’s
light seemed to diminish as the planet passed behind it (see image, page 30).
But identifying a transit means being in the right place at the right time.
“Things have got to be exactly lined up,” says Stapelfeldt of the
Jet Propulsion Lab. In our own solar system, for example, a century may pass
between a transit of Venus because Earth and Venus have offset orbits. “That
just emphasizes how fortunate these events are, and how you can’t count
on them” for planet-finding, he says.
By directly surveying tens of thousands of stars to search for several hundred
planets, Kepler may have a better chance of finding small planets, says William
Borucki, the mission’s principal investigator from NASA Ames Research Center
in Moffet Field, Calif. “If Earths are common,” he says, “then
it may well be that there is life throughout our galaxy. On the other hand,
if Kepler finds none, then Earths are extremely rare,” and, hence, life
may be too.
Pale blue dot
Finding those small, rocky and watery planets orbiting their large stars presents
scientists with the “pale blue dot problem,” says Steven Benner, a
chemist at the University of Florida in Gainesville. “Pale” because
that sought-after planet is small and faint in comparison to its sun, and “dot”-like
because it’s supposed to be small and rocky, like Earth. The “blue”
requirement comes from the assumption that water is necessary for life.
Benner is a member of a committee convened more than a year ago by the National
Academy of Sciences to challenge just those assumptions, by considering “weird
life.” Photosynthesis or water may not be necessary, Benner says. “Organic
chemistry could support life in an atmosphere like Titan,” a moon of Saturn
(see story, page 22 of this issue), he says, and scientists
might have to consider life on planets configured very differently than Earth.
To detect any kind of life at distances of 15 to 20 light years away, scientists
will “need more information than just a barely resolved light such as a
pale blue dot,” Benner says. That information includes biosignatures —
the types of evidence that could come only from life. Earth’s atmosphere,
for example, contains significant amounts of oxygen and methane, which are otherwise
incompatible, and can only be maintained by life forms that consume and exude
both. Therefore, on another planet, finding the spectra of two gases coexisting
that are not normally stable together could be an indication of life.
Tracking such evidence will require directly gathering the spectra of a planet’s
atmospheric gases — starlight reflected from a planet’s surface varies
in strength at different wavelengths according to its composition — or
other such indicators. And collecting the profiles will require long observation
periods of candidate planets. Depending on their environments, planets most
likely will present different spectra. For example, early Earth before the rise
of oxygen in its atmosphere would have had a very different profile from afar
than it does now, even if it did have life. That’s because methane-based
life, for example, would have a different signature than photosynthesizing life
forms.
To test these ideas, researchers in Victoria Meadows’ lab at the Jet Propulsion
Lab have been building virtual planets with various atmospheric, orbital and
other configurations. So far, Meadows’ team has been able to model a Mars-like
planet’s annual cycles of soil and atmospheric temperatures, and have worked
out methane and ozone signals for hypothetical Earth-like planets according
to the radiation they receive from different-sized stars. Such complex interactions
might mislead researchers into thinking they have found life, so the research
group also pursues what kinds of planetary configurations might give “false
positives.”
For now, the search for life continues to be based on what scientists know from
Earth, the only example we have, says Borucki of NASA. “You always work
from the familiar and the understood to the new,” he says. “If you
jump too far, you don’t know where you are.” That means starting with
trying to find planets that have water on their surface, and understanding life
that is carbon-based.
But if researchers throw out the requirements for life as we know it on Earth,
then that opens up other options for finding life, Benner says, such as on larger
planets at distances outside what we think of as the habitable zone. “There’s
the possibility of life in solvents that are not water,” he says, “possible
even in gas giants like Jupiter.”
That possibility throws into question researchers’ ability to recognize
life if it’s in an unfamiliar form, even in more local searches. Benner
cites the controversy that sprung up around the Allan Hills meteorite, a martian
rock found in the Antarctic and identified as bearing possible microbial fossils.
The putative fossils were deemed “too complex to be made by nonliving processes,”
Benner says, but they seemed too small.
“What makes you think cells have to be large? On Earth, proteins and nucleic
acids are necessary for life” — hence the large cell size argument
— “but there’s a perfectly good model that says cells need only
nucleic acids,” Benner says. “You can immediately see the kinds of
arguments you get into even having the sample in hand.”
“We’re trying to understand life as we know it in the extreme,”
says Tarter of SETI, from the various metabolic pathways an organism might use
to eat and breathe to the detritus it could leave behind. Built on extreme life
studies here on Earth in places where bacteria might thrive on methane or microbes
feed on iron sulfide, without need for sunlight or oxygen, the scientific community
is focusing on the martian subsurface for a microbial search.
The recent explorations by the Mars twin rovers “leave no doubt that Mars
could have supported life,” Benner says, as the planet had water at the
same time Earth did long ago. The question is not only whether life was there,
but whether scientists can recognize its traces.
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