Can We Find A Living Planet by 2020?
There was a lot of excitement last week about the discovery
of a “waterworld” planet called GJ 1214b, as reported on Discovery News by my
colleague Ian O’Neill.
This world belongs to an emerging class of planets dubbed “super-Earths.”
It is 6.5 times Earth’s mass and nearly
three times our diameter. Its mass, diameter and density suggest the planet is
largely a ball of water with an icy/rocky core.
Considering that the first exoplanet orbiting a normal star
was discovered only 15 years ago, we have made tremendous strides. We are
finding ever-smaller planets and have reached the threshold of super-Earths.
Yet we can only learn a few basic parameters: their orbital period and mass. Planet diameter
and density can be deduced only by observing transiting exoplanets that have orbits
tilted edge-on to Earth and so they can be measured crossing the face of their
The next big step is to chemically characterize the
atmospheres of exoplanets. And this will inevitably lead us to the confirmation
of life on other worlds — presumably in stellar habitable zones.
Finding evidence for extraterrestrial life is a daunting
task. First you need to measure and dissect the light reflected by the planet. Considering the
parent star could be as much as 10 billion times brighter than the planet, it
would be like trying to see a gnat crawling on the rim of a car headlight aimed
at you. This is beyond the capabilities of the largest planned ground-based
telescopes, and barely doable by immense space telescopes yet to be built.
Secondly, if you can measure the signature of what we
consider biotracers for life –oxygen, methane, carbon dioxide, and others – you
need to convince yourself that these are not produce by some exotic non-biological
process. That debate among scientists could last for many years, as it has with
possible biotracers in the famous Mars meteorite ALH 84001 that was first announced in
1996 as possibly containing evidence for martian microbes.
Last week I received a call from a physician who was
enthralled by the discovery of a suspected water planet and wondered if Hubble
Space Telescope was going to observe the planet. This kind of
observation is a long-shot for Hubble, if doable at all.
But we can accomplish identifying an inhabited planet by
2020 if we get very lucky in finding a big planet orbiting a small nearby star,
according to an article in NATURE magazine published last month by MIT’s Sara Seager and Drake
Deming of NASA’s Goddard Space flight Center. Seager describes how to go find another Earth in a nicely illustrated, concise book she has published online called "Is There Life Out There? The Search For Habitable Exoplanets."
A small star is dim, and a planet bigger than Earth is
easier to detect. There are more than 10,000 red dwarf (M-class) stars within
100 light-years of Earth. It’s predicted that a handful of these will have
super-Earths that (1) lie very close to the star in its habitable zone and (2)
transit the star so that diameter and density can be calculated. The atmosphere
can also be measured during a transit, as Hubble successfully demonstrated in
The James Webb Space Telescope (JWST) scheduled for launch in 2014,
has the potential to find an inhabited planet. This would be possible if a
free-flying “starshade” were placed 9,300 miles behind Webb, it could block out
the star’s light and allow for reflected light from the planet to be collected.
The plastic starshade would be half the width of a football field and would
need a precise (to within one millimeter) flower-petal shape to exactly
block out the starlight but let the planet’s light through. The occulter would
cost under $1 billion, but is not currently planned for JWST.
The odds are all stacked in favor of this kind of planet being the first place we look for extrasolar life. Skeptics with a “Rare Earth Hypothesis” philosophy would
argue that such a planet is uninhabitable because it is so close to the red dwarf to be
in gravitational tide-lock meaning the planet doesn’t have a day-night cycle
and therefore wide temperature extremes. What’s more, the planet could suffer
from titanic red dwarf flares and an anemic planetary magnetic field.
But astronomers living on such a planet might look at Earth
and say: “Bah! who could live on such a puny ball of rock whirling around a super-hot, quickly evolving star?”
My discussion with the physician ended with speculating on
the probability of life in space, given the expected abundance of inhabitable
worlds. “I hope we are preparing to defend ourselves (against alien attack),”
he mused. I shrugged this off. “If they are smart enough to travel here they
are smart enough to kick our a** if they wanted to!”