Signs of Life
Astronomers search for other Earths
Tuesday, 7 March 2000
NEWS 1A
By Jim Erickson
THE ARIZONA DAILY STAR
SIGNS OF LIFE: Third of a three-day series
UA astronomer Neville J. Woolf dreams of armadas of giant telescopes floating through space, scanning the skies for signs of life on Earth-like planets orbiting nearby stars.
Each of the 100 or so telescopes in Woolf's fantasy fleet has a papery-thin plastic mirror the size of a football field.
The space mirrors gather starlight and beam it into an orbiting mother ship, where snapshots of alien worlds are processed.
``This is a dream. Whether it becomes a reality in the 25th century, I don't know,'' Woolf said. ``The important thing, at this time, is to get it started.''
With funding from NASA, Woolf, Roger Angel and other astronomers at the University of Arizona's Steward Observatory are figuring out how to make ultra-lightweight space telescopes that can see other Earths - if they're out there.
Taking pictures of such a planet is a daunting challenge that may take many decades to solve. But finding them and looking for evidence of life in their atmospheres is a more manageable goal, one that can probably be achieved - with steady funding - in about a decade, Woolf and Angel said.
``The question `Is there life on other planets?' is one that people have been asking for a long time. Life is such a magical and remarkable thing, and the sophistication of it and the complexity of it is completely mind-boggling,'' Angel said.
``What's absolutely unique now is that we have some of the tools - and we are developing others - so that we can actually go out and find an answer,'' said Angel, founder and director of the Steward Observatory Mirror Lab, where reflectors are cast for some of the world's largest ground-based telescopes.
In the past five years, 28 Jupiter-sized planets have been discovered around nearby stars. None of the alien worlds is like Earth, and all were found by measuring the wobble of the parent star caused by the gravitational tug of the planet.
Earth-sized planets around nearby stars would be too small to cause a detectable wobble. Astronomers hope to see such planets directly with large telescopes placed above Earth's image-blurring atmosphere.
But even from space, huge challenges remain. The biggest is that Earth-like planets will be small, faint and close to their parent star. Their feeble glow will be swamped by starlight millions or billions of times brighter.
Finding a small planet amid the blinding glare of its star is like looking for a firefly next to a searchlight.
The solution is to block the starlight so you can see the planet. UA astronomers and others at Hawaii's Keck Observatory are developing a technique called nulling interferometry, which combines light from two or more telescopes in a way that cancels the star's light while preserving the planet's glow.
UA astronomers receive about $500,000 a year from NASA's Origins program to study nulling. Graduate student Phil Hinz used some of the money to build a nulling interferometer, tested at Southern Arizona's Multiple Mirror Telescope two years ago.
In the first astronomical use of nulling, Hinz observed dust envelopes around red giant and supergiant stars. A second-generation version of the instrument went to the MMT this month, and future models will be tested at the $83 million Large Binocular Telescope, under construction on Mount Graham.
``Glare is the main technological challenge. It's the main source of noise the instrument would have to overcome,'' said Hinz, who won a three-year fellowship from the space agency last fall to continue his research.
The nulling interferometer is being developed for NASA's proposed Terrestrial Planet Finder, a $2 billion space telescope that could launch by 2011. Woolf, who served as mission scientist early in the project, said a launch around 2025 is probably more likely.
Planet Finder will study 150 nearby sun-like stars in search planets similar to Earth. It will combine light from several space telescopes separated by at least 100 yards.
The telescope array will be sensitive enough to spot an Earth-sized planet's faint glow and will deliver images 100 times sharper than the Hubble Space Telescope. That's the kind of resolution needed to separate a small, close-in planet from its star.
Planet Finder will probe the ``habitable zone'' around other stars, the region where a planet's surface temperature allows liquid water, which is essential to all known forms of life.
If other Earths are found, TPF will do a crude spectroscopic analysis.
Astronomers spread light into its constituent spectrum of colors to determine the chemical composition of stars and planetary atmospheres. When looking for Earth-like planets outside our solar system, they will look in the infrared portion of the spectrum, where the planet shines brightest relative to the star.
At infrared wavelengths, Terrestrial Planet Finder should be able to spot the spectral signatures of what Angel calls ``the three biggies'': water vapor, ozone and carbon dioxide.
Detectable levels of atmospheric water vapor would suggest surface water, possibly oceans.
``We take it for granted, sitting on a planet that's 70 percent covered by oceans, but liquid water is not trivial as a commodity out there,'' said biogeochemist David Des Marais of NASA's Ames Research Center in Mountain View, Calif.
Des Marais is part of NASA's ``astrobiology'' effort to uncover the origins of life on this planet and to look for life beyond Earth.
A key search strategy - one that applies to Mars and Jupiter's moon Europa as well as planets orbiting other stars - is to ``look for places where liquid water was present and could have supported life, then look for life itself,'' Des Marais said.
``We have not yet discovered liquid water outside our own planet, so that would be quite an achievement, to prove that it exists elsewhere.''
Planet Finder should also be able to see carbon dioxide, which denotes a planet with a substantial atmosphere, and ozone, a form of oxygen. Seeing ozone in a planetary atmosphere rich in water vapor would be strong evidence for life.
``Under most circumstances, ozone tells you that there's photosynthetic life on the planet's surface, because most of the oxygen in our own atmosphere comes from photosynthesis,'' said James F. Kasting, a Pennsylvania State University geoscientist and a member of the Planet Finder science working group.
The Earth is about 4.5 billion years old, and for the first 2.5 billion years or so there wasn't much oxygen in the atmosphere. That changed when cyanobacteria, also known as blue-green algae, learned to harness the sun's energy to split water molecules, releasing oxygen in the process.
That microbial innovation, photosynthesis, allowed oxygen-breathing multicellular life forms to emerge. The oxygen buildup also marked Earth's atmosphere with a biological signature that would be obvious to technologically advanced extraterrestrials, Angel said.
``We didn't start broadcasting our presence when the Howdy Doody show went out 50 years ago,'' he said. ``We've been broadcasting our presence for 2 billion years. They - if there are any `they' out there - know that this planet has life on it.''
In 1960, astronomer Frank Drake conducted the first search for radio messages from other solar systems, and the effort to contact intelligent extraterrestrials continues today through the SETI program.
Drake once calculated that our Milky Way galaxy has about 10,000 civilizations capable of interstellar communication.
Others contend that complex life - and especially intelligent life - is a rarity. In their new book ``Rare Earth,'' University of Washington researchers Peter D. Ward and Donald C. Brownlee say modern science is showing that the hospitable, stable conditions that allowed complex life to evolve on Earth are extraordinarily scarce elsewhere in the cosmos.
While reaching other sentient beings by radio would be an astounding feat, spectroscopic analyses of the atmospheres of other Earth-like planets ``is very likely to be the real key to seeing life, if there is any,'' Angel said.
If Terrestrial Planet Finder lives up to its name, a larger space telescope would be needed for the kind of detailed spectroscopy that could confirm the presence of life. One ``acid test'' for life would be the simultaneous presence of ozone and methane in a planet's atmosphere.
In an Earth-like atmosphere, oxygen would quickly eat up all the methane, converting it to carbon dioxide. Our planet's atmosphere is methane-rich because the gas is constantly resupplied, largely by microbes.
Biological sources of methane include bogs, swamps, rice paddies, cows and other ruminant animals, and termites, all of which contain methane-producing bacteria.
``In order to keep the methane in the atmosphere in the presence of oxygen, you have to have a constant source - a pretty big one - which probably requires biology,'' Kasting said.
Seeing oxygen and methane in an Earth-like planet's atmosphere ``would convince me that there was life present,'' he said.
But if the planet hunters' focus is solely on finding oxygen-rich atmospheres, they could miss many living worlds, said oceanographer and microbiologist Ken Nealson of NASA's Jet Propulsion Laboratory.
On Earth, microbial life existed for about 2 billion years before atmospheric oxygen levels rose significantly. The earliest microbes didn't need oxygen, and the same may be true on Earth-like planets in other solar systems, he said.
``The Earth was just full of life back then, but the atmosphere didn't look anything like it does today,'' Nealson said. ``So, if you measured an atmosphere like that from two light years away, would you decide that it wasn't alive because you didn't see oxygen and ozone?''
The long-term key to finding life on other planets is to probe their atmospheres for ``things that shouldn't be there at the same time,'' he said. Like the simultaneous presence of oxygen and methane on present-day Earth, all life forms create chemical imbalances not seen on sterile planets.
``Life, by definition, is a chemical disequilibrium,'' Nealson said. ``It is absolutely different from anything you'd predict on that planet. And it doesn't have to be like life on Earth.''
Atmospheric chemist James Lovelock and microbiologist Lynn Margulis pointed that out in their Gaia hypothesis, developed in the late 1960s and early 1970s.
Nealson heads JPL's astrobiology program. One of his team's long-term goals is to develop a set of ``atmospheric planetary biosignatures'' that could be used to find life on planets around other stars - even if those bodies have atmospheres unlike today's Earth.
Terrestrial Planet Finder is a stepping stone toward the kind of detailed atmospheric studies envisioned by Nealson and others. Planet Finder, in turn, will rely on technologies tested in earlier NASA missions, including the Next Generation Space Telescope, the planned Hubble successor.
Like Hubble, NGST will use a single light-gathering primary mirror. But Planet Finder will use the techniques of interferometry to combine light beams from several reflectors.
The earliest design concepts for Planet Finder called for four 4-meter-diameter mirrors bolted to a 100-meter-long truss.
But maintaining the precise alignment of a beam the size of a football field is tough, and the space agency now favors untethered telescopes that would fly in formation, said Planet Finder project scientist Charles Beichman of the Jet Propulsion Laboratory.
Formation flying in space will be attempted for the first time on the Space Technology-3 mission, set for launch in 2005. Each spacecraft would use a Global Positioning Satellite-style navigation system to maintain its alignment.
Other big uncertainties remain for Planet Finder. For starters, the size, number and configuration of the telescopes is still ``up in the air,'' Beichman said.
By mid-April, NASA will select three industry/university teams to explore various design options. Each team will get $1.5 million to come up with six workable concepts, and the space agency will then select two ideas from each team for detailed study.
``The idea, at the end of 18 months, is to have looked at the waterfront in a lot of detail and then to have homed in on four or five or six architectures that will meet our science requirements,'' Beichman said.
The agency hopes to have a detailed design in hand by 2007.
Woolf, Angel and other UA astronomers teamed up with aerospace giant Lockheed Martin for their Planet Finder proposal, submitted last month.
The UA astronomers are focusing on Planet Finder's optics and have already explored several ideas for the main light-gathering mirrors. Their leading candidate is a super-thin, lightweight concave glass reflector, Woolf said.
But more radical concepts will be offered, including flat sheets of reflective plastic thinner than a human hair. The lightweight, flexible sheets would be stretched across hexagonal or circular frames like Saran Wrap stretched across a bowl.
UA astronomers have been investigating the potential of these so-called membrane mirrors with a grant from NASA's Institute for Advanced Concepts. The grant provides $250,000 this year and an equal amount next year.
UA researchers received $23.7 million from NASA during the last fiscal year. Of that, Steward Observatory astronomers took in $14.2 million, while the Lunar and Planetary Laboratory drew $6.9 million.
Membrane mirrors may not be ready in time for Planet Finder, Woolf said. But the idea could be scaled up for future use on the huge, ultra-lightweight space telescopes that would be needed to do follow-up studies and imaging of Earth-like planets.
Detailed spectroscopic studies might require a dozen or more 8-meter-diameter mirrors that would feed starlight into a collector telescope, Angel said.
Taking even a blurry snapshot of another Earth - a long-term goal of NASA Administrator Daniel Goldin - would require 25 telescopes, each with a mirror 40 meters across, spread over thousands of miles in space, he said.
Capturing a picture that begins to show details like continents, oceans and ice caps would require up to 100 telescopes, each the size of a football field, Woolf said.
``No existing technology even comes remotely close to this kind of hugely ambitious telescope for imaging,'' Angel said.
``Mr. Goldin has this picture in his head that wouldn't it be nice to see a picture of another planet with its blue oceans and clouds and the rest of it,'' he said.
``We've told him that he was crazy, and what it would actually take, which is this enormous array of huge mirrors,'' he said.
``And undaunted, the man has come back and said OK, go figure out the technology that's going to do it.''
UA astronomers Roger Angel, left, and Neville Woolf stretch Mylar for possible use in space-based telescopesUA grad student Phil Hinz built this nulling interferometer to block a star's light so astronomers can see planets orbiting nearby
Painting by William K. Hartmann: Planetary scientist's painting shows an Earth-like planet forming in gas, dust and debris surrounding a newborn star
1999 Star photo: Giant 10-ton glass mirror of the MMT atop Mount Hopkins will test the nulling interferometer.
