The search for habitable planets is a complicated business, and the quest to image them is beyond the reach of current technology.
Searching for signs that they might actually be inhabited is an even tougher prospect.
At a weeklong conference in Oro Valley, “Search for Life Beyond the Solar System: Exoplanets, Biosignatures and Instruments,” about 200 astrobiologists from around the world proposed a variety of techniques for finding and verifying candidates, but cautioned that the search for life on other planets is just getting started.
Jared Males, a Carl Sagan fellow at the University of Arizona’s Steward Observatory who was one of 60 speakers at the conference, said he likes to preface his talks on the subject with a photo of Earth — “the blue marble” — taken by the Apollo 17 mission to the moon.
You can make out the Earth’s hemispheres in the photo, and even the clouds that envelop it.
In astronomical terms, it’s a close-up — taken from a mere 28,000 miles away. But it is not how the largest telescopes on Earth and the most advanced ones in space see exoplanets.
Exoplanets are tiny dots, if they can be viewed at all. Most are too far away to see, and their presence is inferred by a wobble in a star or a diminution of its light.
Some are hidden behind clouds of stellar debris and all are outshone by the stars they orbit.
Exoplanet count growing
To get an idea of what the astrobiologists gathered at the Hilton Tucson El Conquistador Resort face, consider someone from another planet — somewhere in the cosmos — looking for signatures of life on Earth.
From another solar system, Earth is not the “blue marble” viewed by Apollo 17, nor is it the “pale blue dot” seen as a fraction of a single pixel by NASA’s Voyager 1 from 3.7 billion miles away.
Earth is an invisible planet in a vast cosmos. Even if you knew where to look, its puny reflection is eclipsed by a sun that is more than 1 million times bigger and only 93 million miles away.
An alien astronomer might be able to deduce Earth’s presence as it passed by the sun once a year. That would take patience and the good fortune to be viewing it from the right orientation.
What is amazing is that we have actually seen planets near distant stars, including a Jupiter-sized planet that Males imaged in nearly visible light.
Beta pictoris b is 63.4 light-years away. A light-year is about 5.9 trillion miles.
Fifteen years ago, exoplanets were theoretical.
Today, with the recent discovery of 715 planets found by combing through the data of NASA’s Kepler space telescope, NASA counts the existence of 1,692. And by extrapolation, we know that billions more are out there.
new ideas to ponder
The conference presented numerous ideas for finding more planets and fleshing out what we do not know about those we’ve found.
MIT astrophysicist Sara Seager is pushing to have NASA launch a telescope with a sun shield that would allow a space-based telescope to blot out the light of a distant star and reveal the planets orbiting it.
The technique has been used successfully from the ground, but not tried in space.
Astronomer Jeff Kuhn of the University of Hawaii said our current plans for Earth telescopes — behemoths like the Thirty Meter Telescope and the Giant Magellan Telescope, an array of seven 8-meter mirrors — are too small.
He proposes Colossus, an array of 60 8-meter mirrors that would search up to 60 light-years away for planets giving off infrared signatures that would indicate deliberate use of energy sources.
Wesley Traub, chief scientist for NASA’s Exoplanet Exploration Program, is enthusiastic about the WFIRST-AFTA mission, a planned space telescope that recently added a coronagraph to help it directly image exoplanets.
UA astronomer Phil Hinz reported recent success at the Large Binocular Telescope on Mount Graham, where he has linked its two 8.4-meter mirrors as an interferometer in his NASA-funded studies of the stellar dust clouds that impede planet-hunting.
He hopes to gather information about the best and worst places to look.
The chemists and biologists at the conference, presented by Steward Observatory and the Vatican Observatory, said we may have to expand our definition of habitable zone, based on research that stretches our notions of where life can form and persist.
Oceanographer John Baross of the University of Washington said the only limits on carbon-based life are temperature and water, and those limits expand with each new discovery of life-forms known as “extremophiles.”
Barrows studies bacteria and other organisms that live near vents in deep-sea beds where the sun never shines and where escaping magma heats water to well above its boiling point.
Creatures need water and they need a livable temperature, he said, but new discoveries keep pushing the temperature boundaries.
He said the existence of Earth’s extremophiles gives credence to calls to search our solar system for others.
Saturn’s moon, Titan, with its methane lakes, and Jupiter’s moon, Europa, with its salty ocean beneath an icy crust, may harbor such extreme forms of life.
seeking earth’s “twin”
The search for intelligent extraterrestrial life began long before evidence existed that there are exoplanets.
Astronomer Jill Tarter, who has been involved in the search for more than 35 years, said the discovery of exoplanets in the last 15 years has refined her target area.
Tarter recently stepped down as director of the SETI Institute to spend time raising money for the effort. Tarter was the model for Ellie Arroway, the astronomer in Carl Sagan’s book “Contact,” who receives a signal from the cosmos.
SETI now focuses its search for radio waves and flashes of light that can’t originate from natural astrophysical processes on the star field plumbed by NASA’s Kepler, Tarter said.
“Kepler has really transformed our view with the diversity of planets it found,” said UA astronomer Daniel Apai, one of the conference organizers.
The next step, he said, is to find analogs to Earth itself — rocky planets in the habitable zone where liquid water may exist.
“Ideally, we’d like to find Earth twins. It would be our best case that what we see is capable of supporting life.”
Apai won’t predict when that will happen, but said it is achievable. “This is something that people could make happen. It’s a question of will, which often translates to a question of money.”
In a recent review of exoplanet research published in the Proceedings of the National Academy of Sciences, astronomer Adam Burrows of Princeton University cautioned against making too much of our current knowledge about them.
“There’s a lot of hype in this subject, a lot of irrational exuberance,” he said. “Popular media have characterized our understanding as better than it actually is.”
Traub, of the NASA Exoplanet Exploration Program, does not agree with that sentiment, though he admits to feeling that way, especially when it comes to the impending WFIRST-AFTA mission to directly image exoplanets.
“I’m exuberant that they’re letting us do this,” he said. “It responds to a public interest.
“Normal people are interested in exoplanets.”