It was a good week for astrobiology. Within days of NASA's announcement that the necessary ingredients for life likely exist in the Saturn moon Enceladus' subsurface ocean, scientists gathered at Stanford University to discuss discovering life outside the solar system.
Noting how "the search for life in the universe has been transformed from speculation to a data-driven science," speakers like Stanford physicist Peter Michelson offered detailed plans for finding life on exoplanets.
Over the course of two days on April 20-21, dozens of scientists attending the Breakthrough Discuss conference contemplated options for exploring planets in other star systems. These options include using a new generation of powerful telescopes for long-distance observations, as well as advancing a first-of-its-kind technology to visit other star systems — all within the next generation.
What these strategies have in common is a focus on observing habitable-zone planets in our local stellar neighborhood. In this neighborhood alone, within 30 light-years or so of our solar system, astrobiologists have already identified several potentially Earth-like exoplanets and dozens of systems that may harbor Earth-like worlds.
These exoplanets, identified by the effects they have on their parent star, are rocky and roughly the same size and density as Earth. They orbit their stars at a distance that would allow liquid water to exist on the surface, given the right atmospheric conditions. There is, however, at least one major difference between our planet and these potentially-habitable exoplanets: They aren't circling stars like our sun.
On the spectrum of stars, our sun is what's known as a yellow dwarf. It's bright, and not terribly large compared to the largest stars in our galaxy. Yet, even middling stars like our sun aren't all that common. Our local stellar neighborhood — and probably the universe as a whole — is filled with many more low-mass stars. There are 20 yellow dwarf stars like our sun nearby and 250 M-dwarfs, a variety of star so small and dim that, despite their abundance, they can't be seen with the naked eye. Over the last three to four years, every single low-mass star we've studied appears to have at least one planet. Usually, they have more than one.
"How common are planets orbiting low-mass stars? Very common indeed," Courtney Dressing, an astronomer at the University of California, Berkeley, said during a presentation at the conference. "For a typical M-dwarf, there tends to be 2.5 planets. One in four of the stars has a planet the same size and temperature as Earth in the habitable zone."
Dressing's point was that, given the number of M-dwarfs in the local region, there should be at least 60 potentially Earth-like planets in habitable zones within 32 or so light-years from here, and perhaps many more. To date, most of our exoplanet data comes from NASA's Kepler spacecraft, which has focused its search for planets on large M-dwarf stars. In the near future, when the small and medium-sized M-dwarfs are studied, we may discover that closer to one in three stars have an Earth-like planet in the habitable zone.
Apart from their abundance, these low-mass stars offer other advantages to researchers who study potentially habitable exoplanets. M-dwarf planets have tight orbits around their stars because the habitable zones are close in, giving scientists opportunities to view their transits every few weeks. It is during these transits, when the exoplanets pass in front of their stars, that we have the best opportunity to study their atmospheres for signs of life.
Many conference attendees, including Mercedes López-Morales from the Harvard-Smithsonian Center for Astrophysics, explained how we will be surveying the atmospheres of the closest habitable-zone planets for signs of life dwelling on the surface or in an ocean. "We're going to look for oxygen," she said.
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Because the rise of oxygen in Earth's atmosphere corresponded with the appearance of life, we frequently use that particular molecule as a marker for the presence of life elsewhere. Also, oxygen likes to interact with other chemicals. If we discover a planet where oxygen is still hanging around in the atmosphere, something, possibly life, is actively making it. So, the search for life will focus on elements and molecules like hydrogen, oxygen, and methane. However, as López-Morales explained, there is a downside to this approach.
"A planet's atmosphere is only 1 percent the size of the planet," she said. "The size of the signal is tiny. You need to collect at least one trillion photons to be very certain that you are truly looking at oxygen."
The good news is that a new generation of telescopes designed for planetary exploration and astrobiology will soon be coming online to help us gather those photons. Around this time next year, NASA's Transiting Exoplanet Survey Satellite (TESS) will be getting ready for launch. During its two-year mission, TESS will survey 200,000 stars, including the brighter ones in our local systems.
The Giant Magellan Telescope (GMT) in Chile, slated to be operational by 2022, will have a resolving power 10 times greater than NASA's famous Hubble Space Telescope. The GMT will feature a device called the G-CLEF spectrograph, which will be able to see molecules like oxygen in far-off planetary atmospheres. Finally, when the ground-based European Extremely Large Telescope (E-ELT) opens in 2024, it will have more light-gathering-power than all of Earth's current 8- to 10-meter telescopes combined. Astrobiologists are counting on these large telescopes coming online between now and 2024 to identify the prime candidates to look for oxygen and life in our stellar neighborhood.
Even as we anticipate a treasure trove of atmospheric data from these missions, scientists are discovering species that live quite happily without oxygen, light, and other features that we used to believe were required for life. These discoveries highlight how atmospheric biosignatures like oxygen are an imperfect, if tantalizing, way to look for life from afar. The question then becomes: Could there be there another way to look for extraterrestrial life beyond studying exoplanet atmospheres?
Ideally, to definitively identify life on other worlds, we would visit nearby planets like Proxima b, only 4 light-years away, either in person or with a spacecraft. This is the goal of the Breakthrough Starshot initiative. Announced a little more than a year ago, Starshot's goal, according to its founder, is to "literally reach the stars in our lifetimes." The plan to accomplish this feat involves launching a fleet of very small spacecraft. Starshot will then accelerate these craft to about 20 percent the speed of light using powerful ground-based lasers. This strategy should allow scientists and engineers to slash the time, cost, and weight required to gain an up-close look at planets around other stars, project representatives have said.
"The goal is to fly a probe very close to a planet and figure out if it has life," said theorist Avi Loeb, chair of Harvard University's astronomy department. "What is the color of the planet? Is it green? Does it have vegetation? Is it blue, are there oceans? Or is it desert-like?"
At the conference, NASA engineer Ruslan Belikov premiered simulations of what an exoplanet might look like from Starshot's point of view. Even if the craft were moving at 90 percent the speed of light, the onboard cameras should still be able to pick up signs of large oceans, clouds and land masses that an exoplanet might have, Belikov said.
The hope is that, someday, by combining laser acceleration of these very small craft with cameras and other sensors, we might finally be able to take a firsthand look at habitable-zone planets circling nearby stars, and, in so doing, perhaps definitively find life elsewhere in the universe. Combining data from our new generation of very large telescopes with atmospheric observations of nearby exoplanets around M-dwarfs may help us choose the best targets for small Starshot craft to fly by.
"We are going to be the generation that is remembered for finding exoplanets. That's a fact," López-Morales said. "Are we going to be also the generation that will be remembered as the first ones who found life on those planets?"
That, indeed, would be the breakthrough of a lifetime.
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