Will We Know Life When We See It? NASA-led Group Takes Stock of the Science
In the last decade we have discovered thousands of planets outside our solar system and have learned that rocky, temperate worlds are numerous in our galaxy. The next step will involve asking even bigger questions. Could some of these planets host life? And if so, will we be able to recognize life elsewhere if we see it?
A group of leading researchers in astronomy, biology and geology have come together under NASA’s Nexus for Exoplanet System Science, or NExSS, to take stock of our knowledge in the search for life on distant planets and to lay the groundwork for moving the related sciences forward.
“We’re moving from theorizing about life elsewhere in our galaxy to a robust science that will eventually give us the answer we seek to that profound question: Are we alone?” said Martin Still, NASA exoplanet scientist at Headquarters, Washington.
In a set of five review papers published last week in the scientific journal Astrobiology, NExSS scientists took an inventory of the most promising signs of life, called biosignatures. They considered how to interpret the presence of biosignatures, should we detect them on distant worlds. A primary concern is ensuring the science is strong enough to distinguish a living world from a barren planet masquerading as one.
The assessment comes as a new generation of space and ground-based telescopes are in development. NASA’s James Webb Space Telescope will characterize the atmospheres of some of the first small, rocky planets. Other observatories— such as the Giant Magellan Telescope and the Extremely Large Telescope, both in Chile— are planning to carry sophisticated instruments capable of detecting the first biosignatures on faraway worlds.
Through their work with NExSS, scientists aim to identify the instruments needed to detect potential life for future NASA flagship missions. The detection of atmospheric signatures of a few potentially habitable planets may possibly come before 2030, although whether the planets are truly habitable or have life will require more in-depth study.
Since we won’t be able to visit distant planets and collect samples anytime soon, the light that a telescope observes will be all we have in the search for life outside our solar system. Telescopes can examine the light reflecting off a distant world to show us the kinds of gases in the atmosphere and their “seasonal” variations, as well as colors like green that could indicate life.
These kinds of biosignatures can all be seen on our fertile Earth from space, but the new worlds we examine will differ significantly. For example, many of the promising planets we have found are around cooler stars, which emit light in the infrared spectrum, rather than our sun’s high emissions of visible-light.
“What does a living planet look like?” said Mary Parenteau, an astrobiologist and microbiologist at NASA’s Ames Research Center in Silicon Valley and a co-author. “We have to be open to the possibility that life may arise in many contexts in a galaxy with so many diverse worlds — perhaps with purple-colored life instead of the familiar green-dominated life forms on Earth, for example. That’s why we are considering a broad range of biosignatures.”
The scientists assert that oxygen — the gas produced by photosynthetic organisms on Earth — remains the most promising biosignature of life elsewhere, but it is not foolproof. Abiotic processes on a planet could also generate oxygen. Conversely, a planet lacking detectable levels of oxygen could still be alive — which was exactly the case of Earth before the global accumulation of oxygen in the atmosphere.
“On early Earth, we wouldn’t be able to see oxygen, despite abundant life,” said Victoria Meadows, an astronomer at the University of Washington in Seattle and lead author of one of the papers. “Oxygen teaches us that seeing, or not seeing, a single biosignature is insufficient evidence for or against life — overall context matters.”
Rather than measuring a single characteristic, the NExSS scientists argue that we should be looking at a suite of traits. A planet must show itself capable of supporting life through its features, and those of its parent star.
The NExSS scientists will create a framework that can quantify how likely it is that a planet has life, based on all the available evidence. With the observation of many planets, scientists may begin to more broadly classify the “living worlds” that show common characteristics of life, versus the “non-living worlds.”
“We won’t have a ‘yes’ or ‘no’ answer to finding life elsewhere,” said Shawn Domagal-Goldman, an astrobiologist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland and a co-author. “What we will have is a high level of confidence that a planet appears alive for reasons that can only be explained by the presence of life.”
Chandra Scouts Nearest Star System for Possible Hazards
A new study that has involved monitoring of Alpha Centauri for more than a decade by NASA’s Chandra X-ray Observatory provides encouraging news about one key aspect of planetary habitability. It indicates that any planets orbiting the two brightest stars in the Alpha Cen system are likely not being pummeled by large amounts of X-ray radiation from their host stars. X-rays and related Space Weather effects are bad for unprotected life, directly through high radiation doses and indirectly through stripping away planetary atmospheres (a fate thought to have been suffered by Mars in our own Solar System).
Alpha Centauri is a triple star system located just over four light years, or about 25 trillion miles, from Earth. While this is a large distance in terrestrial terms, it is much closer than the next nearest Sun-like stars.
“Because it is relatively close, the Alpha Centauri system is seen by many as the best candidate to explore for signs of life,” said Tom Ayres of the University of Colorado Boulder. “The question is, will we find planets in an environment conducive to life as we know it?”
The stars in the Alpha Centauri system include a pair called “A” and “B,” (AB for short) which orbit relatively close to each other. Alpha Cen A is a near twin of our Sun in almost every way, including age, while Alpha Cen B is somewhat smaller and dimmer but still quite similar to the Sun. The third member, Alpha Cen C (also known as Proxima), is a much smaller red dwarf star that travels around the AB pair in a much larger orbit that takes it more than 10 thousand times farther from the AB pair than the Earth-Sun distance. Proxima currently holds the title of the nearest star to Earth, although AB is a very close second.
The Chandra data reveal that the prospects for life in terms of current X-ray bombardment are actually better around Alpha Cen A than for the Sun, and Alpha Cen B fares only slightly worse. Proxima, on the other hand, is a type of active red dwarf star known to frequently send out dangerous flares of X-ray radiation, and is likely hostile to life.
“This is very good news for Alpha Cen AB in terms of the ability of possible life on any of their planets to survive radiation bouts from the stars,” said Ayres. “Chandra shows us that life should have a fighting chance on planets around either of these stars.”
While one remarkable Earth-size planet has been discovered around Proxima, astronomers continue to search, without success, for exoplanets around Alpha Cen A and B. Planet-hunting around these stars has proved more difficult recently due to the orbit of the pair, which has drawn the two bright stars close together on the sky over the past decade.
To help determine whether Alpha Cen’s stars are hospitable to life, astronomers have run a long-term campaign in which Chandra has observed the system’s two main stars about every six months since 2005. Chandra is the only X-ray observatory capable of resolving AB during its current close orbital approach, to determine which star is doing what.
These long-term measurements have captured the complete ups and downs of the X-ray activity of AB, analogous to the Sun’s 11-year sunspot cycle. They show that any planets in the habitable zone for A would receive a lower dose of X-rays, on average, than similar planets around the Sun. For companion B the X-ray dose for habitable zone planets is higher than for the Sun, but only by a factor of about five.
In comparison planets in the habitable zone around Proxima receive an average dose of X-rays about 500 times larger than the Earth, and 50,000 times larger during a big flare.
Besides illuminating the possible habitability of Alpha Cen’s planets, Chandra’s X-ray history of AB plays into theoretical explorations of our own Sun’s cyclical X-ray activity. Understanding this is a key to cosmic hazards such as Space Weather, which can impact the technology-laden civilization right here on our home world.
Tom Ayres presented these results at the 232rd meeting of the American Astronomical Society meeting in Denver, Colorado, and some of these results were published in January 2018 in the Research Notes of the American Astronomical Society. NASA’s Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA’s Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra’s science and flight operations.
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