Planet discoveries boost LifeSearch 2.0

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Planet discoveries boost LifeSearch 2.0
Illustration of supersized Earth with clouds and terrestrial features. Another planetary body or moon is shown transiting a star in the background.
A newly discovered planet dubbed “mega-Earth” is shown in an artist’s conception. Kepler-10c orbits a sun-like star, has a diameter of about 29,000 kilometers (2.3 times larger than Earth), and weighs 17 times more. The solid planet may have a thin atmosphere that is unlikely to support life. 10c is located about 560 light years away and orbits its star every 45 days.
Photo: David A. Aguilar (Center for Astrophysics)

The search for life in space — boosted half a century ago by a series of grade “C” sci-fi horror films — has been perking again lately, largely due to a gusher of newly found planets.

“I think the field is very upbeat,” says David Black, president of the SETI Institute, a pioneering group devoted to finding intelligent life in space. “The detection of so many planets has buoyed enthusiasm, and more specifically, the detection of objects that are in some sense Earthlike, in the ‘habitable zone.'”

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That zone is defined as the range where liquid water — considered essential to all known life — can exist. In most uses, “habitable planet” implies a rocky one, like Earth or Mars, rather than a gas giant like Saturn or Jupiter.”

Some hype may have entered the picture, Black concedes. “If you are really careful, what we are finding is many Venus-like analogs,” meaning rocky planets in hot orbits. “But the fact that we are finding objects that could potentially lead us to find life is exciting.”

The first exoplanet — planet beyond the solar system — was found in 1995 by two Swiss astronomers. That made headlines. These days, exoplanets barely make news. That changed on April 14, 2014, when scientists announced the discovery of the first earth-size planet in the habitable zone.

Since January, roughly 950 planets have been found, largely by the Kepler Space Telescope, which was launched in 2009 to detect the slight dimming that occurs when a planet “transits” across the face of its star.

These discoveries are helping fill in blanks in the Drake equation, an early effort to predict how many planets had invented powerful radios.

A 2013 study, based on Kepler’s unwavering stare at 42,000 stars, found that 22 percent of sun-like stars in the Milky Way harbor Earth-like planets in the habitable zone.

The habitable zone (green), where liquid water can exist, is farther from hot stars than cooler ones. Rollover image to see an artist’s view of star Kepler-186, about 500 light-years from Earth in the constellation Cygnus. Kepler-186 is an M dwarf star with half the mass of the sun. Kepler-186 has five known planets, including Kepler-186f, which orbits in the habitable zone.

A 2014 study concluded that 17 billion earth-like planets occur in the Milky Way alone — and our galaxy is one of roughly 100 billion others.

Even the numero-numbskulls at The Why Files can calculate that: more than 100 billion billion earth-like planets could exist in the universe.

Planets: Common yes, but alive?

Granted, none of these discoveries prove that life exists beyond Earth, but they sure do jack up the odds.

If planets are indeed so common, scientists are edging toward the conclusion that life is practically inevitable somewhere or other, especially given discoveries of life on Earth above the boiling point, in the sun-less, superheated deep sea, and in rocks buried a kilometer underground. “The likelihood that life exists out there, in my view, is almost certain,” Black says.

Which is not to say that this life can be detected by today’s technology — or even tomorrow’s. Nor is it to say that these lost ecosystems are smarter than a bacteria-infested mudpot on the side of a sleeping volcano.

Photo of engineers in bunny suits examine Kepler, antenna, thrusters and solar cells visible.
Kepler space telescope nears completion in a cleanroom at the Jet Propulsion Laboratory. Although Kepler is now hobbled by a severe guidance glitch, managers have figured out a two-year work-around to take advantage of its remaining capabilities.

We asked Black why the quest for life has fascinated so many smart people. “At the root, we are curious about our origins. You can see this in people who worry about genealogy. This is cosmic genealogy. We are trying find out, not only our roots, but are there others with those roots too.”

Finding life, Black adds, could “affect us in ways I don’t dare speculate. If you go back to the time when everybody thought Earth was the center of the universe, and poor Galileo looked through a telescope and found four moons going around a planet [Jupiter]. We found that we were going around the sun, and that removed us from the center of the universe. That had profound effects, it rippled through religion, and I think it speaks to how we as a species see ourselves in relation to the cosmos. Ultimately, this has to be viewed as one of the most profound questions: Are we alone?”

So how do we search?

The many planets found by the Kepler spacecraft — suitably named for Johannes Kepler, who figured out the laws governing planetary orbits — could aid the design of the terrestrial planet finder, a plan for space telescope that could find and study Earth-like planets orbiting in the habitable zone.

Illustration of telescope with much larger attached starshade with gilded reflectors.
A sunflower-shaped “starshade” placed in front of a telescope could help a planet-finding telescope hunt for another Earth by blocking a blazing-bright star next to a faint planet.
Illustration: NASA/JPL/Caltech

Estimates for various types of stars say they average between 0.1 to 0.5 planets apiece.

If planets are rare, “a greater number of stars would need examination, and they are going to be further and dimmer,” says James Kasting, professor of geoscience at Penn State. That would force an already-expensive telescope to be even bigger.

Kasting says the planet finder’s aim is to find one habitable-zone Earth analog with 95 percent probability. The spacecraft would also use spectroscopic analysis to identify which elements and compounds are present.

Planet Hunting 101

The list of exoplanets is inflating like a Red Giant, to almost 1,800. In February, 2014, NASA announced 715 new exoplanets discovered by Kepler. What techniques are used to find planets beyond our solar system?

ENLARGE
Bar graph of planets found for different detection methods from 1988-2014. Confirmed planets in 2014 dwarf previous years.
Planets can be found by different methods; Kepler space telescope has used the “transit” technique to produce a gusher of discoveries.
Graph last updated Feb. 26, 2014, Aldaron

bullet_earth Radial velocity Gravitational forces between a star and a planet affect the star’s position and alter its velocity relative to Earth. Earthbound telescopes can then detect changes in starlight due to the Doppler effect—the same phenomenon used by radar guns to nail scofflaw drivers. Radial velocity was the prevailing method for finding exoplanets until 2014.

bullet_earth Transit A planet that “transits” (crosses in front of its star) dims the star. Transit only works if the star, planet and viewer are in line.

bullet_earth Direct detection Because the visible light reflected from a planet is extremely faint next to the star’s light, astronomers use infrared imaging to detect a planet’s heat. This method is best for nearby planets that are distant from their star, bigger than Jupiter and very hot.

ENLARGE
Photo of three domed observatories above the cloud layer at dusk.
Keck Observatory studies stars and planets beyond our solar system. From a volcanic peak on Hawaii, Keck benefits from some of Earth’s darkest skies.
Kepler: NASA/JPL

The three designs under study must all cope with one blinding problem: seeing a planet right next door to a star that is vastly brighter (the sun, for example, is 10 billion times brighter than Earth).

One of the two visible-light planet-finder designs would use a 50-meter star shade, flying roughly 50,000 kilometers in front of the telescope. This would entail, Kasting says with a bit of understatement, “a challenge of flying in formation.”

NASA, short of money, cancelled planet finder in 2011, but some astronomers hope it could be resuscitated at a planning meeting in 2020. Don’t expect a launch before 2030, Kasting warns.

The signature of life

Line of golden cows with full udders stand in clean straw
Livestock, including these pampered cows at the 2014 World Dairy Expo, produce about 19 percent of the annual production of methane, a potent greenhouse gas but also a telltale for a living planet.
Courtesy S.V. Medaris

After a rocky planet in the habitable zone is discovered, what then? The closest stars are light-years away. Paying a visit would take decades, or more likely centuries. More feasible — though still difficult — is to search for evidence of life; a “biosignature,” in astrobio lingo. “When you look at Earth’s atmosphere, you see methane and oxygen that are totally out of chemical equilibrium,” says Black. Methane — the major compound in natural gas — is easily oxidized by oxygen atoms and radicals.

The only reason our atmosphere contains 1.8 parts per million of methane is because “something is driving it, and that something is life,” Black says. Although some methane comes from geologic sources, more comes from organic sources, mainly bacteria in cows, rice paddies and other living locations.

A spectroscope mounted on a telescope could, from a suitable distance, detect methane in a planet’s atmosphere. (Spectroscopes analyze the wavelength of light to reveal which elements created that light. They are often used to register elemental composition in space.)

Photo of 2 geologists examining sedimentary rock in a scrubby, rocky landscape in Australia. Rock is banded ochre, gray and white.
While studying iron and uranium isotopes in this jasper (a mixture of iron oxide and chert), Johnson’s team found that the iron had oxidized 3.4 billion years ago, in an ocean with little oxygen. Because mineralized remains of photosynthetic bacteria called stromatolites date to the same era, the team concluded that what the stromatolites were using “is more deeply rooted” type of photosynthesis that made no oxygen (modern photosynthesis, which does make oxygen). Therefore, even though the jasper is not made of organic carbon, its isotopes revealed an ocean with little oxygen and that, combined with the organic origin of stromatolites deposited at the time, reveals the existence of a different type of photosynthesis 3.4 billion years ago. So the jasper carries a biosignature for non-oxygen-producing photosynthetic bacteria on Earth at that early date.
Courtesy Clark Johnson (Marble Bar Chert, Pilbara region, Australia)

Other biosignatures are more subtle and may be impossible to analyze without feeding a sample into a heavy instrument, which entails the expensive and risky return of samples to Earth.

Clark Johnson, professor of geoscience at University of Wisconsin-Madison and head of the Wisconsin Astrobiology Research Consortium, says the search for life on Mars and the moons of Jupiter and Saturn must begin on Earth, which “still remains the only place we can prove that life is.”

The biosignature of a roadkill raccoon or even a million-year-old hominid femur is easy enough to identify. But on Mars, the supposed organic remains would have been bathed by solar radiation for three billion years, so “the likelihood of organic molecules remaining on the surface is very small,” Johnson says.

Ozone in our atmosphere blocks a comparable UV assault, but there are other problems, he adds. “Ancient rocks on Earth have been subject to alteration through metamorphism [long-term heating, pressure and deformation], which tends to destroy organics.”

Photo of Curiosity on barren Mars landscape with dusty atmosphere. Wide wheels and a forest of instruments are seen.
As the Mars rover Curiosity drilled into sandstone, its arm-mounted camera took 55 images that were fused into this self-portrait. Due to clever manipulations of the arm and the camera turret, the robotic arm is invisible in this mosaic.
April and May 2014, NASA/JPL-Caltech/MSSS

Organic material can, however, solidify into stone, and so Johnson’s research group studies 3.4 billion-year-old rocks in Australia and South Africa that have not been buried by tectonic movement or altered beyond recognition by metamorphism. Specifically, they look at isotopes in rocks that may have been formed from legions of microbes. Isotopes are atoms of a particular element with different masses that can be separated in a mass spectrometer. Isotopes allow one to recognize minerals with organic origin even after intense deformation.

Johnson is especially interested in iron, which, scientists have recently deduced, played a major role in microbial metabolism more than three billion years ago.

Microfossils — remains of microbes from the ancient rock record that might “hit you in the face with picture of an ancient microbe” — are extremely rare and highly contentious, Johnson says. By studying iron formations, he says, “We can show multiple locations around the world that were processed by microbes, and this gives a broader feel for the ecosystem.”

The technique, he says, does not just show the existence of life, “but it shows what the life was doing, which sets it in an ecological context.” On Mars, Johnson says, “the earliest microbes could have been iron based, long before oxygenic photosynthesis.”

The biosignature strategy, he says, is this: “Study Earth — it’s the only example of life. What did ancient, primitive life do to change Earth? How do we recognize that, and how would we recognize that when a sample is returned from Mars?”

Mad about Mars!

Mars — Earth’s twin — has long excited and frustrated life-seekers. More than a century ago, astronomer Percival Lowell thought the “canals” on its surface were the work of long-gone Venetian gondoliers.

Since then, NASA and others have mounted an intensive search for life on Mars. Although the planet is dry and cold now, surface features indicate the presence of liquid water a couple of billion years ago. At that time, therefore, Mars was a rocky planet in the habitable zone.

Theoretically, life could persist deep underground — or some fossilized remains may still remain on the surface. But so far, it’s no dice. Despite some early enthusiasm, Mars seems lifeless, dead.

Did you lose their voice-mail? : )

Decades ago, before other telescopes grew acute enough to detect planets, radio telescopes that detect faint signals across the galaxy were the main hope for detecting intelligent life. Although the decades-long search for radio signals from space has had promising moments, confirmed discoveries of extra-terrestrial intelligence are easy to count: zero.

The search for extra-terrestrial intelligence is embodied in the name of the SETI Institute, and “The radio search is ongoing,” says Black, its leader. “We have the Allen array of telescopes at Hat Creek, California, with 42 dishes used for radio signals, and we are upgrading the feeds to make the system more sensitive and get a wider wavelength coverage.”

What does Black make of the empty result? “The most disappointing message is that there is not anybody out there.

Or, they are out there, but everybody is listening and nobody is sending; it’s like a party of strangers, and they sit around the room, and nobody wants say anything. Or they are sending signals, and we have not been able to figure out what they are. We … have an idea what the signal might look like, but maybe we are being bathed in these signals but are not smart enough to notice.”

We figure it’s inevitable, given an uncountable number of planets out there, that life — smart or dumb — must exist beyond Earth, and we asked Kasting if he agreed. “I would rephrase the question,” he responded. “Maybe if you count all the stars, galaxies in the universe, then life is inevitable somewhere, but that’s not testable. What is testable is, ‘Is there life in the region of the universe that we can search telescopically?'”

“I’m not impressed by all these numbers,” Kasting continued. “If we want to be able to search for life in our lifetimes,” we need to know the expected number of planets per star “and look at 30 to 60 nearby stars, and also survey the planets in our own solar system.”

– David J. Tenenbaum

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Kevin Barrett, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David J. Tenenbaum, feature writer

Bibliography

  1. Distant oasis: Could Europa’s hidden oceans hold ingredients for life?
  2. Graphic of confirmed planets that appear to be “just right” for life.
  3. European Space Agency set to build “extreme new telescope” to search for distant life.
  4. Simple and engaging animation explaining why we are searching for planets with infrared.