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Marble in the Sky: the Hunt for Another Earth

Artist's concept of two suns in the pink sky of an exoplanet.
In this artist's concept, a distant, life-bearing world orbits two stars instead of one, seen from the surface in a pink sky – yet in some ways still might be considered similar to Earth. Illustration: NASA/JPL-Caltech/Lizbeth B. De La Torre

Capturing even a faint, blurry image of a distant world that looks something like our own would mark a profound shift in history ­– and our place in the universe.

The hunt for such a world has been described as a search for “Earth 2.0,” an “Earth-like” planet, even an “Earth twin.” But each of these terms implies its own raft of assumptions. We might, or might not, catch a glimpse of an exoplanet – a planet orbiting another star – that looks like present-day Earth. A blue, water-covered world marbled with clouds of white.

Yet even our own planet probably looked very different in the deep past.

The term “Earth-like” also carries some burdensome baggage. That begins with how we define it. Which characteristics, exactly, make Earth Earth-like? How would we recognize these qualities on a planet hundreds or thousands of light-years away?

“The kinds of planets that could be [considered] Earth-like may be very different from modern Earth,” said Giada Arney, an astronomer and astrobiologist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

And suppose life elsewhere is not “life as we know it.” Would we recognize life as we don’t know it?

This all comes down to the one big question we’ve been asking ourselves through the ages: Are we alone?

We’ve taken a journey through NASA’s efforts to answer this question:

As we search the heavens for a possible twin, we’ll have to take into account the stages of growth – infancy to maturity. A rocky planet with the beginnings of life could be shrouded in an orange haze, as Earth once might have been.

Timing is everything. Our own Earth was uninhabitable for millions of years, and we might find a planet at a similar stage – perhaps an unimaginably hot or cold surface that had a liquid water ocean in the past, or might develop one in the future.

“We tend to talk about Earth-like planets as planets like ours is today,” said Douglas Hudgins, program scientist for NASA’s Exoplanet Exploration Program at NASA Headquarters in Washington. “But our planet has been radically different throughout its history, while still supporting an abundance of life.”

Mars, once warm and wet and now cold, dry and forbidding, might reveal evidence of past life. Even present life can’t be ruled out.

A life-bearing world also might be covered in a crust of ice. Jupiter’s moon, Europa, and Saturn’s Enceladus, both hide subsurface oceans sealed inside icy shells.

Titan, another moon of Saturn, also is the only other solar system body with rain, rivers and lakes. The liquid in this case, however, is composed of methane and ethane instead of water, but could be home to “weird life.” Titan also might have a subterranean ocean. If other planets and moons in our solar system harbor life forms, it isn’t obvious – but the possibility is being actively investigated by NASA and other space agencies.

“The more we study our own cosmic backyard, the more surprises we find,” said Morgan Cable, a researcher with the Astrobiology and Ocean Worlds Group at NASA’s Jet Propulsion Laboratory in Southern California. “And I’m excited. We’ll be surprised more and more as we continue to extend our senses to the outer solar system and beyond.”

Scientists have confirmed more than 4,000 exoplanets in our galaxy, many of them likely rocky worlds in Earth’s size-range. Thousands more are expected to be confirmed in the years ahead. But one very special type of planet still eludes us: a world in Earth’s size range orbiting a Sun-like star, at a distance that would give it a year comparable to our own.

While it’s possible that such planets will turn out to be rare, another factor better explains the mystery. Present-day technology used in the hunt for exoplanets, primarily space telescopes and the instruments they carry, has a tough time resolving such systems.

Future, more powerful space telescopes could help bridge this gap, or peer into the atmospheres of Earth-sized worlds that have been found in abundance – those in close orbits around red-dwarf stars – to look for signs of habitability.

“The day we detect life on an exoplanet will be nothing short of a Copernican revolution,” Hudgins said. “It will changes the way human beings view our place in the universe forever.”

We could say that the chances of finding life somewhere else in the galaxy are improving. While scientists have confirmed thousands of exoplanets so far, the Milky Way likely holds trillions. A good percentage of these exoplanets are in Earth’s size range, and believed to be of similar composition.

Yet the cosmos is stubbornly silent on the question. Exoplanet-hunting technology, though developing rapidly, probably is not yet sufficient to detect signs of possible life in exoplanet atmospheres.

We’ve seen or heard no credible indications of a technological species among the sea of stars; a question more than half a century old, “Where is everybody?” still has no answer.

Concepts like the “habitable zone” – the orbital distance from a star allowing a planet, with a suitable atmosphere, to retain liquid water on its surface – are helping astronomers sort through the many possibilities to find likelier candidates for life-bearing worlds. Still, planets with life could be far outside this zone if there were, for example, an ice-covered planet with a deep ocean supporting aquatic organisms.

There's a helpful concept we use to help understand what distance from a given star you might expect to find planets with liquid water on their surface – liquid water being essential for life as we know it. It's called the habitable zone. Every star has a habitable zone, but where that zone lies is different for stars of different sizes and brightness. Credit NASA/JPL-Caltech

“To search for life anywhere, we have this ‘follow the water’ approach,” said Shawn Domagal-Goldman, a research astronomer at Goddard. “Anywhere you find water on Earth, you find life. Whether it’s life on Mars, ocean worlds, or exoplanets, water is the first signpost we’re looking for.”

We’re on the brink of a new era in exoplanet science: sifting through the atmospheres of distant worlds to look for combinations of gases that could reveal a living planet.

First in line is the James Webb Space Telescope, targeted for launch in October 2021. The Webb telescope will be a cosmic multi-tasker, looking deep into the universe – and deep into its past – to discover clues to its origin and early formation.

The Webb telescope also will capture starlight shining through the atmospheres of exoplanets, which provides a kind of profile of the gases present. That will pave the way for future, more powerful space telescopes to look in on small, rocky planets perhaps resembling our own.

Infographic depicting biosignature detection
Technology now under development could reveal signs of possible life – biosignatures – in the atmospheres of exoplanets.
NASA/JPL-Caltech/Lizbeth B. De La Torre

With more advanced technology, astronomers could detect atmospheric chemicals that are considered “biosignatures,” potentially indicating the influence of lifeforms.

This new era of characterizing exoplanets will continue with the launch of the Nancy Grace Roman telescope in the mid-2020s. An intricate instrument onboard called a coronagraph will help blot out the glare of parent stars to reveal orbiting planets.

That will mean direct images of large, gaseous planets. Those targets are unlikely to be habitable, but demonstrating this technology will open the door to future such instruments with greater resolving power. A future telescope might even find a small, rocky world with an atmosphere of oxygen, methane, and carbon dioxide – in other words, an atmosphere that reminds us of home.

“We could actually find out: Is Earth common or rare? Is life common or rare?” said Aki Roberge, an astronomer at Goddard. “We honestly have zero clue what habitable or inhabited planets look like in general. We really need to just take a look.”