How Astronomers Will Find Earth 2.0

By Sarah Scoles | July 16, 2015 4:34 pm

earth sun

While self-aware humans have long wondered whether Earth is the only place like itself, we — and our technology — are finally advanced enough to answer that question. And with that power, astronomy’s quest du jour is to find habitable (and potentially inhabited) Earth-esque planets.

To discover biology from afar, scientists peer into planets’ atmospheres in search of evidence that something on their surfaces breathes and metabolizes. But planets are small (cosmically speaking) and far away, and their stars outshine them. Because of that latter problem, astrobiologists currently favor focusing on worlds orbiting small, dim red dwarf stars. Their meager light still nearly blinds us to their planets’ atmospheres, but visibility is better than it would be near a star like the sun.

But it’s not just the star that matters – it’s the other planets too. Astronomers have generally been looking for solar systems like ours, the only inhabited one we know of. That is to say, tidy solar systems where the planets have regular orbits in a flat disk.

Where to Look

So a logical first step in finding another Earth is to find a red dwarf with an orderly planetary system. According to research by Sarah Ballard of the University of Washington in Seattle and John Johnson of Harvard University, red dwarfs have basically two planet-forming options (which Ballard calls the “choose your own adventure” method of planet formation).

They can do a neat job, with their multiple planets’ orbits (seven, on average) lined up like a bulls-eye. Forty-seven percent end up like this. Telescopes like Kepler can see each of these pass in front of, or transit, its star.

Or they can be antisocial and messy, with just a solo planet or multiple planets in scattershot orbits. Fifty-three percent go this way. In this case, because a planet is alone or multiple planets orbit at a bunch of different angles, only one planet passes in front of the star from our telescope’s perspective.

If we’re looking for habitable worlds, we should look first at the bulls-eye kind. But, short of actually training a telescope on them for months or years, how can we tell which those will be?

Artist's concept of a young, red dwarf star surrounded by three planets. Image Credit: NASA/JPL-Caltech

Artist’s concept of a young, red dwarf star surrounded by three planets. Image Credit: NASA/JPL-Caltech

Searching Smarter, Not Harder

If astronomers can figure out what conditions lead to organized solar systems, they can then point their telescopes preferentially at dwarfs they know will have clean neighborhoods. Ballard and Johnson have a few ideas:

  1. In systems with lots of heavy elements (iron and carbon, for instance), multiple giant planets — like Jupiter — form. These huge balls of gas push and pull on each other, potentially producing scattered orbits. But systems with more light elements (hydrogen, helium) make fewer heavyweight planets, leading to stable, peaceful systems: neighborhoods that look like ours.
  2. When two stars orbit each other, their similar tug of war could also lead to scattered orbits. So, better to focus on single-star systems.
  3. Billions-of-years-long processes could gradually grind planets out of alignment. So a younger star might be more promising than an older one. But that doesn’t really leave enough time for life to arise and evolve before disaster crushes it.
  4. When a planet around a dwarf star has an “eccentric” orbit — one that’s more stretched than circular — gravitational effects can sterilize its surface. So even if a planet seems to be in a habitable zone, it probably isn’t a nice place to live. But solar systems with more planets, like ours, tend to line up and produce nice circular orbits and nice places to live.

One Caveat

That’s a helpful list, though it comes with a big caveat: our own Earthly perspective.

Imagine a system in which a few planets live in lined-up orbits close to each other and to their star — but one or two far-out planets zing at weird angles (like Pluto, which is not a planet, but you know). If the neat inner planets transit, the outer ones will be out of alignment, and we would never know they were there. But if we can see an outer planet transit, chances are the inner ones don’t line up for us. 
So we could getting two views of the same kind of system, and incorrectly believe we see two different varieties.

Still, assuming the two varieties do indeed exist, a solar system’s ultimate habitability seems set from the star’s very beginning. You could also call it the “you can’t fight fate; your life is predetermined” theory of red-dwarf planet formation.

The expected transit signature of a multiple planet system. Credit: NASA

The expected transit signature of a multiple planet system. Credit: NASA

Signs of Life

After astronomers ID some red dwarf systems that seem promising, finding out if they do indeed have life is no easy matter. Astrobiologists seek out tiny signals called “biosignatures,” which are gases in planets’ atmospheres that could give away the presence of microbes, plants, animals, or whatever other branch of life the universe cooked up. Biosignatures are like footprints – when you see a footprint in the mud, you know someone walked there, even if you never see them.

Gases such as oxygen, ozone, methane, ammonia, and nitrous oxide – and other gases that seem to have an outsized presence – could be those footprints. But non-biological processes, including chemical and geological ones, can also make those gases. Astronomer Sara Seager said in a 2014 report that we’ll have to go the statistical route to account for that confusion. “Although we may not be able to point to a planet with certainty and say, ‘That planet has signs of life,’” she wrote, “with enough rocky worlds with biosignature gases, we will inspire confidence that life not only exists in the solar neighborhood but is common in our Galaxy.”

To see a planet’s atmosphere, telescopes watch as its star’s light passes through the atmosphere, illuminating its contents. “Any Earth-like exoplanets within dozens of light years are about as faint as the faintest galaxies ever observed by the Hubble Space Telescope,” Seager continued, and the biosignatures themselves represent just a small portion of that light.

How to See Them

But there’s a catch. While the Kepler Space Telescope has uncovered a huge list of planets, they are all too far away for us to analyze their atmospheres with current technology.

The Transiting Exoplanet Survey Satellite (TESS), which NASA plans to launch in 2017 (although I wouldn’t hold your breath, and not just because you’ll mess up our biosignature) should discover super-Earths (2-5 times as massive as our planet) around red dwarfs. It will survey 400 times as much sky as Kepler did, scanning half a million stars. Then, the space agency’s James Webb Space Telescope, slated for launch in 2018, can zoom in to see if some have promising atmospheres. Seager’s paper estimated that dozens of TESS’s candidates will be suitable for James Webb.

A few planned optical telescopes on the ground – like the Giant Magellan Telescope, Thirty-Meter Telescope, and Extremely Large Telescope – might catch sight of Earth-sized atmospheres. But only if they get lucky.

Farther in the future (in the nebulous region between 2025-2035), the Advanced Technology Large Aperture Space Telescope (ATLAST, get it?) will launch with a mirror 5-10 times wider than the James Webb’s, and with 2,000 times as much sensitivity as Hubble.

But no matter which telescopes we build, or when, scientists won’t have unlimited time to peer into the night sky. And, for such tiny specks of light, quality of planet-scanning will be as important as quantity.

So to find another nice place, where creatures tiny and mute or big and boisterous might live, we will have to look both hard and smart. And that might start with the search for tidy planets around a red dwarf star.


Top image by Johan Swanepoel/ Shutterstock

CATEGORIZED UNDER: Space & Physics, Top Posts
MORE ABOUT: exoplanets
  • 7eggert

    “First step is to find a red dwarf?”

    From what I learned, planets in the habitable zone round red dwarfs are tidal-locked, they’d have one hot and one cold side – not likely to support life.

    • David K

      There was a recent study that said that if the exoplanet had a thin atmosphere (Such as 1 bar) it could have a day/night cycle 0f 30-40 hours, also, a tidally locked exoplanet could support life between the day and night side. They’re also easier to find than exoplanets orbiting sun-like stars.

      • 7eggert

        Even so I’d focus on yellow-green dwarfs if possible.

        Expanding the view on red dwarfs isn’t wrong and we’ll learn a lot, but it’s not my primary choice for finding habitable planets.

        • David K

          Yellow-Green? There is no such thing as a yellow-green star, let alone a completely green star, so do you mean yellow-orange like the sun? But okay, how about this: What about the stars in-between M and G type stars, what about K type stars? They’re bigger and brighter than M type stars (Which means orbital periods between 100 and 300 days, usually 200 days) And live longer (3-10 times longer than the Sun will last) and are more common than G type stars.

          • 7eggert

            As I understand, the smaller the star, the closer the orbit of the habitable zone and the more likely the planet will be in synchronous rotation.

            I think we need not to look at as many small stars as possible, but to find the balance between looking at big stars because the planets there are more likely to have life (we know one G type star …) and looking at small stars because we can see planets there at all.

            (Off cause if the stars are too big, our current theories predict that life either has no time to emerge or would be killed by hard light.)

          • David K

            You’re right that red dwarf stars have small habitable zones, and are more likely to have tidally-locked planets, and that big stars aren’t good candidates for finding a habitable planet since their lifespans are too short, and yes we would need to find the balance between star types (M, K, G, and F) to find decent habitable planets, but the balance is probably K-type stars, and here’s an example: Have you heard of Gliese 570? It’s a K0 type star 19 ly away, and it’s habitable zone is pretty decently sized since a planet with the same rough temp. as Earth (The temp. where if it didn’t have an atmosphere, would have a specific temp., for Earth, it’s 255 kelvin) would have to have a orbital period of around 250 days, or 8 months, Personally, that’s a pretty good orbit, and standing on the surface, you would see a blue sky, pretty awesome right?

          • 7eggert

            I don’t remember the exact numbers, but I heard about some of Gliese’s planets to be quite habitable.

            Anyway, the Article sounds like we should look for only tiny stars and expand only that research, to which I wanted to disagree.

          • David K

            I think you’re talking about Gliese 667c 24 ly away, with 3 habitable exoplanets, and I agree that we shouldn’t limit our search to M dwarfs, and we should look at many candidates, but I think that at first we’ll probably look at M dwarfs since it’s easier to find planets around them, and then look at larger sun-like stars.

          • JR

            I think that’s right, the constraint at the moment is observation technology, which will improve. Larger stars have wider habitable zones and so more margin of error.

          • JR

            Gliese 570 though is a ternary system (quaternary if you count D, a brown dwarf), B & C together having an eccentric orbit around the primary, so the gravitational / orbital stability issues make it a less likely candidate.

          • David K

            I know, but the red dwarfs separated by over 190 AU, and the brown dwarf is separated by 1,500 AU, I think that’s far enough away that the system would make itself stable after awhile, there was also nothing about the system that said it was unstable.

  • James A. DeHart

    Logic tells us we are not alone; why pursue it further than that?

    • bwana


    • g.d. weeks

      Also. What to do if we find life somewhere. Today, 23 July 2015, it was announced the finding of a world so like Earth that it is very possible for life to be there. 1400 light years away. Not likely to have friendly conversations.

  • Ever García

    Excellent article. We do certainly need to “hunt” life on planets in an smarter way which will eventually lead to the finding of life. I hope I see that in the upcoming years…

  • kay Johnston

    Well for one reason – look how we abuse our planet. How much more abuse will it take before we destroy it?
    Then we can start abusing another one.

  • Sue Bumgardner

    Nice timing they announced today they have found one 1400 light years away a little bigger than Earth in the right zone around a similiar sun

  • EquusMtn

    It seems that all of the supposedly “Earth-like” planets discovered so far are actually significantly larger than Earth. The latest one is 1.6 times Earth mass, which means that even if we could get there somehow, just standing on its surface would be exhausting, and impossible for any length of time — like being on a jet pulling 1.X g constantly. Why do scientists and the media always overlook the gravity factor when they announce another “Earth-like” planet?

    I’ll get excited when they find one about the same size as Earth or a little smaller. No doubt some sci-fi writer is way ahead of me on this, but a planet with about 0.8 Earth gravity and a somewhat denser atmosphere (which is entirely possible) would be a great place for the elderly to retire (again assuming we figure out how to get there). We could call it “Resthome” or something.

  • JerryG

    I think it is good to search, but what happens when we find one? Oh boy we found one; Kepler 452b just a mere 1,400 lightyears away. Let’s go and check it out.
    A better and surer way to find a planet with life, just follow one of these UFOs back home. Almost certain to find life there, plus I’d bet they know many other places also.
    Here lies the problem, our technology today straddles us to this planet, we need to develop the technology to teleport ourselves instantaneously to these places in order to make this process worth the money being spent. We can create an endless list of potential planets with life capability, but what good is it?
    Instead of spending $600 billion to put a couple of astronauts on Mars for a 2 hour walk, let’s start some real R & D on teleportation to explore the whole entire Universe. Then when we find one, we will also have the technology to do something with it too, i.e. put real people on it versus just astronauts.

  • Joe White

    Once we find one we settle the oldest question in our history: are we alone?

    Once that is settled we can get off our high horse and realize we’re not the chosen ones that all religions believe we are.

    We can come together as a race and move forward together, instead of fighting each other.

    Maybe this sounds a bit kumbaya-ish, but our future ancestors will likely look back on our pre-we-are-not-alone selves and wonder why we spent so much time killing each other when we could have advanced our species so much faster by working together.


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