How Would Alien Astronomers Find Earth?

How Would Alien Astronomers Find Earth?

This weekend, astronomers announced the discovery of the most Earth-like planet anyone has identified yet. The search of habitable planets is intensifying — and, with it, questions about whether we're looking for the exoplanets the right way. For starters, figuring out how Earth would look to aliens is actually pretty useful.

The search for exoplanets is an incredibly complicated endeavour. In theory, planets with life-sustaining habitats are out there, orbiting around stars other than the Sun. But finding them is another matter entirely. Among other issues, these planets would be outshone by their respective suns, which is why NASA is dreaming up incredible spacecraft like the PlanetQuest, which shields its sensors from the blinding starlight to capture better photos of the planets around it.

But what, exactly, should astronomers be looking for besides the obvious existence of water? It's a question NASA has been asking for years. And to find the answer, it's looking at the single habitable planet it knows best: Earth. Back in 2009, the agency used its Deep Impact/EPOXI spacecraft to look back at Earth to see what it would look like from afar. And just last week, astrophysicists at Harvard proposed that we look for alien pollution, just like the stuff we make here on Earth.

Now, a group of NASA astrophysicists have published a paper that explains how they used an existing spacecraft to find out how alien life could detect Earth. According to Daily Galaxy, they did so by re-using existing data from NASA's Lunar Crater Observation and Sensing Satellite, or LCROSS, which collided with the Moon five years ago. Its mission was to find evidence of water — which it did! But as the research team explains in Detection of Ocean Glint and Ozone Absorption Using LCROSS Earth Observations, LCROSS inadvertently observed some fascinating things about Earth, too.

For example, when Earth is seen as a crescent from the surface of the moon, it reflects light from our oceans — an effect the researchers call Earth glint. "Also, the Earth at crescent phase, thanks to the ocean, can be twice as bright. If it's something you look for in exoplanets, it can be a significant effect," one author told Daily Galaxy. If alien astronomers are looking for Earth, this glint could be a major sign that live exists on our small blue dot. Another hint? Ozone.

Astronomers have wondered if we could observe other planets' "glint" for years, but this new research shows exactly what kind of tools we'd need to do it. For instance, detecting Earth glint is also a matter of choosing the right wavelength of light to look for — which means that this new information could inform how telescopes are built from here on out.

By looking at the Earth as though it were an exoplanet, astronomers are getting a clearer picture of what they should be looking for, millions of miles away. [Daily Galaxy; The Astrophysical Journal]


Comments

    I would start by theorizing what senses this hypothetical alien race has first. If they see with light, what spectrum? Do they hear sound, what frequencies? It's all well and good to make ourselves visible in that tiny percentage of light we look with, but there is a very, very high chance that either we are it, or other races don't interact with the universe like us...

      Think of the title as a thought experiment to encourage new solutions for finding extra-solar planets, so it really doesn't matter what the alien astronomer is, but rather its perspective.

      Also, your comment "but there is a very, very high chance that either we are it" is intriguing. What information is this based on?

        Aye, the poster doesn't seem to understand how few forces there are in physics.

        The facts are:
        1: The electromagnetic force is by far the most easily modulated long-range force in the universe.

        2: All beings intelligent enough to search the stars can reasonably be expected to use instruments vastly more capable than their senses.

        The only real questions are:
        1: If you're going to signal, what is the most logical way to effectively modulate electromagnetic fields to get greatest effective range while still being clearly artificial. (For instance a spread-spectrum encoding technique centred on pi x the emission frequency of hydrogen ).

        2: If we're going to look for non-signalling life, what aspects of the spectrum should we be searching.

    How would aliens find earth? By turning right at the moon. Boom!

    The major assumptions in all of this is that for intelligent life to exist on other planets in the universe, they must require oxygen, water and heat. What if there are lifeforms in other parts of the universe that exist and do not require this? Or have not evolved to the point that they make pollution? Or have evolved beyond that point of making pollution? Or live on planets that do not have water (therefore no "glint")? What if they have not thought about looking for other life forms or dont have the technology to do so? It is tough trying to find intelligent life when the only thing you're using an example of is yourself and the world you know, which is really limiting the possibilities.

    Last edited 01/07/14 1:36 pm

      you do know that "heat" (that is, a positive temperature difference) is energy, and without said energy, a biological life form cannot perform the necessary chemical reactions needed to sustain life. While some basic life forms can survive being frozen, they do not function while they are frozen.

      Also, even if they have "evolved" (by which you probably meant, progressed technologically) beyond pollution, surely any species advanced enough to have completely renewable (or at least waste free) energy sources would have some form of recording history, and would be aware that pollution is something that exists and would be a key thing to look for.

      TL:DR you point out that life on other planets could be completely different to life as we know it, yet assume that said life would only look for life as THEY know it in their present.

        Just thought I'd chime in with a few comments about alternative planet-based life-forms.

        Although carbon and water are marvellous, it's not that hard to find other bases and solvents which may be able to for the basis of alternative life systems.

        But there's a *big* problem with proposing living atmospheres which don't revolve around O2/CO2. For any other atmosphere proposal to be taken seriously you'd need to sketch out an alternative to photosynthesis which works with that atmosphere cycle, which is a huge hurdle.

        you do know that "heat" (that is, a positive temperature difference) is energy, and without said energy, a biological life form cannot perform the necessary chemical reactions needed to sustain life.
        Based on what we as humans know as the signs of life, yes. And there lies the problem or limitation of scope for finding life.

        surely any species advanced enough to have completely renewable (or at least waste free) energy sources would have some form of recording history, and would be aware that pollution is something that exists and would be a key thing to look for
        I dont disagree with that statement but what if the aliens have evolved to the point that they left their home planet after destroying it by polluting it, moved to another solar system and started from scratch with limited records of their history? Or what if they're successfully polluting their own planet but have no technology to record history (such as computers etc...think the human race back in the early 1900's with only pen and paper). Advance lifeform doesnt necessarily translate to being ahead in technology.

        TL:DR you point out that life on other planets could be completely different to life as we know it, yet assume that said life would only look for life as THEY know it in their present.
        You read my comment wrong. My last sentence incorrectly I think. Re-read it as "It is tough (for humans) trying to find intelligent life when the only thing you're using an example of is yourself (ie the human race) and the world you know (Earth), which is really limiting the possibilities. Hope this clarifies which angle I was coming from.

    3rd rock from the sun

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