Why We Shouldn’t Call Exoplanets ‘Earth-like’ Just Yet

By Nathaniel Scharping | October 21, 2016 12:28 pm

(Credit: sdecoret/Shutterstock)

Every time astronomers discover another exoplanet, the first question is,”Does it look like Earth?” Finding an Earth-like exoplanet would certainly increase our chances of finding life, as we know it, on that distant world. We could finally prove that we’re not all alone in this big, cold universe.

But, when we see planets described as Earth-like, we should be skeptical. With our current instruments, it’s hard for us to even find other planets out there (although it’s gotten much easier), much less see if there are oceans, atmospheres, plants or animals. Furthermore, what does it even mean to be “Earth-like?” Does it just need to be in the habitable zone? Or does it need to have liquid water and a similar atmosphere? 

Don’t Hold Your Breath

The question likely won’t be resolved anytime soon. While some progress is being made using the starlight that filters through a planet’s atmosphere to detect what kinds of gases are there, that’s about as precise as we’re going to get for awhile. When, and if, the Breakthrough Starshot Project ever reaches Alpha Centauri, we might have a better idea, but even that mission is decades off. So, for now, let’s put off calling planets Earth-like. Unfortunately, we really have no way of knowing exactly how alike they are to our own precious planet.

We are currently limited to just a few solid observations about planets outside of our solar system, and even those can be tough to glean. The three pieces of information we can obtain with any kind of reliability are the mass of the planet, its orbital period, and how far it orbits from its star. These may seem paltry compared with the detailed measurements collected from Venus or Mars, but astronomers can still derive important information about a planet just by knowing how big and far away it is.

How We Know What We Know

To determine the mass of an exoplanet, astronomers usually look to the star it orbits to measure tiny back-and-forth movements caused by the planet’s gravitational pull as it orbits. We should remember that mass and size are different, and we have no real way to measure size right now — the best we can do is run an approximation based on the mass. To figure out how fast it orbits, all astronomers have to do is observe how often the light of the star dims as the planet passes in front of it. Combining this information with the mass of the star, astronomers approximate how far away the exoplanet should be, and whether that places it in the habitable zone of a star or not.

Being in the habitable zone, where liquid water could theoretically exist, is one of the biggest tests we can do right now for astronomers to determine if a planet could support life or not. If a planet lives outside of that thin ring, the chances of finding life there are pretty much zero.

While the habitable zone of a star may be the first hurdle for a planet to overcome on the path to being like Earth, it’s far from the last. Just because liquid water could exist there, doesn’t mean it actually does. The planet could be full of toxic minerals too, or a complete wasteland. It’s core, the internal dynamo that powers our radiation-deflecting magnetic field here on Earth, could be dead, or it could have lost its atmosphere. It could be blasted by waves of powerful radiation from its star, or it could have been assaulted by asteroids. Point is, there are lots of reasons why a potentially exciting exoplanet could be uninhabited, and our methods of observation aren’t refined enough to explore most of those reasons. Calling an exoplanet “Earth-like” is a little too much of a stretch at this point.

There are lots (and lots and lots and lots) of exoplanets out there though, and we’re only to find more. There’s no reason that we won’t find a planet that looks very much like Earth someday. We’re just going to have to wait awhile.

CATEGORIZED UNDER: Space & Physics, top posts
  • http://www.unsigned.com/SuperPredator Chris H

    I think another variable in them being able to support life would be a moon like ours.

    Seem like the Moon has had a big influence on evolution, just the menstrual cycle of women following it proves that.

    • Mike Richardson

      Maybe, but there are any other number of variables you could introduce from Earth to make “Earthlike” seem rare. The fact is, you could have a world without a moon that might settle into a fairly stable orbital inclination. Or, perhaps life itself could prove more resilient to such seasonal variations. Also, how many habitable moons of gas giants might exist, stabilized in their climate regimes by their mother planets? We can’t use the one example of Earth itself to limit what might be possible for a wide variety of worlds harboring complex life in this universe. One thing we know from studying life on Earth is that it can continually surprise us with what is actually possible. To quote one of my favorite lines from the Jurassic Park movies, “Life finds a way.”

  • Juliska Magyar

    The other one is no sense of the size of the universe. As in that ‘there may be billions of planets similar to the Earth in the Universe’.

    Huh…just one for every 2 trillion stars?

    No, there are not billions, trillions, quadrillions…etc. But likely a thousand octillion planets similar to the earth….

    Last weeks new estimate was 2 trillion galaxies or 10 to the 25th power of stars. If one in a thousand stars has a similar planet…that’s 10 to the 22nd planets similar to Earth…10,000,000,000,000,000,000,000

    • Glenn Beaton


    • Rick Principle

      How remarkable that we are not a parking lot.

  • Mark Thomason

    We need to expand our definition of life. There are forms of life at our own sea floor and deep in the crust that required us to re-think what life required.

    We must take the next step, and consider such things as methane based life in extreme cold, possibly even in our own solar system. There may be more forms.

    This is significant because the next step is to realize that convergent evolution should apply to all forms of life. Intelligent life could arise from forms other than ours, as convergent evolution to an answer that works. This vastly increases the potential numbers for finding intelligent life.

  • OWilson

    Perhaps, in some ways, the universe itself is living.

    It came from a tiny point, grew, defied the laws of entropy to condense from chaos, form stable complex solar systems and planets and ultimately computers and US!

    Maybe we are not the end of everything?

  • Glenn Beaton

    The reason scientists and the media are quick to label a planet ” earth-like” is that they want to leave the impression that there’s life there. NASA scientists like that because it helps them get funding. The media likes it because they think (erroneously in my opinion) that it disproves the existence of God.

  • brianreilly

    Thank you for an article that just begins to describe how much we do not know, and why we should stop claiming we do.

    Please do not take this as a call to stop looking, observing, and developing instruments and analytical tools to learn more. Just don’t claim to have data that you do not.

  • Nixak*77*

    IMO The Rare Earth Principle is likely correct- which would fully explain ‘Fermi’s Paradox’ & SETI’s total silence. IMO there’s likely just a few or perhaps even only one truly Earth-like planet orbiting one / a few Sun-like star(s) w a / a few solar-system(s) like ours- per galaxy or maybe even per group of galaxies. So tho there MAY Be billions of earth-like planets inhabited by ET type civilizations in the entire Universe [maybe], IMO for our Galaxy there are likely just a handful of ‘earth-like’ exo-planets w ET type civilizations- In fact we may even be unique re this Galaxy.

    • OWilson

      We are unique in our neighborhood, and beyond our neighborhood there is only speculation.

      There is obviously no technology out there to communicate with us and time travel is purely wishful thinking.

      We might as well try to get along with each other here on this planet instead of wishing to find the answers elsewhere!

      • Nixak*77*

        That very well may indeed be the case.

  • Lee Chapman

    Indeed, Mars and Venus are Earth like planets.

    But we are not looking for civilizations there. And any possibility of life appears to be eluding us.

  • Luke Ashton

    Great minds think alike.
    There was only one Einstein.


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