The past 12 months has seen a dramatic influx in announcements proclaiming the discovery of Earth-like planets. It was only a few years ago that the detection of any kind exoplanet was cause for celebration, but these days news of a potentially habitable planet has become routine. But as astronomer and astrobiologist Caleb Scharf now notes, we need to relax and take a deep breath; just because these rocky exoplanets reside within a solar system's habitable zone doesn't mean they're anything like Earth.
As Scharf correctly points out, talk of a potentially 'habitable' world conjures images of fantastically blue and green planets blanketed with cloud cover — and just ready to burst with life.
But for astronomers it means something else, merely a planet that has the conditions to support liquid water on the surface, and atmospheric pressure high enough for water to exist without boiling off to vapor. It also implies the presence of an atmosphere that that can alter the transfer of radiation to and from the surface (a la greenhouse effect).
Writing in Scientific American, Scharf elaborates on what this implies: "So strictly speaking ‘habitable' includes a range of environments that we would find appallingly hostile, including high-pressure, high-temperature climates and those in a sub-arctic category with thin atmospheres."
His primary concern is that we simply lack information on the presence or absence of water or an atmosphere on many of these exoplanets, nor do we have any idea about their geophysical history or present state. He writes:
Which gets us to the other point, the cavalier use of the phrase ‘Earth-like'. Utterance of this can evoke all sorts of images. It may make us think of oceans, beaches, mountains, deserts, forests, fluffy clouds, fluffy bunnies, warm summers, snowy winters, the local pub, or the fabulous hubbub of the local souk.
But this is typically far from the meaning attached by scientists. It can simply indicate a planet with a rocky surface, rather than a world with a thick gaseous envelope. It can mean a world that is roughly the same mass and density as Earth. It can mean a planet orbiting a star like the Sun. Or it can just mean that we got bored of saying things like ‘a two-Earth mass object in a close to circular orbit around a roughly 4 billion year old main-sequence star that is similar in mass to the Sun.'
Although at some level this is purely to do with semantics I think it's important to consider. What I believe we really mean when we say ‘Earth-like' is that a planet is Earth equivalent. That is to say that while the planet might feel completely alien to human senses it nonetheless matches many of the same physical and chemical characteristics of Earth. It's a bit like renting a car at an airport where you've reserved the open top red sports-car, only to be told that they've run out but you can have ‘an equivalent' vehicle. It'll have four wheels, an engine, and yes you can wind the windows all the way down if you'd like.
Scharf's point is well taken. Many of the exoplanets that have been discovered are so-called super-Earths — planets that are orders of magnitude larger than our planet. The chemical and geophysical processes on those planets could be astoundingly far removed from what we see here on Earth.
Perhaps it's time, as Scharf points out, for astronomers, astrobiologists, and the media to refrain from using such hopeful and potentially misleading language. But let's face it — it's still damn mind-blowing whenever a planet is found within a solar system's habitable zone. And on that point we all reserve the right to get excited.
Read more at SciAm.
Top image: Image: PHL @ UPR Arecibo, ESA/Hubble, NASA. Inset image: Artist's impression of a sunset from the super-Earth Gliese 667Cc courtesy ESO/L. Calçada. The large sun is the red dwarf, 667C.