‘Earth-like’ exoplanets may exist—but we don’t know of any yet
Scientific American · LC · trust 44/100
Can we stop calling exoplanets ‘Earth-like,’ please?
From what I can gather, Freddie Mercury—the rightfully celebrated lead singer of Queen—was about the same height as I am.
So while I’m certainly not Freddie Mercury’s twin, would you say I’m Freddie Mercury–like? Besides having our heights in common, I also am male, have facial hair and can carry a tune (though at that, I am not a champion ). Weirdly, we both also worked with rock stars named Brian with Ph.D.s. related to physics , too.
You might argue that, if you squint really hard, I am at best extremely vaguely Freddie Mercury–ish. Really, all you can say is that I’m Freddie Mercury– sized —I certainly wouldn’t go so far as to say Freddie Mercury– like .
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That’s why, among many other reasons, I need not worry about ever being described that way by the media. But then again, I’m not an exoplanet.
To date, astronomers have found more than 6,300 of these alien worlds orbiting other stars, with new entrants to our exoplanetary catalogs arriving practically every day.
Many of these are so weird that they’re difficult to imagine. The very first ones discovered were radiation-riddled worlds orbiting a pulsar —the burned-out stellar remnant of a catastrophic supernova explosion and pretty much the last place anyone would have expected a planet to hide. Next, astronomers spied several exoplanets that are gas giants like Jupiter—but in orbits so tight that these worlds practically skim the surfaces of their stars. Others orbit so far out that it’s hard to explain how they got there. Some exoplanets are rocky but so hot that their surfaces are molten; others are so cold that they’re frozen solid.
Each and every one we find isn’t just a world—it’s an additional data point in the grand scientific quest to understand how planets form and how they change over time, putting our own solar system into context with the rest of the galaxy.
There’s an emotional, almost personal aspect of this, too, because the story of planets is also the story of life’s origins—including our own. And the element of this that calls out to us is the search for another world like Earth.
Just finding a single true twin to our planet would be a huge landmark in science, putting a framework around our very existence. It’s clear the public feels this way, too. The media certainly does: any time an exoplanet pops up with any sort of vaguely familiar terrestrial characteristic, press conferences are held, news stories are written and headlines speculate on what that world may actually be like.
And here we run into a problem: the adjective “Earth-like.” It gets used frequently to describe exoplanets we’ve discovered, but in my opinion, it has never been true.
While the term has been around a long time, it started getting used to describe actual exoplanets once technologies and methods for the search for distant worlds improved, which allowed for smaller, harder-to-see planets to be found. Eventually, astronomers uncovered ones with dimensions similar to Earth’s, and these were sometimes dubbed Earth-like worlds.
Yet they weren’t actually like Earth—at least, not necessarily. At best, all we could say with confidence is that they were Earth- sized (roughly the same diameter as Earth) or Earth- mass (about the same heft as our planet), or both. The only other thing we really knew was their distance from their host stars, which let us determine how much starlight they received—and that only gives us a vague idea of any exoplanet’s temperature.
That’s because the mix of gases in their atmosphere—and the presence of one in the first place—can wildly change a planet’s surface temperature. Just look next door at Venus: it’s slightly smaller and less massive than Earth and not that much closer to the sun than our own world is, but its surface conditions are hot enough to melt lead, and it has a sky with a crushing pressure that is 90 times greater than Earth’s at sea level. A runaway greenhouse effect caused by Venus’s atmosphere of nearly all carbon dioxide is to blame. So even if the planet’s surface were somehow more charitable toward humans, we still couldn’t breathe there.
This makes Venus very Earth-sized indeed but absolutely not Earth-like in any real sense of the word.
Despite this object lesson right on our interplanetary doorstep, the “Earth-like” term endures. Even if we expand what we mean by the phrase—which admittedly has no consensus definition, though attempts have been made to establish one—the label keeps getting slapped onto exoplanets for which it clearly doesn’t apply.
The latest chapter in this saga is the exoplanet LHS 1140b, which is a very interesting world regardless of labels. Its host star is a dim bulb of a red dwarf about 49 light-years from our sun. Astronomers with the MEarth project discovered this world in 2017, and current measurements put it at 1.73 times Earth’s size and 5.6 times our planet’s mass. Despite its bulkier build, this gives LHS 1140b a density very similar to Earth’s, which implies its composition is similar as well.
Well, this world is a mere 14 million kilometers from its host star—closer than Mercury is to the sun (the planet this time, not the singer)—and even though its star is so faint and cool, the planet is still not likely to be a nice place for life as we know it. While it’s technically in the star’s habitable zone , or within the…
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