What's the Deal With 'Super-Earth' Exoplanets?


The big news in the universe this week is that NASA’s Spitzer space telescope has managed to capture light from an exoplanet — an alien "super-Earth" to be exact, which makes this the first time astronomers have managed to detect light from a small, rocky body.

But don't let the name fool you. Super-Earths have very little in common with our own planet. That's not to say the discovery isn't fascinating, but it's more science as usual than the discovery of our next home.

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There are many parts to this story, like the one about 55 Cancri e. It isn't a new find. Astronomers discovered it in 2005 and have since determined that it's one of five planets in a solar system orbiting the star 55 Cancri about 41 light-years away in the constellation Cancer. They also know that 55 Cancri e orbits extremely close to its parent star and its star-facing side is heated and emits a thermal glow — that's the infrared emission Spitzer picked up.

This isn't the first planet Spitzer has observed directly either; usually astronomers "see" planets by a telltale dip in the brightness of the parent star as they pass in front of it (i.e., the "transit method"), or the "wobble" of a star as an exoplanet orbits it (i.e., the "radial velocity method"). But the $770 million telescope, which was launched in 2003, is searching for exoplanets directly by detecting their glow in the infrared spectrum.

ANALYSIS: 'Earth-like' Planets May Be Nothing Like Earth

Hot Jupiters are gas giants that orbit their parent stars closer than Mercury orbits our sun. These are big planets with big surfaces that reflect a lot of light and generate a lot of heat, making them easier for telescopes to find. That Spitzer measured 55 Cancri e directly is impressive, since as a super-Earth, it's much smaller than a hot Jupiter and gives off a lot less light.

It's 55 Cancri e's classification as a super-Earth that's a little problematic. The term may be intuitive to astronomers, but to the rest of us it's incredibly misleading.

With the familiar name "Earth," it's easy to see why people read "super-Earth" and get really excited. Our home planet Earth is pretty super. We have a temperate climate, fresh water, ample sunlight, a protective magnetic field and a balanced atmosphere. Earth has everything that makes life possible. By extension, and by linguistic convention, a super-Earth should be better — a bigger Earth with more fresh water, beach weather year-round, a thicker or richer atmosphere, and anything else that could improve our own planet.

In actuality, super-Earths are quite different from our Earth. Super-Earths have masses roughly 10 times greater than Earth's, but 55 Cancri e shares its rocky nature with Earth — that’s where the similarities end, however. But that isn't to say it’s not a really neat planet.

ANALYSIS: White Dwarfs Are Eating 'Earth-like' Planets for Dinner

Exoplanet 55 Cancri e stood out to astronomers in 2005 because of how close it is to its star — about 26 times closer than Mercury is to the sun, which gives it an 18-hour day. At that distance, the temperature on the star-facing side reaches up to a staggering 3,140 degrees Fahrenheit. Measurements of the planet tell astronomers that its likely a dark world without the substantial atmosphere needed to warm its nighttime side. It's also oozing. The extreme heat keeps material on its surface, which includes some amount of water, in a supercritical fluid state, meaning the liquid is likely topped with a layer of steam from the heat.

So 55 Cancri e isn’t at all like Earth — so what should we take away from this latest development from Spitzer? That very cool science happens all the time, and that "firsts" are exciting. And that we still haven’t found a true outstanding Earth other than our own.

Image: Artist's impression of a "hot Jupiter" — the heat of which may be detectable by an infrared telescope like Spitzer. Credit: NASA/JPL-Caltech/R. Hurt (SSC).

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