Super-Earths: Mirrors of our world?

by Carolyn Gramling
Thursday, January 5, 2012

An artist's conception of the hot Jupiter HD 189733 b, which orbits too close to its star to be habitable. The planet's atmosphere may contain water vapor and methane. NASA

The Gliese 581 system. Only Gliese 581 d, about eight times the mass of Earth, is firmly in the habitable zone (blue), where liquid water oceans might exist. ESO

Fifty light-years from Earth, in the constellation Pegasus, burns a yellow star not unlike our sun. The star, called 51 Pegasi, was one of 142 stars under the watchful gaze of Swiss astronomers Michel Mayor and Didier Queloz of the University of Geneva in 1994. From the La Silla Observatory at the southern end of Chile’s vast Atacama Desert, Mayor and Queloz were tracking how these stars move in the sky, hoping to determine whether the stars were alone — or whether any of them might be accompanied by a planet or two.

Less than a year later, Mayor and Queloz had amassed enough data on the movements of 51 Pegasi to make an astounding announcement. Rather than blinking motionless in the sky, they said, the star wobbled in a periodic, predictable way: 51 Pegasi was being tugged by an orbiting planet.

The discovery electrified astronomers. It was the first time that a planet had been discovered orbiting a distant star similar to our own sun. In the 15 years since then, the universe has become rapidly more crowded: Scientists now know of more than 300 such “exoplanets” in orbit around stars outside our solar system. But most of these, including the planet orbiting 51 Pegasi, are inhospitable gas giants like Jupiter and Neptune — and for many astronomers, the primary driver behind the hunt for exoplanets is finding worlds that can harbor life.

“We’re looking for habitable worlds,” says planetary scientist Alan Boss of the Carnegie Institution in Washington, D.C. The “gold standard,” he says, is finding planets that closely resemble Earth, “because we know Earth has life.”

Boss is convinced that there are thousands of Earth-like planets out there. But we haven’t found them yet. The closest we’ve come are a handful of exoplanets that seem potentially Earth-like, with perhaps two to 15 times Earth’s mass and possibly a similar rocky surface and thin atmosphere. Some of these “super-Earths” might have liquid water at their surface or even have plate tectonics. But with only the scantest of information so far, it’s hard to say whether these planets are even terrestrial. And as new spacecraft prepare to scan the stars for more new worlds, whether super-Earths will turn out to be much like Earth at all is still not clear.

New worlds: The habitable zone

Astronomers have traditionally thought that galaxies and solar systems have a theoretical habitable zone. Loosely defined, a galaxy’s habitable zone is a region close enough to the center of the galaxy that there is an abundance of heavier elements such as carbon, oxygen and nitrogen — all needed to form rocky, terrestrial planets as well as complex molecules — but far enough out that life forms won’t be damaged by dangerous radiation (likely emitted by a black hole at the galaxy’s center). In a solar system, a planet is in the habitable zone if it’s close enough to the star for liquid water to be present on its surface.

Astronomers haven’t had much luck finding exoplanets in such habitable zones, however. Mayor and Queloz’s exoplanet, officially named 51 Pegasi b (and unofficially named Bellerophon, after the hero of Greek mythology who rode the winged horse Pegasus), certainly isn’t likely to harbor life. The planet circles its star so closely that its orbital year lasts only four days and it has an atmosphere somewhere in the thousands of degrees Celsius — far too hot to be habitable.

Of a similar size and composition to Jupiter, the biggest gas giant in our own solar system, 51 Pegasi b became the first planet dubbed a “hot Jupiter.” Many more hot Jupiters were discovered over the next decade. But in 2005, another team of scientists announced they had found a new kind of exoplanet. A mere 15 light-years from Earth, in the constellation Aquarius, is a red dwarf star called Gliese 876. Two hot Jupiters were already known to orbit the star, but scientists discovered a third, smaller planet. At only about six times the mass of Earth, it was small enough to possibly be terrestrial rather than a gas giant. It was the first so-called super-Earth.

Still, both the new planet and several subsequently discovered super-Earths orbit their stars too closely to be habitable. In February, for example, the European Space Agency’s CoRoT spacecraft detected a super-Earth just twice the size of Earth, called Exo-7b, but the planet completes its orbital circuit in only 20 hours, suggesting temperatures on its surface are likely to soar above 1,000 degrees Celsius.

In recent years, however, scientists have identified a few super-Earths that may be habitable, including at least one of three super-Earths known to be orbiting the star Gliese 581. The most recently discovered planet, Gliese 581 e, discovered in April, is a bit too close to its star to be habitable. But the fourth planet from the star, Gliese 581 d, is thought to orbit firmly in the system’s habitable zone, so that it may have liquid water on its surface — and may even have a large, deep ocean.

Elusive exoplanets

Part of the problem with finding habitable planets is that the smaller a planet is, and the farther from its star, the harder the planet is to detect. Boss says he is convinced that there are many, many Earth-sized planets out there — they’re just very difficult to see. The bright glow of stars tends to obscure any faint light reflected by planets, making it very difficult to spot them directly. Scientists, therefore, have turned to indirect detection methods that favor finding large planets close to their stars. Thus, it’s much easier to detect gas giants than super-Earths.

Although we tend to think of the planets in our solar system as orbiting the sun, in fact the planets and the sun alike are in orbit around the same thing: the center of mass of the solar system, Boss says. Because the sun is so massive, the center of mass happens to be pretty close to the sun’s location. But Jupiter, although much smaller, is pretty massive too — so even as Jupiter circles the sun in its 12-year orbit, it exerts a gravitational pull on the sun. As a result, the sun, too, travels around in a little circle.

Large planets orbiting a star in other solar systems exert a similar pull on their stars — and the star’s resulting wobble is one way to determine whether it has planets around it. Spotting the star’s actual wobble is difficult, because the distance it travels in orbit might not be much bigger than the diameter of the star itself. But by measuring changes in the star’s light emissions over time, it is possible to measure the speed of the wobble, as the star is pulled toward the planet (and either toward or away from Earth).

To measure this wobble, scientists study changes in the frequency and wavelength of light waves emanating from distant stars. The technique, called Doppler spectroscopy, takes advantage of a similar phenomenon based on sound waves: Anyone who’s heard the wail of an ambulance siren rise and fall as it passes by is familiar with the Doppler effect. As an observer moves relative to the source of a wave (whether a sound wave or a light wave), it appears to change in both frequency and wavelength.

It was the Doppler method that found most of the known exoplanets to date, including the hot Jupiter 51 Pegasi b and super-Earth Gliese 876 d. Both planets have very short orbital periods; to identify them, scientists had to observe the wobble often enough to be sure it was statistically significant.

The Doppler method is useful for finding possible planets, but it doesn’t provide any information about the planets' radius or mass — for that, scientists use a second technique, the transiting method. As a planet crosses in front of a star, the star’s brightness may appear to dim slightly; how much it dims depends on how big both the star and planet are, as well as the position of the observer relative to the transit. With those uncertainties, the transiting method is more useful to identify large planets than smaller, Earth-sized planets. Scientists have also used this method as a way to determine the mass and radius of previously discovered hot Jupiters.

A third technique, called gravitational microlensing, takes advantage of an effect predicted by Einstein: If you have two stars, one more distant and one closer, and the nearer star passes in front of the more distant star, the gravity of the foreground star will act as a giant lens, “bending” the light of the more distant star in the direction of the observer and amplifying that light. If a planet is circling the nearer star, the background star will become even brighter – and that brightening reveals an otherwise too-faint planet.

Plate tectonics: The key to life?

Plate tectonics on Earth recycles greenhouse gases like carbon dioxide between the atmosphere and the lithosphere, helping to regulate surface temperatures on the planet. Thus, this process may be one of the key ingredients for habitability — at least for life as we know it, says Diana Valencia, a planetary scientist now at the Observatoire de la Côte d’Azur in Nice, France.

The recipe for plate tectonics includes the rupture of the lithosphere, deformation and subduction — and on super-Earths, Valencia says, the ingredients for plate tectonics are right. “The conditions [for plate tectonics] that these planets would have would be [even] more favorable than Earth,” Valencia says. “The bigger you are, the easier it is to maintain plate tectonics. The smaller a planet is, the more it needs other agents to help plate tectonics happen.” In fact, Valencia and colleagues at the Harvard-Smithsonian Center for Astrophysics wrote in The Astrophysical Journal Letters in 2007, plate tectonics on larger terrestrial planets is not only possible, it’s “inevitable.”

It comes down to size. Earth, the largest of the four terrestrial planets in our solar system, is also the only planet in our solar system with active plate tectonics. Two of the other rocky planets orbiting our sun, Mercury and Mars, have so-called “stagnant-lid convection” — meaning that, relative to the churning mantle forces driving plate tectonics, the lithosphere is too strong to rupture. Venus, meanwhile, is similar in mass to Earth — and indeed, may have once had plate tectonics. But its dense, carbon dioxide-rich atmosphere may have made the surface of the planet too hot for too long; if heat from the interior of the planet cannot escape efficiently, plate tectonics may shut down.

On larger planets like super-Earths, however, the lithosphere is thinner and convection is stronger, Valencia says. And the planet’s size isn’t the only factor at play. Other researchers have pointed out that bigger planets have more gravity, which also strengthens the resistance of the plates, possibly creating a stagnant-lid situation. The age of the planet matters as well, Valencia says. Younger planets, with more radioactive elements producing heat in their interiors to help drive mantle convection, are more likely to have plate tectonics.

But even if super-Earths are tectonically active, they may still not harbor life, Valencia says. To some extent, plate tectonics is an indicator, simply because the process is important to Earth. “It makes sense to look for life in a planet that might resemble our own,” she says. “That doesn’t mean life of a certain form couldn’t spring up on a planet with a stagnant lid — we don’t know because we don’t really know the whole variety of life. But what we do know is that certainly on Earth plate tectonics has played an important role in regulating the climate, which is important for the emergence of life.”

Super-Earth or mini-Neptune?

Although the larger size of super-Earths may make them more amenable to plate tectonics, other scientists say their size may actually make them too big to be habitable. In fact, there isn’t yet enough evidence to be sure they’re even terrestrial, says John Johnson, an astrophysicist at the University of Hawaii’s Institute for Astronomy in Manoa.

A lot of people in the scientific community think of super-Earths as simply scaled-up Earths, with solid surfaces, a thin atmosphere and plate tectonics, Johnson says. But “we don’t know what the physical composition of these planets is,” he says. “If I were to put my money on any idea, I’d think that these are going to be mini-Neptunes,” planets with a large, thick gaseous atmosphere and no well-defined solid surface.

Johnson says he’s placing his bet based on what we know of how planets form. Solid cores build up in an accretion process; once a planet reaches a high enough mass, it has enough gravity to start to pull in hydrogen gas from the surrounding solar system disk material. If it’s big enough — perhaps 300 times the mass of Earth — runaway gas accretion leads to a Jupiter-sized planet: a big core and a whole bunch of gas.

Even if the disk contains a little less gas, and a planet is a bit smaller (maybe five to seven Earth masses, the size of a super-Earth) the accreting planet is still “going to do its thing,” Johnson says: It will “pick up gas and material, get a mantle of rock or gas, and contain 5 to 10 percent, by mass, hydrogen or helium.” In other words, it’ll look exactly like Neptune.

Earth-sized planets may also initially have an envelope of hydrogen — ours did at one time. But the planet lacked sufficient mass, and therefore gravity, to hold onto it. However, for a planet the size of a super-Earth not to hold onto its thick, gassy atmosphere — to become a true rocky super-Earth — “you’d have to invoke a different [planetary formation] process” than scientists are currently familiar with, Johnson says.

There is a way to solve the question of whether a planet is a super-Earth or a mini-Neptune: Determine both its mass and radius. Terrestrial planets will have a significantly higher mass-to-radius ratio than gas giants. Scientists have done this for hot Jupiters and hot Neptunes using the transiting method. But because super-Earths are relatively tiny compared to their stars, the dip in brightness during a super-Earth’s transit across its star has been too difficult to detect.

Johnson and his colleagues are working on that problem. Last fall, they published a paper in Nature that showed how astronomers can obtain vastly improved photometric precision — enabling them to see differences in the brightness of stars down to 0.05 percent. That, it turns out, is the same percentage of change in a star’s brightness that occurs when a super-Earth-sized planet passes in front of it. So, for the first time, astronomers might be able to detect a smallish planet passing in front of a star, and from that, determine whether it’s a super-Earth or a mini-Neptune.

Now that they have the photometric precision to use the transiting method on these smaller planets, Johnson’s team is aiming its telescopes at a half-dozen or so super-Earths. “We’re rolling the dice really; it’s a probability game,” he says. “For each planet around a star, there’s a 10 percent chance you’ll [be in the correct orientation to] see a transit. A handful [of super-Earths] times 10 percent — that gives us a 60 percent chance we’re going to find one. I like those odds.”

New missions: The view from space

Astronomers may not yet know for sure what kind of planet super-Earths are, but that doesn’t mean they aren’t interested in finding more. NASA, ESA and other space agencies have a variety of exoplanet-finding projects in the works, which astronomers hope will help them not only answer questions about super-Earths, but also find even smaller planets.

The first off the ground was ESA’s CoRoT mission. CoRoT, with its very sensitive light-detecting instruments, can spot the tiny dips in brightness of a planet transiting across a star. NASA’s Kepler spacecraft, which launched in March, will also hunt for planets transiting across their stars — but it will be able to find much smaller planets than even CoRoT can find, smaller even than the mass of Earth. “Kepler’s the first step to finding out about true Earths,” says Sara Seager, an astronomer at MIT in Cambridge, Mass., and a member of the Kepler science advisory team.

During its three-and-a-half year mission, Kepler will look at 100,000 stars, essentially taking a census of how common Earth-sized planets are around stars like our sun. Kepler will provide an “unambiguous” statement of just how frequent Earth-like planets are across the galaxy, Boss says.

Another NASA project, called the Space Interferometry Mission or SIM Lite, may be the best hope of finding an Earth-like planet right next door. Currently in development, SIM Lite will have enough precision to detect visible wobbles in a star’s movement (rather than the spectroscopic wobble that Doppler looks for) — so scientists hope it will be able to detect planets just slightly larger than Earth.

Another project, the Microlensing Planet Finder, could also find many potentially habitable planets. The telescope would look for “free-floating” Earth-sized planets that were ejected from their solar systems due to the gravitational influence of other planets. Led by David Bennett of the University of Notre Dame in South Bend, Ind., the proposed four-year mission would monitor 100 million stars in the “galactic bulge,” the tightly packed group of stars at the center of a spiral galaxy. The mission would be able to detect planets one-tenth the mass of Earth, with orbits bigger than Mercury’s around the sun — making them at the right distance to support life. The mission’s designers say the project could discover as many as 150 planets as massive as Earth, and 5,000 or so Jupiter-mass planets.

Other space missions, however, are meant to do more than just find planets. The Hubble and Spitzer Space Telescopes (launched in 1990 and 2003, respectively) have analyzed the atmospheres of about a dozen hot Jupiters, identifying molecules such as water vapor and methane, and possibly carbon dioxide. These are gas giants, so “it’s not a sign of life,” Seager says. “But just the fact that we can identify molecules on distant planets is a really big step forward.”

And when it launches in 2013, the James Webb Space Telescope will do for super-Earths what the Hubble and Spitzer telescopes have done for hot Jupiters: stare at their atmospheres and see what molecules it can detect. This upcoming launch is partially why “there’s a big push to find more super-Earths,” Seager says, “to have candidates to follow up” and search for signs of life.

Super-Earth: What’s in a name?

Definitions of the habitable zone in both galaxies and solar systems are rapidly changing. Some scientists have recently challenged the idea that stars stay put in their original location in the galaxy — suggesting that habitable zones, too, might migrate. And the solar system concept of a habitable zone leaves out places, such as an ocean possibly buried under the ice on Jupiter’s moon Europa, that might be acceptable to at least some forms of microbial life.

“The real goal is finding anything that must have [originated separately] from life on Earth,” Boss says. “We’re not necessarily talking about having to find worlds … with dinosaurs and then humans,” he says. “We’d be quite happy to find worlds where things are a little more dicey, where slime mold grows. It’s easier to find evidence for slime mold than humans.”

Defining the habitable zone is a question that is “always in an astronomer’s mind,” Valencia says. Super-Earths have captured people’s imagination because they seem so much more similar in some ways to our own planetary home, she says. “The gas giants are not seen as habitable because we do not experience life in a planet like this. We only understand life in our planet so we look for planets similar to ours ... it’s a safer route. That doesn’t mean that we won’t be surprised.”

Johnson has another perspective on the search for life on other worlds and the hunt for super-Earths: Focusing the quest too narrowly on habitable planets may mean missing the forest for the trees, he says. “People project their own hopes and dreams of finding the next Tatooine,” aka Luke Skywalker’s home planet in the “Star Wars” movies. “In our haste to get to a habitable planet,” he adds, “I think we’re overlooking something really neat.” From a planetary formation perspective, for example, super-Earths or mini-Neptunes both have interesting implications, he says. “We’re jumping the gun a little bit. We’ll get there. But they’re very interesting astronomical objects in their own right.”

Seager, however, says super-Earths are really just a stepping stone to the real prize: other Earths. “We’re interested in all questions about exoplanets, but there are some people who are stuck on super-Earths,” who view them as a replacement for Earth, she says. “That’s not really the case. If we could find Earths, people wouldn’t care about super-Earths.” As for habitability, Seager says, “we have to be optimistic that [Earth-like] planets are habitable. We can do all the calculations we want, but ultimately, nature will have something in store for us.”

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