Just 23 years ago, our solar system’s planets were alone in the universe.
Scientists naively presumed if we ever did discover planets around other stars, these worlds would look, well, familiar. “We imagined we were going to find other planetary systems in our own image,” says Andrew Howard, an astronomer at the University of Hawaii.
Boy, were we wrong. Among the 1,900-and-counting confirmed alien planets found so far, we’ve seen everything from bizarro, jumbo versions of Jupiter in scorchingly tight orbits to exoplanets dozens of times farther out than Neptune, and even worlds circling two stars, like Tatooine in Star Wars.
Yet perhaps the biggest exoplanetary surprise of all? The super-Earth. This class of planet — loosely defined as any world with up to 10 times Earth’s mass — is like nothing in our solar system. Super-Earths fall smack dab into a size and mass gap between Earth and the gassy worlds Uranus and Neptune. Talk about terra incognita.
Nor do super-Earths appear to be outliers. Astonishingly, this species of planet is the most common in the Milky Way, making up some 77 percent of the planetary quarry snagged by our biggest survey to date, with the Kepler space telescope. “We see these planets around every kind of star we look at,” says Zachory Berta-Thompson, an observational astronomer at the Massachusetts Institute of Technology. “Clearly nature likes to make them.”
For any real insight into these worlds’ essences, astronomers needed to find a transiting super-Earth, which would yield a size estimate. Once they knew a planet’s size and mass, high school physics would provide its density. (From your old notes: Density equals volume divided by mass.) Knowing an object’s density is akin to holding it in your hand as you gauge its weight in relation to its size, explains Berta-Thompson.
“At a very gut level here on Earth, if I want to figure out what something is, I pick it up,” he says. “I can say, ‘This is made of water, of wood, this is a balloon.’ ” With densities, scientists could judge super-Earths as fluffballs or medicine balls, as dead or possibly as living worlds. “Bulk density goes a long way to telling you the character of a planet,” says University of Hawaii’s Howard.
The wait ended in 2009, when astronomers divined the densities of two super-Earths. The first, named CoRoT-7b after the spacecraft that witnessed the transits, weighs about five Earth-masses, measuring about one-and-a-half times Earth’s width. The derived density figure confirmed CoRoT-7b as the first truly rocky exoplanet, heralded then as the most Earth-like known, though given the infernal proximity to its star, its surface must be molten.
The pendulum swung the other way for the second, to a lightweight called GJ 1214 b, still the most studied super-Earth. “We found it in my first year of grad school,” recalls Berta-Thompson, who, daunted by undergrad physics courses at Princeton, nearly became an art history major. “We’d just started this project, and I thought, ‘Wow, we’re finding planets!’ ” GJ 1214 b’s tale of the tape: about five Earths wide, with six-and-a-half times the mass, and a density several times lower than CoRoT-7b’s. The puffy world likely has a huge, gassy atmosphere, perhaps full of scalding water vapor.
In the past few years, a flurry of research has begun shedding light on these unprecedented planets. And the emerging picture is striking. This type of world is the planetary equivalent of Starbucks — everywhere you go, full of near-endless variations. Some, we think, are gaseous orbs, better described as mini-Neptunes. Solid, rocky super-Earths, on the other hand, could be covered completely in oceans of water — or lava. Super-Earths’ insides could contain hypercompressed ices that are paradoxically hot or be bejeweled with layers of carbon crushed into diamond. Ice and bling aside, some super-Earths could be just that: supersized Earths, largely indistinguishable from our own Blue Planet, at least from the surface.
This last possibility becomes even more pulse-quickening because super-Earths will be the first worlds we can telescopically probe for alien life. Plain ol’ Earth-size worlds, the first of which are now trickling into our exoplanet catalogs, will remain too small for our telescopes to study in any detail for years to come. So ongoing research is delving into super-Earths, from clouds tops to cores, to see if they have the right stuff for life.
“Super-Earths might be just as good as Earth [for life], if not better,” says Dimitar Sasselov, director of the Harvard Origins of Life Initiative in Cambridge, Mass. “Super-Earths as a family are the places where we should be looking for living planets.”
Strange New Worlds
The newfound significance of super-Earths is ironic, for these worlds have been right under our nose since the beginning. The very first exoplanets, discovered in 1992, are members of this class, although they don’t orbit a normal star. Instead, they make laps around a pulsar, the city-size remnant of a colossal star gone supernova, and send out beams of radiation. Discrepancies in these beams from the pulsar PSR B1257+12 suggested the presence of two interfering bodies — planets? — each with a mass about three times Earth’s.
The finding gobsmacked researchers, including Sasselov, who grew up ogling Jupiter’s moons through a backyard telescope in Bulgaria. “We were all wondering, ‘What kind of weird things are these?’ ” he says.
Scientists still debate the pulsar planets’ origins, and back then few people took these freakish would-be worlds seriously, anyway. The true exoplanet gold rush didn’t kick off until 1995 with the discovery of a so-called hot Jupiter in an infernally close orbit around a typical sunlike star. Finally, a (relatively) normal-looking planet!
Buoyed, astronomers began planning for the planet-harvesting mission that would launch 14 years later as Kepler. Over the space telescope’s first run, cut short due to a component failure in spring 2013, Kepler patiently stared at 150,000 stars, looking for the tiniest of flickers as planets crossed their faces — so-called “transits.” These crossings not only betray an exoplanet’s presence but also reveal its size, based on how much starlight the world blocks.
In 1999, while writing up the Kepler proposal, Sasselov wondered if we might find bigger versions of Earth. For lack of a better term, he blurted out “super-Earth.” “I said at the time, ‘I don’t necessarily want to use that word, so if you have a better option. . . .’ ” Sasselov recalls. “But people started using it, and now it’s become so entrenched.”
For years afterward, though, even as scores of hot Jupiters piled up, super-Earths remained elusive. Nevertheless, Sasselov, his student Diana Valencia and their colleague Richard O’Connell went out on a limb. In 2004 they submitted a paper speculating on theoretical super-Earths’ interior structures. The concepts were so unheard of that the journal editor struggled to drum up peer reviewers with relevant expertise.
A year later, these stabs in the dark paid off when researchers proved super-Earths are not just a funky phenomenon around pulsars. Prior scrutiny of the typical star Gliese 876 had rustled up two Jupiter-size companions, and further research revealed a third body, dubbed Gliese 876 d, pegged at 7.5 Earth-masses — the smallest-mass exoplanet then known.
“Gliese 876 d was really an important threshold event,” says Sasselov. The long-in-limbo interior structure paper he co-authored with O’Connell and Valencia was finally published in the journal Icarus in 2006, and super-Earth science was born.
For Valencia, this finding came in the nick of time. A physicist from Colombia, she was captivated by the idea of super-Earths, but “there was no data,” says Valencia, now an assistant professor of physics at the University of Toronto Scarborough. A colleague “teased me that I was studying imaginary planets.” Seeking a potential backup plan, Valencia took a summer seismology internship at Shell Oil. She was planning to return to Harvard, but the Gliese 876 d discovery sealed the deal. She left the oil industry and returned to her passion, never looking back. “I was lucky,” Valencia says. “The stars aligned.”
What Are Ye?
Valencia’s excitement proved justified, as ecstatic planet hunters added more super-Earths to the rolls. Yet for several years, scientists knew nothing else about these worlds except their masses. Without a direct analog in the solar system, no one could guess if these newfangled planets were predominantly rocky (Earth-like), gassy (Neptune-like), something in between (water worlds?) or all of the above. “That’s our first big question about super-Earths,” says MIT’s Berta-Thompson. “What the heck are they made of?”
For any real insight into these worlds’ essences, astronomers needed to find a transiting super-Earth, which would yield a size estimate. Once they knew a planet’s size and mass, high school physics would provide its density. (From your old notes: Density equals mass divided by volume.) Knowing an object’s density is akin to holding it in your hand as you gauge its weight in relation to its size, explains Berta-Thompson.
“At a very gut level here on Earth, if I want to figure out what something is, I pick it up,” he says. “I can say, ‘This is made of water, of wood, this is a balloon.’ ” With densities, scientists could judge super-Earths as fluffballs or medicine balls, as dead or possibly as living worlds. “Bulk density goes a long way to telling you the character of a planet,” says University of Hawaii’s Howard.
The wait ended in 2009, when astronomers divined the densities of two super-Earths. The first, named CoRoT-7b after the spacecraft that witnessed the transits, weighs about five Earth-masses, measuring about one-and-a-half times Earth’s width. The derived density figure confirmed CoRoT-7b as the first truly rocky exoplanet, heralded then as the most Earth-like known, though given the infernal proximity to its star, its surface must be molten.
The pendulum swung the other way for the second, to a lightweight called GJ 1214 b, still the most studied super-Earth. “We found it in my first year of grad school,” recalls Berta-Thompson, who, daunted by undergrad physics courses at Princeton, nearly became an art history major. “We’d just started this project, and I thought, ‘Wow, we’re finding planets!’ ” GJ 1214 b’s tale of the tape: about five Earths wide, with six-and-a-half times the mass, and a density several times lower than CoRoT-7b’s. The puffy world likely has a huge, gassy atmosphere, perhaps full of scalding water vapor.
Kepler’s recent haul of super-Earths has built on these findings and offered clarity on where super-Earths enter into lifeless mini-Neptunehood. A study last year co-authored by Howard brought the number of super-Earths with known densities to around four dozen. A study later in 2014 by California Institute of Technology’s Leslie Rogers concluded that a good rocky cutoff point is a width 3.2 times that of Earth. Below that girth, the planet is dense for its size, and likely rocky. At or above that figure, densities start to drop, despite bigger planetary sizes. Lighter wares — such as water, ice and gases rather than rock — must take up a swelling share of the volume of these larger, less-dense super-Earths.
The Air Up There
Pegging a world as rocky or gassy is, of course, only a first step toward assessing if life could call it home. Astronomers are now taking the next step of studying super-Earths’ atmospheres directly. During a transit, light from a host star filters through the atmosphere of an exoplanet before being eclipsed by the planet’s opaque bulk. Based on the colors of light that reach us, scientists can detect the “fingerprints” of specific molecules. With enough data, they can theoretically reconstruct an atmosphere’s overall makeup. The amounts and kinds of gases they observe offer clues not only to whether super-Earths can support life, but if in fact life is already there.
So far, exciting finds such as water vapor, carbon dioxide and methane have been spotted mostly in the mammoth atmospheres of super-Jupiters, which, like super-Earths, are gargantuan versions of worlds familiar to us. Rockier super-Earths have considerably smaller atmospheres, translating to less light reaching our telescopes. The results to date from the Hubble and Spitzer space telescopes have admittedly been underwhelming. Light collected sporadically from nearby GJ 1214 b and from another super-Earth, HD 97658b, are devoid of specific molecules’ fingerprints.
But the interpretation of these seemingly boring readings is stirring: These worlds are likely cloud-swathed, like Venus. High cloud decks apparently block light from individual molecules lower in their atmospheres, making it harder to identify them. Astronomers are still working on untangling the clouds’ signatures. Overall, it’s been good practice for what’s to come: Picking apart the molecular makeup of exoplanet atmospheres will actually be a chief goal of the next generation of telescopes, such as the successor of Hubble and Spitzer, the James Webb Space Telescope, set to launch in 2018.
Before JWST goes to work, astronomers want to be sure they can understand the data it will gather. Fortunately, the inaugural decade of super-Earth science has seen plenty of geophysical model-making, simulating the internal mechanics of an Earth on steroids.
Getting Under Super-Earths’ Skins
The most critical issue in determining a rocky super-Earth’s geophysics is its inherent beefiness. All that extra mass creates internal pressures far exceeding terrestrial squeezing, with implications for three life-critical planetary properties: the maintenance of oceans, climatic “thermostats” and magnetic fields.
These three phenomena all relate to what’s happening inside a planet. Take Earth, for instance. As the fledging world cooled from its initial molten state over hundreds of millions of years, its outermost layer solidified into a crust. This then cracked apart into plates, which bump and grind atop a warm, denser mantle region, surrounding a still-denser, molten metal layer. Beneath everything hides a solid iron core. Heat spewing from this region roils the mantle, like a burbling fondue pot. The crust’s plates dive underneath each other, plunging back into the mantle (triggering earthquakes) and melting down. Likewise, ocean water recycles through Earth’s mantle at a sufficient rate to maintain our world-spanning seas for eons. Both rock and water return to Earth’s surface through the volcanic cracks between the plates, perpetuating the cycle.
So far, so Earth-centric. What of super-Earths? Taking the matter of oceans first, models of super-Earth geology in a study co-authored by Sasselov earlier this year found that, yes, super-Earths could be hulking Blue Planets. They should preserve their oceans for billions of years, as well as or better than Earth, owing to adequate mantle recycling of water.
This cycling, enabled by plate tectonics, also influences whether super-Earths can have livable climates over long epochs. The key here is carbon dioxide, a greenhouse gas that traps heat from efficiently escaping into space. Rocks and seawater both absorb carbon dioxide from the atmosphere, sequestering away the heat-trapping carbon and cooling the planet. As these surface materials cycle into the mantle, the carbon is converted back into carbon dioxide gas and is returned to the atmosphere via volcanoes in a self-regulating process: When carbon dioxide levels in the atmosphere climb, more gas gets soaked up by rocks and water, curbing the literal degrees of planetary warming. This carbon sequestration diminishes, however, when lower carbon dioxide levels prevail, preventing a planetary chill from getting too deep. The upshot: Earth self-regulates its global temperature.
Do super-Earths also possess this thermostat? In October 2007, Valencia and her Harvard colleagues published a paper theorizing super-Earths have more active plate tectonics. Higher internal heat should overall create faster convection — that fonduelike mantle circulation. “The convection is more vigorous and the forces are larger, so it seems like it’s easier to have plate tectonics compared to Earth,” says Valencia. Such “super” tectonics would keep atmospheric carbon levels in check, meaning these worlds have more even-keeled climates than Earth. That same month, however, another paper suggested the opposite: Super-Earths’ stronger gravity dominates and keeps the crust from cracking into separate plates in the first place. Ergo, no tectonics, and quite possibly, no life. Eight years later, the matter remains unsettled, with subsequent research supporting both conclusions, though Valencia notes that more researchers suggest plate tectonics are possible.
Yet another big question mark on super-Earth habitability, stemming from planetary interiors, is the presence of a magnetic field. Earth’s field deflects much of the sun’s radiation that likely would have ended any upstart life. The sloshing of our world’s interior liquid-iron layer generates this shield. Higher pressures in super-Earths, however, would lead to higher melting temperatures. The planets’ interiors might stay solid and not separate out into Earth-style layers, according to a 2011 study. No liquid metal layer equates to no magnetic field, and no life.
But a separate study that year pointed to another possibility: The higher heat might melt magnesium oxide, a common mineral used in ceramics, and one that would be expected in ample quantity within super-Earths, too. This mineral, when liquefied and churning, could crank out a magnetic field.
Clearly, we need a better grasp of super-Earths’ inner workings to size up their habitability, and Sasselov’s research group continues to explore the possibilities through computer simulations. “We’re not simply running Earth-like interiors for bigger planets,” he says. “It involves some very interesting new physics.” New papers in the works will also sketch out how super-Earths’ insides influence the release of detectable gases into the atmosphere. As one example, learning the carbon dioxide abundance in a super-Earth’s atmosphere would help astronomers gauge whether it’s a temperate place or more like Venus, whose thick carbon dioxide atmosphere conspires with its solar proximity in raising its surface temperature to 900 degrees Fahrenheit.
Life’s Signs
Theories and models of livable climates are one thing, but Sasselov and his colleagues ultimately seek far bigger quarry: actual evidence of alien life. To find that, they need to figure out the combinations of gases, known as biosignatures, that could plausibly be produced only by life. A common example is methane in the presence of ample oxygen, as in Earth’s atmosphere. Typically, oxygen breaks down methane rapidly, and it also seeps into rocks (like carbon dioxide), so for both gases to endure in an atmosphere, something — likely biological — must keep putting them there.
“It’s this jewel of an idea, that life can really profoundly influence an exoplanet’s atmosphere,” says Berta-Thompson. “That’s so compelling when linked with the fact that we know how to study the atmosphere of a planet many tens of light-years away.”
By knowing which super-Earths are rocky and have geophysics conducive to life, astronomers can choose ideal targets for biosignature studies with next-generation instruments. And “targets” is the name of the game with the Transiting Exoplanet Survey Satellite (TESS), launching in 2017 and spearheaded by MIT. TESS will zero in on exoplanets transiting nearby bright stars — the easiest to study. Perhaps 20 objects in TESS’ anticipated planetary windfall should be super-Earth-caliber planets in the “habitable zone.” This is the not-too-hot, not-too-cold orbital distance from a star where life has a chance. “TESS is going to be a fire hose of incredible new planets,” says Howard. “It’s going to be a great machine.” JWST, meanwhile, will focus on the best candidates pinpointed by TESS and other surveys. New, huge ground observatories with mirrors a hundred feet across (nearly four times the size of today’s largest) will also join the party when they see first light in the 2020s.
Berta-Thompson can’t wait. “Even if these telescopes don’t tell us, ‘This is a planet covered in green slime,’ they will push us much farther down the road to that ultimate goal of finding life around other planets,” he says. “My wife is a microbiologist. She studies photosynthetic microbes in the ocean. My hope is that by the time we finish our careers, we’re working in the same field.”
If the history of exoplanet investigation is any guide, we should also expect surprises aplenty as we sink our teeth into super-Earths. “Nature is much more imaginative than we are,” says Valencia. “These planets really are a testament to that.”
[This article originally appeared in print as "Super Earths."]
