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.