Are 'Super-Earths' and 'Habitable Zones' Misleading Terms?
There’s been a lot of buzz lately about planets in habitable zones, particularly with the recent discovery of an Earth-sized planet orbiting a star in a zone where liquid water could exist.
But Geoff Marcy, an exoplanet researcher at the University of California at Berkeley, urges the astronomy community to adopt caution over the term “habitable zone.” Earth is still the only place in the Universe that we know has life. His team recently wrote about this in a survey paper looking at worlds close to Earth’s size around Sun-like stars.
The paper, “Occurrence and core-envelope structure of 1–4x Earth-size planets around Sun-like stars,” is available in preprint version on Arxiv and has been accepted for publication in the Proceedings of the National Academy of Science.
“The key result is that planets that are between the size of the Earth and let’s say, twice the size of the Earth are extraordinarily common,” said Marcy, whose team has discovered more than 250 planets around other stars.
“Something like 26 percent of all of Sun-like stars have a planet between the size of the Earth and twice the size of the Earth orbiting at a distance from the star that’s within about half the Earth-Sun distance.”
Looking at planets close to Earth’s size
We not only can detect these worlds, but can also characterize them. NASA’s Kepler Space Telescope allows researchers to measure the diameter of each planet by seeing how much light it blocks from the star it passes across. Mass is determined by measuring the gravitational “wobbles” a star has as the planet orbits around it.
Since the density of the planet is simply its mass divided by its volume — a calculation any elementary school child learns — researchers are able to characterize if the planet is solid, gas, icy or a combination of these states.
Marcy’s survey of the research shows an interesting change when a planet is 50 percent larger than the Earth. Our planet’s larger cousins show increasing amounts of gas, with smaller densities. In real terms, that means that these large planets have a rocky core and are swaddled in immense amounts of gas, similar to Neptune. This turns the idea of a “super-Earth” on its head, Marcy said.
Moving to mini-Neptunes
The term “super-Earth” is a misleading term because it makes researchers think that these larger planets would be similar to Earth in terms of gravity, atmosphere, and overall habitability. That’s not actually the case, Marcy contends.
“I strongly recommend we abandon the term ‘super-Earth,’ and we have to abandon it because the term implies something like the Earth, but just bigger. Frankly these aren’t,” he said. “These are planets that have a lot of gas. They are much closer kin to Uranus and Neptune in our solar system that have a rocky core, but large amounts of gas and even some water.”
Marcy said the better term for these worlds would be “sub-Neptune,” because their structures are more similar to Neptune than they would be to Earth. He warned that scientists have a tendency to “over-dramatize” their discoveries, because the next best thing to finding an Earth is finding something that is close to Earth.
That said, astronomers recently spotted a planet that is close to Earth’s size, located within what many say is the habitable region of its star. Called Kepler-186f, the planet is about 500 light-years from Earth and within a five-planet system. Its star is a red dwarf, which is slightly cooler than that of our own sun, but Kepler-186f is theoretically close enough for liquid water to be on its surface.
It’s the “theoretically” part that makes Marcy cautious about making any life-friendly predictions at all, however.
‘Split brain’ on habitability
The “burdensome” problem for the scientific community in describing a habitable zone is that scientists know little about where life forms, Marcy said.
“There’s a split brain that we scientists have right now. Half of our brain says there’s a habitable zone and it lies between a region inward of where the Earth is and a region outside the Earth’s orbit [for stars our size],” he said. “The other half of our brain knows perfectly well that excellent destinations for our search for life lie elsewhere in the Solar System.”
Marcy mentioned Jupiter’s icy satellite Europa, as well as Saturn’s hydrocarbon-swaddled moon Titan and its fountaining Enceladus. These moons contain either water or compounds similar to Earth’s atmosphere. They receive energy from their gravitational interactions with a massive gas giant planet.
Even the processes by which life arose are under debate. Did it come to be through chemical processes on Earth, or was it transported through comets or other small bodies? How did amino acids became proteins or complex DNA? For Earth itself, was the presence of the Moon important? Does life have to be carbon-based like the life forms on our own planet?
“If we don’t understand how life started here on Earth, how would we know if life could start around any other star?” Marcy said.
The value of studying other planets
One driver of exoplanet research is assessing habitability. If life isn’t the driving force behind that search, what else could the planets teach us? Marcy says there is still a lot we can learn.
His survey shows two kinds of small planets besides Earth. There are ones that are 50 percent larger with rock and lots of gas, and ones 50 percent smaller that are purely rock. Marcy has an idea of what may make those larger ones so much more replete with gas, with a thicker, Neptune-like atmosphere, than the smaller ones. (Atmosphere could be one of the metrics linked to habitability, although Marcy believes one can’t say for sure.)
“Perhaps nature makes rocky planets roughly the size of the Earth fairly commonly, but some of the time there’s still some gas floating around in the early planetary nebula, and in that case these rocky Earth-sized planets might gravitationally attract some of that gas.”
This means that the “sub-Neptunes” Marcy describes could be nothing more than Earth-like planets that found themselves surrounded by hydrogen and helium late in their formation. Marcy suggests the planet would have then formed in a two-step process: first coalescing into something close to Earth, and afterwards picking up the blanket of gas over times.
“The key question for astrobiology is how common are planets that just have a bare rocky surface like the Earth, with a very, very weak atmosphere,” he added.
Helium can’t evaporate away due to sunlight because it is too light, and even hydrogen is unlikely to have left easily because the star must be at a certain proximity to make the atmosphere hot enough for the hydrogen molecules to achieve escape velocity.
Marcy frames the search for habitable exoplanets as a challenge to the coming generation of astronomers, planetary scientists, biologists and molecular chemists. It’s not only about the search for planets like our own, but also the processes that lead to life itself.