Results from NASA’s Kepler mission — launched to discover Earth-like terrestrial planets that orbit within or near the habitable zone of their host stars — have indicated that the most common planets in the galaxy are so-called super-Earths. Characterized as being larger in size than Earth but smaller than Neptune, there are no super-Earths in our own solar system, so astronomers have been using powerful space telescopes to learn more about these mysterious worlds.
Most recently, the Hubble telescope was used to study the planet HD 97658b, which resides in the Leo constellation. With results from the Kepler mission and observations from Hubble, astronomers are making more accurate inferences as to not only the planetary composition of our galaxy, but also its potential habitability.
Click here to skip to Kepler’s new findings.
What is a super-Earth?
In short, a super-Earth is a planet with a mass larger than that of the Earth and smaller than that of gas giants like like Uranus (15 Earth masses) and Neptune (17 Earth masses). Since there is not a super-Earth in our own solar system, super-Earths must also be exoplanets, or extrasolar planets, which means that they do not revolve around our own sun, but instead orbit some other star, stellar remnant, or a brown dwarf.
|Example of a super-Earth between Earth (left) and Neptune (right)|
Denoting only the size and mass of the planet, super-Earths have also been called “gas dwarfs” when referring to a planet at the higher end of the mass scale, although “mini-Neptunes” has become much more common.
Mini-Neptunes can be up to 10 Earth masses, have thick hydrogen-helium atmospheres, deep layers of ice and rock, and they also have liquid oceans made of water, ammonia, a mixture of both, or heavier volatiles. Several exoplanets have been discovered meeting this description, such as Kepler-11f (see image right), which has about 2.3 Earth masses, yet has about the same density as Saturn. The ratio of size to density of Kepler-11f tells us that it is a gas dwarf with a liquid ocean surrounded by a thick hydrogen-helium atmosphere and only a small rocky core.
A common misconception is that a super-Earth is potentially habitable. The term super-Earth refers only to the size and mass of an exoplanet with no implications for the surface conditions or its habitability.
An exoplanet is a planetary body of any size that orbits any star that is not our own. As of the exoplanet catalog update on October 10, more than 1,800 exoplanets have been discovered in over 1,100 planetary systems including 469 multiple planetary systems, or stars beyond our solar systems with at least two confirmed planets.1 Additionally, the Kepler mission has identified a four thousand candidate planets of which only 11% could be false positives, and that’s while it’s been looking in only one section of the sky.
Throughout the Milky Way, every star typically has at lease one planet2 and 1 in 5 Sun-like stars have an Earth-sized star in its habitable zone (also called the “Goldilocks” zone — it’s not too hot and not too cold, but ‘just right’ for liquid water), the nearest of which is within 12 light-years of Earth. Assuming there are 200 billion stars in the Milky Way, that would mean 11 billion potentially habitable Earth-sized planets, which would rise to 40 billion if red dwarf stars are included. Red dwarf stars are the most common in the universe, but they’re small, dim, and hard to detect because they put off very little light and heat.
There are also free floating planets, which do not orbit any star, that are considered separately from exoplanets and could number in the trillions in the Milky Way alone.
Planetary habitability and complex life
In astronomy and astrobiology, the habitable zone, or the Goldilocks zone, is defined as the region around a star (band-shaped in diagrams) in which planetary-mass objects with sufficient atmospheric pressure can support liquid water, the presence of which we now consider the first step toward the development of life. The idea of the habitable zone is based on the Goldilocks principle, which states that something should fall within certain margins as opposed to reaching certain extremes; when these effects are observed, it’s called the Goldilocks effect.
The bounds of the habitable zone, where liquid water would either evaporate from a star’s heat or free from being too distant, are determined using three things: known requirements of Earth’s biosphere, its position in the Solar System, and the amount of radiant energy, or energy from electromagnetic radiation, it receives from the Sun.
In terms of habitability, we must also be conscious of an exoplanets position within the galaxy itself. Dense galactic centers have much higher levels of radiation than the outer edges of galaxies, such as where our solar system is located within the Milky Way (see image right). The galactic habitable zone is a function of distance from the galactic center. As you venture further from the center, there is less effect from X-ray and gamma ray radiation from the black hole at the galactic center, and gravitational and orbital disruptions become less as stars and planets become fewer and farther between. It also becomes less likely for a planet to be struck by a meteorite or large bolide.
When Kepler was launched March 7, 2009, with the intent of searching for distant exoplanets, it also reinvigorated the search for extraterrestrial life. With regard to the likelihood of complex multicellular life existing elsewhere in the universe, there are two camps of theory. The Rare Earth hypothesis argues that the emergence of life on Earth required a highly improbably combination of astrophysical and geological events and circumstances. The term “Rare Earth” comes from Rare Earth: Why Complex Life is Uncommon in the Universe, written in 2000 by Peter Ward, a geologist and paleontologist, and Donald E. Brownlee, an astronomer and astrobiologist, both faculty members at the University of Washington.
The alternative view has been argued by the likes of Carl Sagan, Frank Drake, and others. Given the principle of mediocrity (also called the Copernican principle), it holds that because Earth is a typical rocky planet in a typical planetary system located in an unexceptional region of a common barred-spiral galaxy, it is possible, even probable, that complex life exists elsewhere in the universe.
Size and habitability
It’s not just our liquid water that gives us our stable atmosphere here on Earth. According to Nicolas Cowan, a researcher at Northwestern University, part of what gives us a temperate climate here on Earth is our exposed continents, which act as a sort of geological thermostat and stabilizing the climate.
The problem with super-Earths is their size. A super-Earth only twice the size of Earth would have 10 times the mass and 10 times the amount of water and the planet’s gravity would be triple that of Earth. The gravity would cause squash the planet’s topography by a factor of three, creating a mostly-level planet with very shallow basins. Given so much water and no place to contain it, this means it would be an ocean planet, inundated and entirely covered in water.
On Earth, we store a significant amount of our water in the mantle, but due to the higher gravity of a super-Earth and the compacted core, the negligible amount of water that could seep into the mantle still wouldn’t allow there to be any land. So for a while, many thought super-Earths with liquid water would be entirely aquatic. However, Cowan and his colleagues crunched numbers and realized the high gravitational pull would create massive pressure on the seafloor and force water into the exoplanet’s mantle.
With this model, we can put 80 times more water on a super-Earth and still have a surface that has continents like Earth’s. If exoplanets of this size, which are very common throughout the galaxy, turn out to have stable climates, then their potential to support life would be much higher than we thought. And the increase in gravity doesn’t rule out life. “Three Gs isn’t a problem for habitability,” Cowan said, adding, “fighter pilots can handle it.”
Kepler’s new findings
NASA’s Kepler spacecraft launched in 2009 and has since searched just one small patch of sky, yet has found more than 4,000 candidate exoplanets. According to the NASA site, Kepler’s mission is to:
Determine the percentage of terrestrial and larger planets that are in or near the habitable zone of a wide variety of stars
Determine the distribution of sizes and shapes of the orbits of these planets
Estimate how many planets there are in multiple-star systems
Determine the variety of orbit sizes and planet reflectivities, sizes, masses and densities of short-period giant planets
Identify additional members of each discovered planetary system using other techniques
Determine the properties of those stars that harbor planetary systems.
The results have shown that smaller planets, but larger than Earth, are the most common, the so-called super-Earths. While our observations have told us a lot about the sizes of these exoplanets, we’ve been able to learn very little about their composition.
There are a number of possibilities. A super-Earth could be just that: a bigger version of Earth — mostly rocky, with an atmosphere. Then again, it could be a mini-Neptune, with a large rock-ice core encapsulated in a thick envelope of hydrogen and helium. Or it could be a water world — a rocky core enveloped in a blanket of water and perhaps an atmosphere composed of steam (depending on the temperature of the planet).
“It’s really interesting to think about these planets because they could have so many different compositions, and knowing their composition will tell us a lot about how planets form,” says Heather Knutson, assistant professor of planetary science at Caltech. For example, because planets in this size range acquire most of their mass by pulling in and incorporating solid material, water worlds initially must have formed far away from their parent stars, where temperatures were cold enough for water to freeze. Most of the super-Earths known today orbit very close to their host stars. If water-dominated super-Earths turn out to be common, it would indicate that most of these worlds did not form in their present locations but instead migrated in from more distant orbits.
To date, nearly two dozen planets have been characterized with this technique. These observations have shown that the enormous gas giant exoplanets known as “hot-Jupiters” have water, carbon monoxide, hydrogen, helium — and potentially carbon dioxide and methane — in their atmospheres. Although hundreds of super-Earths have been found, only a few are close enough and orbiting bright enough stars for astronomers to study in this way using currently available telescopes.
The first super-Earth that the astronomical community targeted for atmospheric studies was GJ 1214b, in the constellation Ophiuchus. Based on its average density (determined from its mass and size), astronomers didn’t think that the planet was entirely rocky. However, its density could also be explained by either having a primarily water composition or a Neptune-like composition with a rocky core surrounded by a thick gas envelope. Information about the atmosphere could help astronomers determine which one it was: a mini-Neptune’s atmosphere should contain lots of molecular hydrogen, while a water world’s atmosphere should be water dominated.
Disappointingly, after a first Hubble campaign led by researchers at the Harvard-Smithsonian Center for Astrophysics, the spectrum came back featureless — there were no chemical signatures in the atmosphere. After a second set of more sensitive observations led by researchers at the University of Chicago returned the same result, it became clear that a high cloud deck must be masking the signature of absorption from the planet’s atmosphere.
Now Knutson’s team has studied a second super-Earth, HD 97658b, in the constellation Leo. But again the data came back featureless.
One explanation is that HD 97658b is also enveloped in clouds. However, it’s also possible that the planet has an atmosphere that is lacking hydrogen. Because such an atmosphere could be very compact, it would make water vapor and other molecules very hard to detect.
Part of the reason why exoplanets and super-Earths are such a hot topic right now is that they’re easier to find and much easier to study. Although scientists would love to study more Earth-sized exoplanets on their search for habitable Earth-analogs, right now super-Earths are on the edge of our technological capability. But that shouldn’t be the case for long. NASA’s James Webb Space Telescope, touted as the premier observatory of the next decade and scheduled to launch in 2018, will provide the first opportunity to study more Earth-sized exoplanets.
“But super-Earths are a good consolation prize,” says Knutson. “They’re interesting in their own right, and they give us a chance to explore new kinds of worlds with no analog in our own solar system.”
You can read the original Caltech article here.