When it comes to exoplanets, the most exciting for many people are, of course, the ones which may be the most Earth-like, since these are regarded as the most likely to possibly support some form of life. Now, two new findings announced today will help astronomers to find these worlds and narrow down the best places to search for evidence of life in other solar systems.
The new results, announced this morning at the annual meeting of the American Astronomical Society (AAS), focus on super-Earth and other potentially Earth-like exoplanets.
In the first update, new theoretical models for super-Earth exoplanets were presented, which show that such planets should be able to maintain water oceans for billions of years in some cases.
According to lead author Laura Schaefer of the Harvard-Smithsonian Center for Astrophysics (CfA): “When people consider whether a planet is in the habitable zone, they think about its distance from the star and its temperature. However, they should also think about oceans, and look at super-Earths to find a good sailing or surfing destination.”
She found that some super-Earths should be even better than maintaining oceans than Earth. Planets two to four times the mass of Earth could hold onto their oceans for at least 10 billion years, unless they were boiled away by a red-giant star. That’s quite a bit longer than Earth’s oceans have been around, and would allow more time for life to evolve.
“This suggests that if you want to look for life, you should look at older super-Earths,” Schaefer said.
In some cases, it may take longer for oceans to first form, but once they do, they should be able to remain on the surface for a very long time.
As on Earth though, water would likely need to be recycled from below the surface. Earth’s mantle is now thought to hold several oceans’ worth of water below ground, due to plate tectonics and subduction of the ocean seafloor. If it weren’t for volcanism releasing water back to the surface, Earth’s oceans probably would have disappeared a long time ago. Even so, water makes up a lot less of the planet’s bulk that you might think; only about a 10th of a percent is water, while the rest is rock and iron.
“Earth’s oceans are a very thin film, like fog on a bathroom mirror,” noted study co-author Dimitar Sasselov, also of CfA.
So older super-Earths might be some of the best planets to look for life, as Sasselov explained:
“It takes time to develop the chemical processes for life on a global scale, and time for life to change a planet’s atmosphere. So, it takes time for life to become detectable.”
Of course, as we’ve seen with moons like Europa and Enceladus in our Solar System, there may also be many more similar “water worlds” out there, where the oceans are underground, but could still support life of some kind. No exomoons have been confirmed yet, but astronomers think we are getting close as they continue to find smaller and smaller exoplanets.
In other research, scientists have come up with a “recipe” for other Earth-like exoplanets, the details of which were also presented at the AAS meeting.
“Our Solar System is not as unique as we might have thought,” said lead author Courtney Dressing of the Harvard-Smithsonian Center for Astrophysics (CfA). “It looks like rocky exoplanets use the same basic ingredients.”
The “recipe” for Earth suggests that it also applies for other rocky planets orbiting distant stars. The new findings were made using the HARPS-North instrument on the 3.6-meter Telescopio Nazionale Galileo in the Canary Islands. HARPS measures the masses of terrestrial rocky planets similar in size to the Earth. From that data, densities and compositions of those planets can then be determined.
As Harvard astronomer David Charbonneau explained, “Our strategy for using HARPS-North over the past year has been to focus on planets less than two times the diameter of Earth and to study a few planets really well.” Charbonneau, also of CfA, leads the HARPS-North Science Team.
The most recent exoplanet to be studied was Kepler-93b, which is 1.5 times the size of Earth and orbits its star in just 4.7 days. Using HARPS-North, astronomers determined the mass of the planet to be 4.02 times that of Earth, which means it has a rocky composition like Earth.
The analysis was the extended to the 10 known exoplanets with diameters less than 2.7 times that of Earth. Five of those planets, with diameters less than 1.6 times that of Earth, showed a close correlation between diameter and mass, similar to Venus and Earth in our own Solar System. This indicates that they likely also have rock and iron compositions.
There are some exceptions: low-mass planets with low densities, such as the planets in the Kepler-11 system, but they don’t seem to be as common. Larger and more massive exoplanets, similar to Uranus or Neptune or bigger, tend to have lower densities with compositions of water or other volatiles, hydrogen, and/or helium. With our Solar System, a common saying is that Saturn could theoretically “float on water” because its density is so low.
“To find a truly Earth-like world, we should focus on planets less than 1.6 times the size of Earth, because those are the rocky worlds,” suggested Dressing. She also noted that planets with 1-6 Earth masses are more likely to have compositions similar to Earth, while heavier ones would tend to be more like Neptune.
The new paper will be published in The Astrophysical Journal.