Why Does Earth Have No Super-Earth Cousins?

In this artist's conception, gas and dust-the raw materials for making planets-swirl around a young star. The planets in our solar system formed from a similar disk of gas and dust captured by our sun. Credit: NASA/JPL-Caltech

In this artist’s conception, gas and dust-the raw materials for making planets-swirl around a young star. The planets in our solar system formed from a similar disk of gas and dust captured by our sun. Credit: NASA/JPL-Caltech

New discoveries of many super-Earth planets orbiting very close to sun-like stars have started to make Earth’s solar system look a little lonely. The mystery of why Earth ended up without any super-Earth cousins continues to puzzle astronomers as they try to better understand the unusual history of the Solar System and the evolution of planets in general.

The uniqueness of our solar system first became clear when astronomers began discovering hundreds of extrasolar planets orbiting other stars. They recently found that around 30 to 50 percent of stars similar to the Sun have hot super-Earths—planets orbiting very close to their parent stars with up to 10 times as much mass as Earth. Now, researchers hope to develop computer simulations that can show how the Solar System evolved and also explain the other possible paths of planetary formation.

“The Solar System is just one possible outcome of planet formation and it might be an unusual case,” said Sean Raymond, an astronomer at the Laboratoire d’Astrophysique de Bordeaux in Bordeaux, France. “Still, we have way more data on the Solar System than on any other system so it’s not easy to come up with a model that matches what we see.”

One big unknown is how hot super-Earths form in the first place, a mystery that has spawned at least two competing theories among astronomers. Raymond discussed such mysteries and controversies surrounding planetary formation during a talk titled “Formation and Volatile Content of Terrestrial Exoplanets,” at the Space Telescope Science Institute at Johns Hopkins University on April 28.

Birth of the super-Earths

Some astronomers favor an “in-situ formation” model, which suggests hot super-Earths grew very fast near their parent stars because they had enough mass from huge disks of rocky debris. But Raymond doesn’t buy this idea for several reasons.

First, Raymond notes that the scenario requires a very large amount of mass close to the star —something that conflicts with observations about dusty debris disks surrounding other stars, and the current understanding of disk physics. Second, many studies have shown that Earth-sized planets within such disks tend to migrate toward their parent stars from farther out rather than stay still.

Kepler-186f is within ten percent the size of Earth. Kepler-186f orbits its star once every 130-days and receives one-third the energy that Earth receives from the sun, placing it on the outer edge of the habitable zone (shaded region in green). The system is also home to four inner planets, all smaller than 1.5 times the size of Earth. Because Kepler-186 is cooler and dimmer, the habitable zone is located closer in. All five planets in this system have orbital distances to their star less than Mercury’s distance to the Sun. Credit: NASA/Ames/JPL-Caltech/T. Pyle

Kepler-186f is within ten percent the size of Earth. Kepler-186f orbits its star once every 130-days and receives one-third the energy that Earth receives from the sun, placing it on the outer edge of the habitable zone (shaded region in green). The system is also home to four inner planets, all smaller than 1.5 times the size of Earth. Because Kepler-186 is cooler and dimmer, the habitable zone is located closer in. All five planets in this system have orbital distances to their star less than Mercury’s distance to the Sun. Credit: NASA/Ames/JPL-Caltech/T. Pyle

“Hot super-Earths are still not well understood. A decent number of people have jumped on the ‘in-situ formation’ bandwagon and I spend a decent amount of time persuading them to jump off,” Raymond said.

Instead, Raymond favors the “inward migration” model that suggests super-Earths first formed farther out from their parent stars. That means the planets originally formed in a colder region beyond the so-called “frost line,” the point where water and other chemical compounds that contain hydrogen will condense into solid ice. By comparison, Earth probably formed inside the frost line and later received its water in the form of asteroids or other debris from beyond the frost line.

Understanding the origins of super-Earths would represent a crucial first step toward unraveling their many mysteries. Astronomers also want to figure out why hot super-Earths have become so common around other stars, as well as understanding the structure of super-Earth planets.

Angry gas giants

Another big mystery about planetary formation involves the evolution of gas giants such as Jupiter and Saturn. About 15 percent of all known stars have such gas giants, but only 10 percent of these planets circle their parent stars in “well-behaved” Jupiter or Saturn orbits.

“We think that Jupiter and Saturn formed far faster than Earth even though they are hundreds of times more massive,” Raymond said. “We think that the gas giants first formed 5 to 10 Earth mass cores and then gas gravitationally accreted on top.  Exactly how the cores grew is not known.”

An artist's rendition of Kepler 186-f, the first Earth-sized planet found in its star's habitable zone. Credit: NASA/Ames/JPL-Caltech/T. Pyl

An artist’s rendition of Kepler 186-f, the first Earth-sized planet found in its star’s habitable zone.
Credit: NASA/Ames/JPL-Caltech/T. Pyl

The other 90 percent of known gas giants occupy “angry gas giant” orbits that loop in very close to the parent star and go far out beyond the parent star in elliptical orbits. Astronomers suspect the eccentric “angry gas giant” orbits come about because of gas giant planets having orbits that crisscrossed one another. That allowed the planets to exert their gravitational pull on one another until one ended up being ejected completely from the star system, leaving a survivor with its loopy orbit.

Raymond cautioned that the number of known gas giants is still too low to really say whether “well-behaved” Jupiter and Saturn represent the rare few in a sea of “angry gas giants.” But astronomers do have ideas about how the formation of such gas giants could have influenced the Solar System’s unique array of planets.

Jupiter’s “Grand Tack”

So how did the unique configuration of Earth’s solar system come to be? Raymond and several colleagues came up with one possible explanation in a paper published in the July 14, 2011 issue of the journal Nature. Called the “Grand Tack” model, the idea suggests that Jupiter once migrated inward toward the Sun as far as the current orbit of Mars, before retreating back to the Outer Solar System.

Figure showing the beginning of the sequence of events in The Grand Tack model developed by Walsh et al (Nature, 2011). Credit: Kevin Walsh, Walsh et. al., 2011, Nature

Figure showing the beginning of the sequence of events in The Grand Tack model developed by Walsh et al (Nature, 2011). Credit: Kevin Walsh, Walsh et. al., 2011, Nature

Jupiter’s movement would have acted like a gravitational shovel in the early Solar System by pushing about half the dust and rocky material in front of it and scattering the rest behind it. Jupiter’s quirky journey would help explain why Mars ended up with less planet-making material in its region and eventually ended up smaller than Earth or Venus. As Jupiter retreated back into the Outer Solar System beyond the frost line, its “shovel” could have also tossed icy asteroids and other debris over its shoulder into the Inner Solar System—perhaps delivering water to Earth and other planets in the process.

Jupiter’s visit to the Inner Solar System could also explain why Earth has no super-Earth cousins in its neighborhood, Raymond said. He and other astronomers suspect fast-forming gas giants such as Jupiter can block super-Earths from migrating inward toward their parent stars. In the Solar System’s case, the planetary cores of Uranus, Neptune, and even Saturn may have originally become super-Earths if not for Jupiter.

Simulating planet formation

Raymond and his colleagues—an international team mostly located in the U.S. and France—hope to develop more supporting evidence for that theory in the near future. They’re working to refine their computer models so that they can eventually predict how planets will form rather than just explain what astronomers already see in the Universe.

Sean Raymond, astronomer at the Laboratoire d'Astrophysique de Bordeaux in Bordeaux, France. Credit: Sean Raymond

Sean Raymond, astronomer at the Laboratoire d’Astrophysique de Bordeaux in Bordeaux, France. Credit: Sean Raymond

“What sets my group apart is that we try really hard to have a coherent story,” Raymond said. “We try to tie the initial conditions we use to observations or models, and the outputs of our simulations to something that is measured (like the orbital configuration of a planetary system).”

The astronomers also continue looking to the stars for more observational evidence that can improve their simulations. Raymond and his colleagues have focused in particular on the disks of cold, dusty debris surrounding older stars as possible “signposts” for star systems that could eventually form terrestrial planets such as Earth or its super-Earth cousins.

“We are trying to test this now by searching for low-mass planets around stars known to have debris.”