Super-Earths forming around alien stars may be pushed close to their stellar companions through the influence of gravity, new research from Penn State reveals. Computer simulations showing this behavior could answer some basic questions about the development of solar systems around alien stars.
Planets form from dust and small grains of matter orbiting in proto-planetary disks surrounding nascent stars. Within our Solar System, the smallest planets (Mercury, Venus, Earth and Mars) are found close to the Sun, while the gas giants Jupiter and Saturn are further out, and the ice giants of Uranus and Neptune are further still from our stellar companion.
However, astronomers searching for exoplanets are finding that other stellar systems come in a wide range of designs. Super-Earths, planets much like our own rocky world but larger, are found close to their suns, whizzing around their stars in a small fraction of the time taken by our own world. The question this research brings up is — why do these super-Earths orbit so close to their suns?
“With the first discoveries of Jupiter-size exoplanets orbiting close to their host star, astronomers were inspired to develop multiple models for how such planets could form, including chaotic interactions in multiple planet systems, tidal effects and migration through the gas disk. However, these models did not predict the more recent discoveries of super-Earth-size planets orbiting so close to their host star,” said Eric Ford, director of Penn State’s Center for Exoplanets and Habitable Worlds.
Hitting the Dusty Trail
Penn State researchers utilized computer simulations to study how super-Earths are affected by gravity as the coalesce out of the dusty disk from which they are born. The models showed that the orbits of these young planets can fall into sync, drawing planets, in a line, closer to their sun. Gravitational attraction between planets, and the effects of gas on outer planets, could pull super-Earths toward their stars in developing solar systems, the study revealed.
“When stars are very young, they are surrounded by a disc that is mostly gas with some dust — and that dust grows into the planets, like the earth and these super-Earths. But the particular puzzle for us is that this disc doesn’t go the all way to the star — there’s a cavity there. And yet we see these planets closer to the star than the edge of that disc,” said Dr. Daniel Carrera, assistant research professor of astronomy at Penn State’s Eberly College of Science.
Our own solar system also experienced planetary migrations. Jupiter likely formed further out in the solar system than it is today, and migrated toward the sun. Neptune may have once been closer to the Sun than Uranus, before both worlds migrated out to the extreme reaches of our planetary system.
“The migration of planets can be directed inward towards their star, or outwards away from their star. Planets with masses similar to Neptune and Jupiter, called Jovian planets, typically migrate inward. This inward migration of Jovian planets can explain the existence of “hot-Jupiters,” planets as massive as Jupiter with orbits smaller than that of Mercury because they are close to the sun,” explains Karna Desai of Indiana University.
As you will no doubt be aware, the plans for development of the outlying regions of the Galaxy require the building of a hyperspatial express route through your star system… All the planning charts and demolition orders have been on display at your local planning department in Alpha Centauri for 50 of your Earth years… What do you mean you’ve never been to Alpha Centauri? Oh, for heaven’s sake, mankind, it’s only four light years away, you know. I’m sorry, but if you can’t be bothered to take an interest in local affairs, that’s your own lookout. Energize the demolition beams.
— Douglas Adams, The Hitchhiker’s Guide to the Galaxy
Looking at it Another Way…
Planets are much smaller and dimmer than their parent stars, yet are only separated from their suns by a small angle as seen from Earth. This means that light from alien worlds is drowned out by their stellar companions, and photographing these planets is like trying to take a picture of a firefly, in front of a car headlight, from miles away.
Therefore, astronomers usually use one of two methods to find exoplanets around alien stars. For those worlds that pass between their sun and the Earth, light from the star dims as the planet passes “in front” of the star as seen from our world. Another method involves measuring the wobble produced by the star as planets orbit the massive body. Using either of these methods, it is easier to find larger planets, and those closer to their stars, then finding smaller worlds traversing larger orbits.
“Although it is true that large close-in planets are easier to detect, we know how to account for this selection bias and we can still say with confidence that close-in super-Earths are quite common. The most recent estimate is that 30% of Sun-like stars have close-in super-Earths. But that doesn’t mean that they are the most common type of planet in the universe. For example, if every star had a Mars-like planet, we wouldn’t know because we cannot detect a planet like that,” Carrera explained in an exclusive interview with The Cosmic Companion.
I Can See Clearly Now…
The Kepler Space Telescope discovered several thousand planets orbiting alien stars, and this work will be carried on the astronomers utilizing the TESS and James Webb space telescopes. Many planets, especially smaller worlds located far from the parent stars, are likely awaiting discovery by the next generation of space telescopes.
Like the Kepler spacecraft and the Hubble Space Telescope, these new observatories are likely to produce vast amounts of scientific data that we can now predict before data starts rolling in from these orbiting outposts.
“Both missions will vastly improve our understanding of super-Earths. TESS will dramatically increase the number of planets for which we know both the planet radius and its mass (thanks to follow up observations from the ground). This is a key step in understanding what these planets are made of (Are they rocky? Are they water worlds? Do they have large atmospheres like Neptune?). In contrast, James Webb will only study a handful of planets, but it will tell us a lot about the atmospheric chemistry of those planets,” Carrera tells The Cosmic Companion.
Fancy Meeting You Here!
Although these simulations describe one reason super-Earths may found close to their sun, it is not the only possible explanation to explain the positions of these worlds. Further research is required to determine other factors which may play roles in the migration of alien super-Earths, and further observations need to be conducted to confirm these findings.
“Super-Earths in very close orbits are by far the most common type of exoplanet that we observe, and yet they don’t exist in our own solar system and that makes us wonder why,” Carrera inquires.
So far, no one is certain why our own solar system seems to be so different from the systems we are finding around other stars. Perhaps, by learning more about distant solar systems, we may learn more about our family of planets right here at home.