Gas Giants Bounce Around — and Collide — in Alien Solar Systems

The Cosmic Companion
Oct 31 · 5 min read

Gas giants around other stars often travel along highly-elliptical orbits, contrary to common thought, and massive collisions and interactions between gas giants may be to blame, a new study finds. The Cosmic Companion talks to lead researcher Renata Frelikh of UC Santa Cruz.

More than 4,000 worlds are now known to orbit stars other than our sun, and a fraction of these are giant worlds, like Jupiter and Saturn, orbiting close to their parent star. Basic laws of physics (as well as common intuition) would indicate that such a world should have a largely-circular orbits, due to the forces acting on the bodies.

Observations of large exoplanets near their stars, however, reveal just the opposite — that many of these worlds are tracing out highly-elliptical orbits as they race around the stellar companion.

A rocky planet is struck by a smaller body. Image credit: NASA/JPL-Caltech
A rocky planet is struck by a smaller body. Image credit: NASA/JPL-Caltech
A period of collisions between planets and other bodies during the formation of solar systems could have unexpected results, new simulations suggest. Image credit: NASA/JPL-Caltech

“A giant planet is not as easily scattered into an eccentric orbit as a smaller planet, but if there are multiple giant planets close to the host star, their gravitational interactions are more likely scatter them into eccentric orbits,” Renata Frelikh, a graduate student in astronomy and astrophysics at UC Santa Cruz, stated in a press release from UC Santa Cruz.

Get Together or Go Rogue

A new series of simulations show that massive planets which formed close to stars can interact with each other, radically altering each other’s orbits. During a giants-impact phase of planetary evolution, massive planets collide, building up even larger worlds. Our own Moon was likely formed as our budding solar system passed through this stage of development billions of years ago.

“Exoplanetary systems host giant planets on substantially noncircular, close-in orbits. We propose that these eccentricities arise in a phase of giant impacts, analogous to the final stage of solar system assembly that formed Earth’s Moon,” researchers describe in an article published in Astrophysical Journal Letters.

Some gravitational interactions between massive worlds are capable of sending planets out of their solar system, to soar free among the stars as rogue planets.

SImulated images of what it might be like to fly past hot Jupiters, massive worlds orbiting so close to their suns that they are tidally locked to their parent star, like the Moon is with the Earth. Columns represent various temperatures on the star-lit side of the planet, and rows represent various possible compositions of clouds, Image credit: NASA/JPL-Caltech/University of Arizona/V. Parmentier

As inertia rises with mass, it should be harder to alter the orbit of a more massive world than it would be to act on a smaller world. So, large worlds close to their local stars should tend to trace out near-circular orbits.

Smaller planets should, therefore, be more susceptible to this gravitational scattering than larger worlds. But, astronomers have detected giant worlds tracing out highly-elliptical paths around their parent star(s). These patterns are far different than that seen among our own family of worlds, where the inner solar system is filled with small planets, traveling along highly-circular orbits.

“Gas giant planets with orbital periods less than 400 days occur around about 5% of stars… We call these planets warm Jupiters,” Frelikh explains.

Exoplanets are usually found using one of two techniques. In systems where exoplanets travel in front of their star as seen from Earth, light from that star appears to dim as the planet passes between its sun and our home world. A regular pattern of dimming and brightening, unrelated to stellar processes, can reveal the presence of an exoplanet. The radial velocity method looks at the tiny gravitational tug a planet has on it’s parent star as a tell-tale sign of an alien world.

A look at how exoplanets may be found orbiting distant stars. Credit: NASA Ames Research Center

Using either of these methods, it is easier to find massive worlds close to their Sun. Highly-elliptical orbits also assist astronomers in finding gravitational pulls from undiscovered worlds, but this method works best for exoplanets close to their parent stars.

“It becomes difficult to detect planets this way beyond the distance of about Jupiter from the Sun. Planets smaller than Neptune are actually thought to be the most common type of exoplanet, but, especially at larger distances from the star, they become a lot harder to detect. The most massive planets would initially appear to be more common to us, and this is why when working with observational data sets it is crucial to consider the observational biases,” Frelikh tells The Cosmic Companion.

Practically A Planetary Mosh Pit

The team created a computer model based on a system containing 10 worlds. The total mass of each planet, as well as the total mass of the solar system, was altered each time a different simulation was conducted. Each simulation was run for 20 million (simulated!) years.

The planets modeled during this virtual investigation were much like Jupiter or Saturn, holding on to vast quantities of gases. Smaller worlds orbiting close to their stars can lose their atmospheres to space due to pressure coming from the nearby star. However, massive planets like the ones modeled in this study are able to retain their atmospheric cover.

“They will not lose a substantial amount of their atmospheres over their lifetimes because they are massive enough and far enough away from their host stars. For Jupiter-sized planets, atmospheric escape can become significant when they are extremely close to their host stars (closer than the orbit of Mercury from our Sun),” Frelikh describes for our readers.

Simulations showed planets interacting with each other and colliding, often forming larger bodies which continued to orbit near their parent star.

The largest planets produced in the simulations were produced at distances from the star between one and eight times greater than the distance between the Earth and Sun.

The final results of the study showed the systems with the greatest amount of total mass produced the largest worlds near the central star, and those planets had the greatest eccentricities seen in the virtual model.

This finding helps to answer mysteries of exoplanets, and could help researchers better model climates of distant worlds, some of which may be home to life.

Did you like this article? Subscribe to The Cosmic Companion Newsletter!

The Cosmic Companion

Exploring the wonders of the Cosmos, one mystery at a time

The Cosmic Companion

Written by

Writing about space since I was 10, still not Carl Sagan. Mailing List/Podcast:

The Cosmic Companion

Exploring the wonders of the Cosmos, one mystery at a time

Welcome to a place where words matter. On Medium, smart voices and original ideas take center stage - with no ads in sight. Watch
Follow all the topics you care about, and we’ll deliver the best stories for you to your homepage and inbox. Explore
Get unlimited access to the best stories on Medium — and support writers while you’re at it. Just $5/month. Upgrade