What if a beefed-up version of Earth were suddenly dropped into the solar system between Mars and Jupiter? That’s the mind-boggling question posed in a new study that seeks to understand how “super-Earths,” a class of planets that is very common in other star systems, might affect our own solar neighborhood.
As it turns out, an exoplanet in this part of our solar system could doom life on Earth in numerous ways, including straight-up tossing our planet into the Sun or causing it to become lost in interstellar space. Super-Earths, which can be anywhere from 1.5 to 10 times as massive as Earth, could even disrupt bigger planets, such as the ice giants Uranus or Neptune, sending them reeling out of the solar system.
The thought experiment is not just about speculatively shaking up our solar system, though that is certainly part of the fun. On a broader level, though, these imaginary super-Earths can “provide important insights into the question of how typical our solar system architecture and evolution is compared with other planetary systems,” according to a recent study published in the Planetary Science Journal authored by Stephen Kane, a professor of planetary astrophysics at University of California Riverside.
“There are many planets in-between the size of Earth and Neptune—something which is maybe twice the size of the Earth—and we call these planets super-Earths,” Kane said in a call with Motherboard. “Because we don't have an analog to that kind of planet within our solar system, it has been a source of great lament amongst planetary scientists and exoplanet folks who wish that we did have a super-Earth, say between the orbits of Mars and Jupiter” that “we could study in detail.”
“This comes up so often in conversation—wishing that we had a super-Earth within the solar system—that I really wanted to explore this and say, ‘okay, let's just say our wish came true,’” he continued. “What would that look like? And what would the consequences of that be?”
Some scientists have suggested that there may be a planet lurking about 20 times farther from the Sun than Neptune, due to some weird gravitational effects that have been spotted in the outer reaches of the solar system. This hypothetical world, called Planet Nine, could be a large super-Earth, but it’s not at all clear that it exists. Even if a super-Earth was hidden far beyond the known planets, it would be very difficult to send a space mission to visit it across such a vast distance.
Our solar system’s lack of any known super-Earth is a bummer for scientists who study these curious worlds, because they present so many unanswered questions that might be resolved by a close-up look. For instance, it’s not clear at what point rocky planets like Earth transform into small gas giants, like Neptune and Uranus, or if some super-Earths might be habitable to life—potentially even more so than Earth-like planets.
While it appears that super-Earths are abundant in the Milky Way, with many found in our closest neighbor systems, the environments on these planets largely remain a mystery. So wouldn’t it be nice to just wave a wand and add one to our solar system so we could take a closer look?
The answer is that “we should be careful what we wish for,” said Kane, who ran thousands of simulations that add super-Earths of various masses to our present-day system in order to game out the results over 10 million years.
Kane placed these new worlds between two to four astronomical units (au) from the Sun (one au is the distance between Earth and the Sun). At this distance, the super-Earths ended up in the sprawling gap between the orbits of Mars and Jupiter that is currently occupied by the main asteroid belt. The presence of these worlds in this location frequently created what are known as “mean motion resonances (MMRs),” which are relationships between orbital frequencies of various planets that can influence their trajectories around a star.
“The inner solar system planets are particularly vulnerable to the addition of the super-Earth planet, resulting in numerous regions of substantial system instability,” Kane noted in the study. “The broad region of 2–4 au contains many locations of MMRs with the inner planets that further amplify the chaotic evolution of the inner solar system. There are also important MMR locations with the outer planets within the 2–4 au region, with potential significant consequences for the ice giants.”
For instance, some of Kane’s super-Earths end up catapulting the inner planets, including Earth, into interstellar space—or worse, directly into the Sun. If this scenario played out, humans and all other life on Earth would either be thrust into cosmic darkness as we yeeted away from the Sun, or vaporized as our star swallowed our world whole.
In other scenarios, Earth might remain in the solar system, but be thrown into an unstable orbit that could put it on a collision course with other planets, an outcome that would probably eradicate life on our world. We could also end up in a new eccentric orbit that could produce wild climatic swings that would make life much more challenging.
“The climate starts to become very, very chaotic” in some orbits, Kane said. “When you have the Earth moving really close to the Sun and then really far out, you get periods where the Earth will freeze over when it's further out, and then will thaw out and get flash-heated by the Sun during the closest approach.”
In some scenarios, the added super-Earth itself gets booted from the solar system in short order, while in others, Neptune and Uranus are ejected into interstellar space. Only Jupiter and Saturn, the biggest giants of the solar system, remain relatively unperturbed by the presence of a new super-Earth.
The results emphasize the delicate balance at play between planets in our solar system, which could help scientists understand the role that super-Earths play in their planetary neighborhoods across the galaxy.
For instance, our solar system seems to be an outlier both because of the absence of these jacked-up Earths, and because of the presence of a huge gas giant, Jupiter, at a significant distance from the Sun, a situation that is relatively rare in other systems. Both of these factors have played a role in the habitability of Earth—and potentially other worlds during the lifespan of our solar system, including ancient Mars—making them important clues in the search for life elsewhere in the universe.
“What I'm trying to show here is that [adding a super-Earth] would have serious consequences,” Kane concluded. “I didn't fully appreciate, until I dug into it myself and ran my own simulations, how finely tuned our solar system is.”