​How Jupiter Could Have Killed Earth’s Older Siblings

By kicking them straight into the Sun.

Mar 24 2015, 3:20pm

Long before our planet came on the scene, super-Earths may have roamed the solar system. Our ancient siblings would have been larger, hotter, and probably not very hospitable. But we'll never know for sure, because before these ill-fated planets could grow up, Jupiter punted them into the Sun.

"Our work suggests that Jupiter's inward-outward migration could have destroyed a first generation of planets," said Caltech planetary scientist Konstantin Batygin, co-author of a new report on planetary dynamics in the early solar system, in a statement.

That hypothesis—one which Batygin and Gregory Laughlin of UC Santa Cruz worked out through a series of calculations and models—appears this week in the Proceedings of the National Academies of Sciences. The notion that Earth, along with Mercury, Venus and Mars, is actually a second-gen planet, and that super-Earths briefly encircled our Sun before their untimely demise, could help resolve a question that's puzzled astronomers for years: Why our solar system, cosmically speaking, is so damn weird.

Twenty years ago, astronomers could count the number of confirmed exoplanets on a single hand, but in the past five years, we've uncovered thousands more. Thanks to NASA's Kepler mission, we now know that roughly half of the sun-like stars in our galactic neighborhood probably harbor planets.

But as far as we can tell, most of these star systems look nothing like our own. "Typical" planetary systems consist of one or more super-Earths—planets larger than Earth but smaller than Neptune—in very tight orbits around their star. It's worth noting, however, that our datasets are incomplete, and our ability to see more distant worlds is limited by the current resolution of our telescopes. Still, by today's galactic standards, Earth is way off in the boonies.

"It appears that the solar system today is not the common representative of the galactic planetary census," said Batygin. "But there is no reason to think that the dominant mode of planet formation throughout the galaxy should not have occurred here. It is more likely that subsequent changes have altered [our solar system's] original makeup."

Batygin and Laughlin's new hypothesis—of ancient super Earths that were destroyed by a wrathful Jupiter—helps flesh out the picture of how our solar system evolved.

During the first few million years after our star was born, it was encircled by a thick, protoplanetary disk of gas and dust. It was from this disk that the planets eventually emerged, but they didn't form just anywhere. A older model, developed by researchers at UCLA, shows that for all of the terrestrial planets to end up in their current orbits, they would have needed to form in a very particular region of the disk—a narrow band of gas that sat 0.7 to 1 astronomical units away from the Sun. (1 AU is the distance between the Earth and the Sun today.)

Artist's impression of a protoplanetary disk. Mercury, Venus, Earth and Mars may have all formed from a narrow strip of such a disk close to the sun. Image:NASA/FUSE/Lynette Cook

What could have restricted planetary formation to this narrow strip of gas? The outer edge, it turns out, can be accounted for by the gas giants Jupiter and Saturn, which emerged very quickly following the birth of our Sun. Shortly after their formation, these giants were sucked in close to the Sun; a pair locked in a gravitational embrace. As they migrated inwards, their massive combined gravity forced a large gap in the surrounding disk of gas and dust, essentially clearing out the region all the way up to Earth's current orbit.

The notion that Jupiter and Saturn acted as the outer solar system's primordial dust-busters has been around for over a decade. But what delineated the inner edge of that planet-forming band has been a mystery. If Batygin is correct, the young Sun's close orbital neighborhood could have been cleared out by primordial super-Earths that were later destroyed. As for why these super-Earths vanished, that's where Jupiter gets really interesting.

According to Batygin and Laughlin's new model, as young Jupiter migrated inward toward the Sun, it swept rocky, 100 kilometer planetesimals along with it. As Jupiter thrust these planetesimals toward the Sun, they set off a violent chain of collisions, breaking other rocky bodies apart for up to 20,000 years. If a population of super-Earths were hanging about the inner solar system during this time, Jupiter's rocky avalanche would have cracked them open and kicked their remains straight into the Sun.

"It's a very effective physical process," said Batygin. "You only need a few Earth masses' worth of material to drive tens of Earth masses' worth of planets into the Sun."

Snapshot from Batygin and Laughlin's new simulation, showing the orbits of Jupiter (white) and the planetesimals (turquoise) that the gas giant helped sling into inner solar system, where super-Earths once roamed (yellow). Image: K. Batygin/Caltech

When Jupiter eventually backed off, some of the planetesimals it held in its gravitational sway settled down into circular orbits. These little nuggets of ancient destruction, would, over hundreds of millions of years, coalesce into the modern terrestrial planets—Mercury, Venus, Earth, and Mars. This scenario fits with other measurements which suggest the Earth formed 100 to 200 million years after the Sun.

If Batygin and Laughlin's scenario is correct, it would imply that the formation of gas giants such as Jupiter and Saturn—a process planetary scientists believe to be rare—helps to dictate whether a planetary system ends up looking like our own, or like the more typical variety with super-Earths in tight orbits around a parent star. As we continue to identify more exoplanet-harboring stars, new data will help us to contextualize our own solar system and understand just how unusual (or not) it is.

When all's said and done, it seems we ought to be thanking Jupiter for annihilating the first generation of terrestrial planets and setting the stage for our world to form. Still, I'm happy our friendly neighborhood gas giants decided to settle down far, far away. If there's one lesson to be learned here, it's that starting a cosmic turf war with Jupiter is basically suicide. You might as well fling yourself into the Sun and have it over with.