Home Science

Where Did Earth's Trojans Go? Ask the Moon – Sky & Telescope

The Essential Guide to Astronomy
The Essential Guide to Astronomy
Solar System
By: Govert Schilling May 2, 2022 comment 0

Why doesn’t Earth have Trojan asteroids of its own? Large impacts in the early years of the solar system may be to blame.
Giant impacts on our newly formed planet, like the smash-up that led to the formation of the Moon, may have evicted small asteroids from stable regions in Earth’s orbit around the Sun, according to a University of Michigan team led by Kevin Napier.
Thousands of Trojan asteroids are known to populate similar stable regions in Jupiter’s orbit — the L4 and L5 Lagrange points, which are located 60° ahead and behind the planet as it goes around the Sun. Neptune and Mars also host a fair number of Trojans. In contrast, Earth’s L4 and L5 points appear to be largely empty, except for two small objects that are probably temporary denizens.
According to Napier’s colleague Larissa Markwardt, Earth Trojans are “some of the hardest objects to study from Earth,” because they always hang out in the vicinity of the Sun from our point of view. Existing surveys indicate that there could be at most something like 100 of them larger than a few hundred meters across. But the gravitational stability of the L4 and L5 regions implies many “primordial” Trojans ought to have survived since the earlies days of Earth’s formation, when small bodies known as planetesimals filled the solar system. So where are they all?
Napier, Markwardt, and their colleagues argue that large collisions like the one that created the Moon may have been responsible. Their numerical simulations show that “such collisions can be highly disruptive to the primordial Trojan population and could have eliminated it altogether,” the authors write in a paper accepted for publication in The Planetary Science Journal (preprint available here).
As Napier explains, a giant collision can produce small changes in the shape of Earth’s orbit: both its overall size and its ovalness, or eccentricity. “These changes, in turn, destabilize the Trojans,” he says. “Several relatively small and less severe impactors also cause Earth to lose most, if not all, of its Trojans.” All in all, the authors conclude that if Earth acquired the final 1% of its mass through a dozen or so major collisions, those collisions would have unbound about 99% of the previously bound Trojans.
Then again, debris from a big collision can also end up in a stable Lagrange point. According to Markwardt, that seems to have happened in the case of Mars. However, the resulting number is much smaller than the expected number of primordial objects.
That said, Bill Bottke (Southwest Research Institute) isn’t sure there ever was a large primordial population of Earth Trojans. He thinks growing protoplanets may have stirred up leftover planetesimals into eccentric and inclined orbits; few, if any, would have remained in the L4 and L5 regions.
“It’s not clear to me that the lack of Earth Trojans is an actual ‘problem’ per se,” Bottke says. “To make the case that their model explains why Earth Trojans are missing, [the authors] probably need to show results from a terrestrial planet formation simulation that leaves the Earth with Trojans prior to the Moon-forming event.”
A more thorough survey of the Earth’s L4 and L5 points may turn up more small members of our planet’s remaining Trojan population, which would certainly tell us more about its evolution. Markwardt is especially looking forward to NASA’s mid-infrared NEO Surveyor, slated for launch in 2026. “It’s my best bet” she says.
You must be logged in to post a comment.
Sky Tour Astronomy Podcast
By: J. Kelly Beatty May 1, 2022
Black Holes
By: Camille M. Carlisle April 30, 2022
This Week’s Sky At a Glance
By: Alan MacRobert April 29, 2022
Solar System
By: Arwen Rimmer April 28, 2022
Explore the Night with Bob King
By: Bob King April 27, 2022
Astronomy in Space with David Dickinson
By: David Dickinson April 26, 2022
Stellar Science
By: Govert Schilling April 25, 2022
Spacecraft and Space Missions
By: Monica Young April 24, 2022
People, Places, and Events
By: Diana Hannikainen April 22, 2022
Subscribe to Sky & Telescope Magazine
Sky & Telescope is part of AAS Sky Publishing, LLC, a wholly owned subsidiary of the American Astronomical Society. Sky & Telescope, Night Sky, and skyandtelescope.org are registered trademarks of AAS Sky Publishing LLC. Sky & Telescope maintains a strict policy of editorial independence from the AAS and its research publications in reporting developments in astronomy to readers.
Copyright ©2022 AAS Sky Publishing LLC. All rights reserved.


Previous articleYes, you can take a break from Netflix. Here’s how – TechHive
Next articleUpdate 'designed to improve user experience' takes down the Microsoft 365 Admin Portal – The Register