Was there ever a fifth rocky, terrestrial planet in our Solar System? If so, it was located beyond the orbit of Mars in what is now the asteroid belt. John Chambers (Carnegie Institute of Washington) likes to call the hypothetical world ‘Artemis,’ and at the 2006 Astrobiology Science Conference in Washington DC this March, he described how the planet might have formed.
The trick, of course, is to account for the orbits of the giant planets, which some believe underwent a shift from almost circular to more highly eccentric (elliptical) orbits. If you set up a simulation with Jupiter in a circular orbit, terrestrial worlds form out to about 2.2 AU. From an abstract of Chambers’ presentation:
Artemis could have formed in a region that was stable before the giant planets’ shift, but unstable thereafter, probably between 1.8-2.2 AU. We simulate the giant planets’ orbital shift to explore Artemis’ demise, varying Artemis’ mass and starting location. In each simulation, the giant planets’ eccentricity jump causes a increase in the terrestrial planets’ eccentricities, sometimes causing their orbits to cross and collisions to occur.
What Chambers finds is that in most scenarios involving collisions or ejection of a planet from the Solar System, the remaining terrestrial worlds are pushed into orbits that are more elliptical than we observe today. If Artemis ever existed, it must have been destroyed quickly to make today’s orbits possible. An abstract of Chambers’ paper “Planet Artemis: the case for the formation and delayed destruction of a fifth Solar System terrestrial planet” can be found here.
Nah. Anybody with enough science fiction novels under his or her belt knows that the planet that was there before the asteroid belt was called Lucifer. It was destroyed by the Old Ones of Mars when it was decided they would be trouble down the road.
;)
As far as I know, the current theory behind the formation of Earth’s moon is that a Mars-sized planetesimal slammed into the earth about 4 billion years ago, and that a bunch of earth’s crust was knocked off into orbit, eventually coalescing into the moon. If Chambers’ simulations are correct, then Artemis fits the profile of the planet that hit the earth so long ago.
I don’t quite understand this. How can the asteroids be in the same stable orbit that a planet cannot? Doesn’t Jupiter’s gravity attract them just the same? Wouldn’t their low mass tend to make them even less stable in this environment?
Sorry — I was clumsy in explaining Chambers on that point. What he’s saying is that in the early era when (and if) Artemis was a factor, it would have had effects on the other planets that would today be obvious. Here’s what he says in the abstract:
“In simulations where Artemis is 1/3 Earth mass or larger, Mars is typically destroyed via ejection or a collision, and Mercury often falls into the Sun. In cases with a Mars-mass Artemis, either Mars or Artemis is ejected. However, the remnant terrestrial planets often have higher orbital eccentricities than observed today because of multiple close encounters. Rapid destruction of Artemis is needed to keep terrestrial eccentricities low.”
As I read that, we can infer that Artemis could not have been long-lived or we would see much more evidence of its existence than we do. The asteroid belt, on the other hand, would not have the gravitational pull to cause these severe disruptions in the orbits of the other planets.
I love Fred’s reference above to the science fictional Lucifer! Another science fiction reference, apropos to robot_guy’s comment, is the movie When Worlds Collide (from the 1932 Wylie and Balmer novel). Good idea about Artemis as a potential moon-forming object!
Ah, Bronson Alpha and Bronson Beta. Good pair of novels (When Worlds Collide/After Worlds Collide). Too bad they never got around to writing the third novel.
It’s interesting to think about all the visions of the Belt that SF has come up with and compare them with reality. For example, the news that Ceres might have more water than Earth–how about that for a target for a manned mission? Talk about opening up the solar system!
Hi All
Artemis couldn’t have been Theia (the Moon-making impactor) because the angular momentum balance would be all wrong. Apparently to get the Earth-Moon system of today the impactor had to have almost zero hyperbolic excess when it collided with proto-Earth. The best guess is that Theia formed in one of the Earth-Sun L4/5 Trojan points, then when its mass was high enough it became unstable and started ‘walking’ around its orbit until it collided.
Perhaps perturbations by Artemis got it going?