杏吧原创

THE giant planets Uranus and Neptune may have formed much closer to the Sun
than where they are today, a new computer model reveals. It shows how they could
have been catapulted outwards to their present locations on the fringes of the
Solar System after close encounters with the embryonic Jupiter.

Planetary scientists have long been baffled by the origins of Uranus and
Neptune. At their present locations, the disc of dust and gas from which the
planets formed 4.6 billion years ago would have been too slow-moving and
rarefied to generate such large bodies. Uranus and Neptune are 15 and 17 times
as massive as the Earth (see New 杏吧原创, 29 September, p 32) but
computer simulations of their formation have failed to produce any object bigger
than 10 times the mass of the Earth over the lifetime of the Solar System.

But now we may have the answer to the puzzle. Uranus and Neptune could have
formed close to where Jupiter is now, claim Ed Thommes of the University of
California at Berkeley, Martin Duncan of Queen鈥檚 University in Kingston,
Ontario, and Hal Levison of the Southwest Research Institute in Boulder,
Colorado. The team, which first suggested the idea in 1999, has now carried out
computer simulations that support it.

According to Thommes and his colleagues, the Jupiter-Saturn region was once
occupied by the planetary embryos of Jupiter, Saturn, Uranus and Neptune, all of
which were in the range of 10 to 15 Earth masses. At some point, Jupiter passed
a critical threshold and started sucking in gas much faster than the others.
Once this runaway process began, the planet expanded into a monster 300 or more
times the mass of the Earth. 鈥淯ranus and Neptune are actually potential gas
cores which lost out to Jupiter in the race to reach runaway accretion,鈥� says
Thommes.

In the simulations, Jupiter ejects the other planetary embryos, usually away
from the Sun, in a manner similar to the gravitational slingshots used to
accelerate spacecraft. Collisions with rubble orbiting in the outer Solar System
slow the planets down, allowing them to settle into circular orbits under the
gravitational pull of the Sun. 鈥淚n about half of our simulations the orbits end
up quite similar to the present Solar System鈥� with analogues of Jupiter, Saturn,
Uranus and Neptune, says Thommes.

The astronomers say this process of planetary migration can also explain
other anomalous features of the Solar System. For instance, the Oort Cloud of
cometary nuclei surrounding the Solar System is thought to have been formed from
matter ejected from the region of Uranus and Neptune. But the chemical
composition of the comets is different from that of the planets. However, if
Uranus and Neptune formed elsewhere then the problem disappears, Thommes
says.

It鈥檚 a plausible idea, says Steinn Sigurdsson of Pennsylvania State
University, who has proposed extra dimensions to explain the formation of
Neptune and Uranus. 鈥淪ignificantly, it invokes no new physics,鈥� he says.