A FIFTH terrestrial planet may once have orbited between Mars and Jupiter. Although gravitational disturbances would have sent the planet hurtling into the sun or out into space long ago, traces of this long-gone world may still be visible in part of the asteroid belt today.
Recent simulations have suggested that the gas giants of our solar system formed with circular orbits but moved into their more elongated paths about 4 billion years ago – 700 million years after the solar system formed. While the gas giants were in circular orbits, rocky planets should have formed in stable orbits out to a distance of 2.2 astronomical units (1 AU = 1 Earth-Sun distance).
However, there are no planets between Mars, which lies at 1.5 AU from the sun, and Jupiter at 5.2 AU. That puzzled Sean Raymond of the University of Colorado in Boulder and John Chambers of the Carnegie Institution in Washington DC. “There’s room for another planet between Mars and Jupiter,” says Chambers. “Given that planets formed everywhere else, why couldn’t another planet have formed there?”
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“There’s room for another planet between Mars and Jupiter. Why couldn’t another planet have formed there?”
The researchers modelled what would have happened in that region, and found that a planet about the size of Mars could have formed 2 AU from the sun and remained stable there until the orbits of the gas giant changed.
Their simulations show that the migration of Jupiter and Saturn greatly disturbed the orbits of other planets, and this could have kicked the fifth rocky planet out of its orbit – either into the sun or out of the solar system altogether. If the planet was swallowed by the sun, it was probably too small to leave any measurable trace in the sun’s composition. “But it’s possible the orbits of the asteroids today show some memory of having had a planet in the asteroid belt,” says Chambers.
Raymond believes this evidence lies in a family of asteroids called Hungaria, which are clustered at 1.9 AU. These bodies orbit in a plane tilted by about 25 degrees to the main disc of the solar system, which suggests they may have been swept off course in wake of the lost planet as it ploughed through the asteroid belt, says Raymond. “They’re in this tiny little area that’s just barely stable – I don’t know how else they would have gotten there.”
If it can be confirmed that the planet once existed, it would imply that planetary systems are dynamic environments, says Chambers.
The researchers presented their results at the Astrobiology Science Conference in Washington DC last week.
Sun’s twin is strong candidate for life
Astronomers have found a twin of the sun, the first such star to be spotted in a decade and only the second ever. They say that these stars are our best bets for finding Earth-like planets with life on them.
Jorge Meléndez, Katie Dodds-Eden and José Robles of Mount Stromlo Observatory near Canberra, Australia, identified the new solar twin, called HD98618, among a sample of 16 sun-like stars whose spectra they observed in fine detail with the 10-metre Keck I telescope on Mauna Kea in Hawaii. They say HD98618 is the second closest solar analogue after 18 Scorpii.
Like the sun, both HD98618 and 18 Scorpii orbit about 26,000 light years from the centre of the Milky Way. The stars are 1 per cent hotter and spin a little faster than the sun, making them about half a billion years younger. Both have roughly the same concentration of heavy elements as the sun. These elements are crucial to the formation of Earth-like planets and the emergence of life ().
Another cause for optimism is the absence of “hot Jupiters”, massive gas giants orbiting close to each star whose gravity could destabilise the orbits of Earth-like planets. However, it is possible that there are Jupiter-mass planets further out and close to the star’s habitable zone, says Meléndez.
The astronomers urge researchers searching for extraterrestrial intelligence to give top priority to the two stars. “Both are 4 billion years old, so hypothetical terrestrial planets may have had enough time to produce complex life, assuming a timescale for its development similar to Earth,” says Meléndez.
Marcus Chown
