Early Jupiter Feasted on Super-Earths

* Super-sized Earths may have smashed into young Jupiter, causing it to lose part of its core.
* Scientists don't know why Jupiter's core is less massive relative to Saturn's.
* NASA plans to launch a new mission to Jupiter called Juno next summer.


NASA plans to launch a spacecraft called Juno next summer to flesh out details about Jupiter's internal structure, gravity and magnetic fields. Click to enlarge this image.

Mysteries abound on a bizarre moon orbiting Jupiter.

* Moons of 'Hot Jupiter' Planets Unlikely to Exist
* Jupiter's Cloud Band Vanishes from View

Jupiter sits today as the solar system's king planet, but it has a troubled past.

In a new study, scientists say baby Jupiter was so strongly bashed by giant Earths that it lost part of its core.

That would explain why Jupiter's core is disproportionately smaller today than sibling planet Saturn, says astronomer Douglas Lin, with the University of California in Santa Cruz.

Scientists suspect that gas giant planets like Jupiter and Saturn started off with rocky and/or icy bodies about 10 times the mass of Earth. At that point, their gravitational muscles would be strong enough to begin pulling gas from the surrounding region, causing them to balloon in size over time.

"In principle, all gas giants need to get to a critical core mass before they can start to accrete gas," Lin told Discovery News. "So the question is: Why is Jupiter's core mass so much smaller than Saturn's even though its total mass is so much bigger?"

Working with computer models, astronomers Shu Lin Li of Peking University in China, Craig Agnor with Queen Mary University of London, and Lin came up with one possible answer: Perhaps Jupiter started off like Saturn, but lost part of its core as it evolved.

The scientists calculated that if an object roughly five times the mass of Earth hit Jupiter, the impact would strike all the way to the planet's core. Add another three or four crashes and Jupiter's core would be eroded to roughly what it is today.

"Jupiter has always had this problem because if we believe the numbers... the core seems to be too small -- so small that it cannot accrete gas efficiently in the solar nebula. This scenario gets around the problem," Lin said.

NASA plans to launch a spacecraft called Juno next summer to flesh out details about Jupiter's internal structure, gravity and magnetic fields.

"Planets like Jupiter are known to be common in the universe so we would like to understand how they form," astronomer Dave Stevenson, with the California Institute of Technology in Pasadena, told Discovery News.

"They need not all form the way our Jupiter formed, but certainly if we can understand how our Jupiter formed we'd be a long way toward understanding how our solar system formed. Jupiter is the 800-pound gorilla in the room. Once formed, it has a major effect on the architecture of the solar system," Stevenson said.

Li's paper appears online at Cornell University's arxiv.org website.