Jupiter may be the solar system’s oldest planet

Jupiter was an early bloomer. A close look at the ages of rock and metal fragments from the birth of the solar system suggests the giant planet formed early on. Probably within the solar system’s first million years. If so, Jupiter’s presence could help explain why the inner planets are so small. It may even be responsible for Earth’s existence, a new study suggests.

Previously, astronomers estimated Jupiter’s age with computer models. These simulations show how solar systems form in general. Gas giants like Jupiter grow by piling on more and more gas. This gas comes from spinning disks of gas and dust around a young star. The disks typically don’t last more than 10 million years. So astronomers inferred that Jupiter formed by the time that the sun’s disk disappeared. It had to have been born at least 10 million years after the solar system started to form.

“Now we can use actual data from the solar system to show Jupiter formed even earlier,” says Thomas Kruijer. He is a geochemist. He studies the chemical composition of rocks. Kruijer did the research while at the University of Münster in Germany. He is now at Lawrence Livermore National Laboratory in California. To study Jupiter, one of the biggest objects in the solar system, he and colleagues turned to some of the smallest: meteorites.

Meteorites are lumps of material from space that land on Earth. Most meteorites come from the asteroid belt. This is a ring of rock currently located between Mars and Jupiter. But those lumps of rock and metal were probably born elsewhere.

Luckily, meteorites carry a signature of their birthplaces. The gas and dust disk that the planets formed from contained different neighborhoods. Each had the equivalent of its own “zip code.” Each is enriched in certain isotopes. Isotopes are atoms of the same element that have different masses. Careful measurements of a meteorite’s isotopes can point to its birthplace.

Kruijer and colleagues selected 19 samples of rare iron meteorites. The samples came from the Natural History Museum in London, England, and the Field Museum in Chicago, Ill. These rocks represent the metal cores of the first asteroid-like bodies to congeal as the solar system was forming.

The team put a gram of each sample into a solution of nitric acid and hydrochloric acid. Then, the researchers let it dissolve. “It smells terrible,” Kruijer says.

They then separated out the element tungsten. It is a good tracer of both a meteorite’s age and birthplace. They also took out the element molybdenum. It is another tracer of a meteorite’s home.

The team looked at the relative amounts of certain isotopes of the elements: molybdenum-94, molybdenum-95, tungsten-182 and tungsten-183. From the data, the team identified two distinct groups of meteorites. One group formed closer to the sun than Jupiter is today. The other formed farther from the sun.

The tungsten isotopes also showed that both groups existed at the same time. The groups existed between about 1 million and 4 million years after the start of the solar system. The solar system was born about 4.57 billion years ago. That means something must have kept the two groups separated.

The most likely candidate is Jupiter, Kruijer says. His team calculated that Jupiter’s core had probably grown to about 20 times the mass of the Earth in the solar system’s first million years. That would make Jupiter the oldest planet in the solar system. Its early existence would have created a gravitational barrier: That barrier would have kept the two rock neighborhoods segregated. Jupiter would then have continued growing at a slower rate for the next few billion years. The planet topped out at 317 times the mass of the Earth.

The team reports Jupiter’s new age in the Proceedings of the National Academy of Sciences. The paper was published the week of June 12.

“I have high confidence that their data is excellent,” says Meenakshi Wadhwa. She works at Arizona State University in Tempe. She is a cosmochemist. That means she studies the chemistry of the matter in the universe. The suggestion that Jupiter held the different groups of space rocks apart is “a little more speculative, but I buy it,” she adds.

Jupiter’s early birth could also explain why the inner solar system lacks any planets larger than Earth. Many planetary systems far beyond the sun have large, close-in planets. These can be rocky planets a bit bigger than Earth, known as super-Earths. They are about two to 10 times the mass of Earth. Or, there can be gassy mini-Neptunes or hot Jupiters.

Astronomers have puzzled over why our solar system looks so different. If Jupiter formed early, its gravity could have kept most of the planet-forming disk away from the sun. That means there was less raw material for the inner planets. This picture is consistent with other work. That research suggests a young Jupiter wandered through the inner solar system and swept it clean, Kruijer says.

“Without Jupiter, we could have had Neptune where Earth is,” Kruijer says. “And if that’s the case, there would probably be no Earth.”