Fingerprinting the formation of giant planets

Difference in chemical composition between the stars 16 Cyg A and 16 Cyg B, versus the condensation temperature of the elements in the proto-planetary nebula. If the stars had identical chemical compositions then the difference (A-B) would be zero. The star 16 Cyg A is richer in all elements relative to star 16 Cyg B. In other words, star 16 Cyg B, the host star of a giant planet, is deficient in all chemical elements, especially in the refractory elements (those with high condensation temperatures and that form dust grains more easily), suggesting evidence of a rocky core in the giant planet 16 Cyg Bb. Credits: M. Tucci Maia, J. Meléndez, I. Ramírez.

A team of Brazilian and American astronomers used CFHT observations of the system 16 Cygni to discover evidence of how giant planets like Jupiter form.

One of the main models to form giant planets is called “core accretion”. In this scenario, a rocky core forms first by aggregation of solid particles until it reaches a few Earth masses when it becomes massive enough to accrete a gaseous envelope. For the first time, astronomers have detected evidence of this rocky core, the first step in the formation of a giant planet like our own Jupiter.

The astronomers used the Canada-France-Hawaii Telescope (CFHT) to analyze the starlight of the binary stars 16 Cygni A and 16 Cygni B. The system is a perfect laboratory to study the formation of giant planets because the stars were born together and are therefore very similar, and both resemble the Sun. However, observations during the last decades show that only one of the two stars, 16 Cygni B, hosts a giant planet which is about 2.4 times as massive as Jupiter. By decomposing the light from the two stars into their basic components and looking at the difference between the two stars, the astronomers were able to detect signatures left from the planet formation process on 16 Cygni B.

The fingerprints detected by the astronomers are twofold. First, they found that the star 16 Cygni A is enhanced in all chemical elements relative to 16 Cygni B. This means that 16 Cygni B, the star that hosts a giant planet, is metal deficient. As both stars were born from the same natal cloud, they should have exactly the same chemical composition. However, planets and stars form at about the same time, hence the metals that are missing in 16 Cygni B (relative to 16 Cygni A) were probably removed from its protoplanetary disk to form its giant planet, so that the remaining material that was falling into 16 Cygni B in the final phases of its formation was deficient in those metals.

The second fingerprint is that on top of an overall deficiency of all analyzed elements in 16 Cygni B, this star has a systematic deficiency in the refractory elements such as iron, aluminum, nickel, magnesium, scandium, and silicon. This is a remarkable discovery because the rocky core of a giant planet is expected to be rich in refractory elements. The formation of the rocky core seems to rob refractory material from the proto-planetary disk, so that the star 16 Cygni B ended up with a lower amount of refractories. This deficiency in the refractory elements can be explained by the formation of a rocky core with a mass of about 1.5 – 6 Earth masses, which is similar to the estimate of Jupiter's core.

"Our results show that the formation of giant planets, as well as terrestrial planets like our own Earth, leaves subtle signatures in stellar atmospheres", says Marcelo Tucci Maia (Universidade de São Paulo), the lead author of the paper. "It is fascinating that our differential technique can measure these subtle differences in chemical abundances; we achieve a precision that was unthinkable until now", adds team member Jorge Meléndez (Universidade de São Paulo). Ivan Ramírez (University of Texas) concludes: "16 Cyg is a remarkable system, but certainly not unique. It is special because it is nearby; however, there are many other binary stars with twin components on which this experiment could be performed. This could help us find planet-host stars in binaries in a much more straightforward manner compared to all other planet-finding techniques we have available today."

The team is composed of the PhD student Marcelo Tucci Maia, Prof. Dr. Jorge Meléndez (Universidade de São Paulo) and Dr. Iván Ramírez (University of Texas at Austin). This research will appear in the paper “High precision abundances in the 16 Cyg binary system: a signature of the rocky core in the giant planet”, by M. Tucci Maia, J. Meléndez and I. Ramírez, in the  Astrophysical Journal Letters.  

Contact information:

Media contact

Dr. Daniel Devost
Canada-France-Hawaii Telescope
(808) 885-3163
devost@cfht.hawaii.edu

Science contacts

Marcelo Tucci Maia
Universidade do São Paulo
marcelotuccimaia@usp.br

Prof. Jorge Meléndez
Universidade de São Paulo
jorge.melendez@iag.usp.br

Dr. Ivan Ramírez
University of Texas
ivan@astro.as.utexas.edu

Source: Canada-France-Hawaii Telescope (CFHT)

Giant planet ejected from the solar system

Artist's impression of a planet ejected from the early solar system
Image courtesy of Southwest Research Institute.
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This animation shows the evolution of giant planets from 20 million years before the instability to 30 million years after the instability (the actual simulation covered a much longer time span). Five initial planets are shown by red circles, small bodies are in green. The fifth planet is ejected at the instability causing a general disorder. The system of the remaining four planets stabilizes after a while, and looks like the outer solar system in the end, with giant planets at 5, 10, 20 and 30 astronomical units. This is just one of more than 6,000 simulations performed to study the likelihood of planet ejection. Animation courtesy of Southwest Research Institute.

Boulder, Colo. — Nov. 10, 2011 — Just as an expert chess player sacrifices a piece to protect the queen, the solar system may have given up a giant planet and spared the Earth, according to an article recently published in The Astrophysical Journal Letters.

"We have all sorts of clues about the early evolution of the solar system," says author Dr. David Nesvorny of the Southwest Research Institute. "They come from the analysis of the trans-Neptunian population of small bodies known as the Kuiper Belt, and from the lunar cratering record."

These clues suggest that the orbits of giant planets were affected by a dynamical instability when the solar system was only about 600 million years old. As a result, the giant planets and smaller bodies scattered away from each other.

Some small bodies moved into the Kuiper Belt and others traveled inward, producing impacts on the terrestrial planets and the Moon. The giant planets moved as well. Jupiter, for example, scattered most small bodies outward and moved inward.

This scenario presents a problem, however. Slow changes in Jupiter's orbit, such as the ones expected from interaction with small bodies, would have conveyed too much momentum to the orbits of the terrestrial planets, stirring up or disrupting the inner solar system and possibly causing the Earth to collide with Mars or Venus.

"Colleagues suggested a clever way around this problem," says Nesvorny. "They proposed that Jupiter's orbit quickly changed when Jupiter scattered off of Uranus or Neptune during the dynamical instability in the outer solar system." The "jumping-Jupiter" theory, as it is known, is less harmful to the inner solar system, because the orbital coupling between the terrestrial planets and Jupiter is weak if Jupiter jumps.

Nesvorny conducted thousands of computer simulations of the early solar system to test the jumping-Jupiter theory. He found that, as hoped for, Jupiter did in fact jump by scattering from Uranus or Neptune. When it jumped, however, Uranus or Neptune was knocked out of the solar system. "Something was clearly wrong," he says.

Motivated by these results, Nesvorny wondered whether the early solar system could have had five giant planets instead of four. By running the simulations with an additional giant planet with mass similar to that of Uranus or Neptune, things suddenly fell in place. One planet was ejected from the solar system by Jupiter, leaving four giant planets behind, and Jupiter jumped, leaving the terrestrial planets undisturbed.

"The possibility that the solar system had more than four giant planets initially, and ejected some, appears to be conceivable in view of the recent discovery of a large number of free-floating planets in interstellar space, indicating the planet ejection process could be a common occurrence," says Nesvorny.

This research was funded by the National Lunar Science Institute and the National Science Foundation. The paper, "Young Solar System's Fifth Giant Planet?" by Dr. David Nesvorny was published online by The Astrophysical Journal Letters.

Editors: An image and animation to accompany this release are available at: http://swri.org/press/2011/giant-planet.htm.

For more information, contact Maria Martinez, (210) 522-3305, Communications Department, Southwest Research Institute, PO Drawer 28510, San Antonio, TX 78228-0510.

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