Astronomers discover planet that shouldn't be there

This is an artist's conception of a young planet in a distant orbit around its host star. 

The star still harbours a debris disk, remnant material from star and planet formation, interior to the planet's orbit (similar to the HD106906 system). 

Credit: NASA/JPL-Caltech

An international team of astronomers, led by a University of Arizona graduate student, has discovered the most distantly orbiting planet found to date around a single, sun-like star.

It is the first exoplanet – a planet outside of our solar system – discovered at the UA.

Weighing in at 11 times Jupiter's mass and orbiting its star at 650 times the average Earth-Sun distance, planet HD 106906b is unlike anything in our own Solar System and throws a wrench in planet formation theories.

"This system is especially fascinating because no model of either planet or star formation fully explains what we see," said Vanessa Bailey, who led the research.

Bailey is a fifth-year graduate student in the UA's Department of Astronomy.

It is thought that planets close to their stars, like Earth, coalesce from small asteroid-like bodies born in the primordial disk of dust and gas that surrounds a forming star.

However, this process acts too slowly to grow giant planets far from their star. Another proposed mechanism is that giant planets can form from a fast, direct collapse of disk material.

However, primordial disks rarely contain enough mass in their outer reaches to allow a planet like HD 106906b to form.

Several alternative hypotheses have been put forward, including formation like a mini binary star system.

"A binary star system can be formed when two adjacent clumps of gas collapse more or less independently to form stars, and these stars are close enough to each other to exert a mutual gravitation attraction and bind them together in an orbit," Bailey explained.

"It is possible that in the case of the HD 106906 system the star and planet collapsed independently from clumps of gas, but for some reason the planet's progenitor clump was starved for material and never grew large enough to ignite and become a star."

According to Bailey, one problem with this scenario is that the mass ratio of the two stars in a binary system is typically no more than 10-to-1.

"Every new directly detected planet pushes our understanding of how and where planets can form," said co-investigator Tiffany Meshkat, a graduate student at Leiden Observatory in the Netherlands.

"This planet discovery is particularly exciting because it is in orbit so far from its parent star. This leads to many intriguing questions about its formation history and composition."

"Discoveries like HD 106906 b provide us with a deeper understanding of the diversity of other planetary systems."

More information: The research paper, "HD 106906 b: A Planetary-mass Companion Outside a Massive Debris Disk," has been accepted for publication in The Astrophysical Journal Letters and will appear in a future issue. An online version is available for download at arxiv.org/abs/1312.1265