Astronomers Discover Planet That Shouldn't Be There

[rhoenix]

he discovery of a giant planet orbiting its star at 650 times the average Earth-Sun distance has astronomers puzzled over how such a strange system came to be.

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 106906 b 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 106906 b 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.

"In our case, the mass ratio is more than 100-to-1," she explained. "This extreme mass ratio is not predicted from binary star formation theories -- just like planet formation theory predicts that we cannot form planets so far from the host star."

This system is also of particular interest because researchers can still detect the remnant "debris disk" of material left over from planet and star formation.

"Systems like this one, where we have additional information about the environment in which the planet resides, have the potential to help us disentangle the various formation models," Bailey added. "Future observations of the planet's orbital motion and the primary star's debris disk may help answer that question."

At only 13 million years old, this young planet still glows from the residual heat of its formation. Because at 2,700 Fahrenheit (about 1,500 degrees Celsius) the planet is much cooler than its host star, it emits most of its energy as infrared rather than visible light. Earth, by comparison, formed 4.5 billion years ago and is thus about 350 times older than HD 106906 b.

Direct imaging observations require exquisitely sharp images, akin to those delivered by the Hubble Space Telescope. To reach this resolution from the ground requires a technology called Adaptive Optics, or AO. The team used the new Magellan Adaptive Optics (MagAO) system and Clio2 thermal infrared camera -- both technologies developed at the UA -- mounted on the 6.5 meter-diameter Magellan telescope in the Atacama Desert in Chile to take the discovery image.

UA astronomy professor and MagAO principal investigator Laird Close said: "MagAO was able to utilize its special Adaptive Secondary Mirror, with 585 actuators, each moving 1,000 times a second, to remove the blurring of the atmosphere. The atmospheric correction enabled the detection of the weak heat emitted from this exotic exoplanet without confusion from the hotter parent star."

"Clio was optimized for thermal infrared wavelengths, where giant planets are brightest compared to their host stars, meaning planets are most easily imaged at these wavelengths," explained UA astronomy professor and Clio principal investigator Philip Hinz, who directs the UA Center for Astronomical Adaptive Optics.

The team was able to confirm that the planet is moving together with its host star by examining Hubble Space Telescope data taken eight years prior for another research program. Using the FIRE spectrograph, also installed at the Magellan telescope, the team confirmed the planetary nature of the companion. "Images tell us an object is there and some information about its properties but only a spectrum gives us detailed information about its nature and composition," explained co-investigator Megan Reiter, a graduate student in the UA Department of Astronomy. "Such detailed information is rarely available for directly imaged exoplanets, making HD 106906 b a valuable target for future study."

"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."

Huh. That's interesting, to be sure. I've heard conjectures about Jupiter being essentially a "brown dwarf," as it emits more energy (in infrared) than it receives from Sol - but this planet's case seems like a much more extreme version, though still with an order of magnitude less mass than what was needed to ignite and become a star.

[LadyTevar]

There are more things Horatio than ever dreamed of in your philosophies....


So, this exoplanet is sitting at 640 AU, 1/100th the size of it's star, and still a "baby" as planetary systems go. They see a accretion disk still floating, so does that mean a possibility of more planets forming?

[Josh]

http://www.imdb.com/title/tt1832668/ <--------- Awesome series that covers planetary formation among other things, and is available on Netflix.

If there's still an accretion disk, all kinds of stuff is likely banging into each other around there. There were originally upwards of a hundred planet-type objects in our own system. The NINE worlds we have today are the battered, hardened survivors of what was an insane fratricidal struggle for survival.

[Lys]

Man, if you're going to include Pluto then you can't leave out Eris, which is actually bigger. So it's eight worlds or ten worlds. Pick one, nine is no longer an option.

[rhoenix]

Man, if you're going to include Pluto then you can't leave out Eris, which is actually bigger. So it's eight worlds or ten worlds. Pick one, nine is no longer an option.

Let's go with eight. If we count Pluto & Eris, we have to count Sedna, Ceres, Haumea, Makemake, and many other objects in the Kuiper Belt besides, and the number quickly gets ridiculous.

[Lys]

Sedna, Ceres, Haumea, Makemake, and all known Trans-Neutonian bodies besides Eris are smaller than Pluto. You could use mass to define "planet" such that Pluto counts but the others don't. Functionally, that was the definition of planet for a long time, because nobody expected to find anything bigger than Pluto. Then they found Eris in 2005, and it is bigger than Pluto, so you can't define a planet by mass and not include Eris. The only way we can have nine planets in the Solar System is if you include an AU limit in the definition.

As it is, I like the definition established by astronomers: an object that orbits the Sun, is massive enough to be made round by its gravity, and has cleared its neighbourhood of smaller objects around its orbit. That makes eight planets.

[Josh]

I'm sticking with Pluto under a grandfathering clause and sentimentality. I was always fascinated by that little rock, orbiting out there in the middle of nowhere.

Also, Pluto makes the case for itself:

http://brunching.com/conversationpluto.html

http://brunching.com/morepluto.html

[LadyTevar]

Its hard to think of "Eight Planets & the Dwarves" after Pluto's been part of "The Nine Planets" for 70+yrs. That's 70yrs of millions of schoolchildren learning about the tiny Pluto so far out, and the weird way it orbits. It's something that's gonna take more than a decade to unlearn.

[rhoenix]

True, and nearly everyone I speak to feels the same way. Knowledge evolves over time, as with all things - people will come around eventually.