Exoplanet formation surprise: Evidence of farthest planet forming from its star

Jun 13, 2013
This graphic shows a gap in a protoplanetary disk of dust and gas whirling around the nearby red dwarf star TW Hydrae, which resides 176 light-years away in the constellation Hydra, sometimes called the Sea Serpent. The gap's presence is best explained as due to the effects of a growing, unseen planet that is gravitationally sweeping up material and carving out a lane in the disk, like a snow plow. In the left image, astronomers used a masking device on the Hubble Space Telescope's Near Infrared Camera and Multi-Object Spectrometer to block out the star's bright light so that the disk's structure could be seen. The Hubble observations reveal that the gap, which is 1.9 billion miles wide, is not completely cleared out. The illustration at right shows the gap relative to the star. The Hubble observations were taken on June 17, 2005. Credit: NASA, ESA, J. Debes (STScI), H. Jang-Condell (University of Wyoming), A. Weinberger (Carnegie Institution of Washington), A. Roberge (Goddard Space Flight Center), G. Schneider (University of Arizona/Steward Observatory), and A. Feild (STScI/AURA)

A team of researchers has discovered evidence that an extrasolar planet may be forming quite far from its star—- about twice the distance Pluto is from our Sun. The planet lies inside a dusty, gaseous disk around a small red dwarf TW Hydrae, which is only about 55 percent of the mass of the Sun. The discovery adds to the ever-increasing variety of planetary systems in the Milky Way. The research is published in the Astrophysical Journal.

Astronomers using NASA's Hubble Space Telescope have found compelling evidence of a planet forming 7.5 billion miles away from its star, a finding that may challenge current theories about planet formation.

Of the almost 900 planets outside our solar system that have been confirmed to date, this is the first to be found at such a great distance from its star. The suspected planet is orbiting the diminutive red dwarf TW Hydrae, a popular astronomy target located 176 light-years away from Earth in the constellation Hydra the Sea Serpent.

Hubble's keen vision detected a mysterious gap in a vast protoplanetary disk of gas and dust swirling around TW Hydrae. The gap is 1.9 billion miles wide and the disk is 41 billion miles wide. The gap's presence likely was caused by a growing, unseen planet that is gravitationally sweeping up material and carving out a lane in the disk, like a snow plow.

The planet is estimated to be relatively small, at 6 to 28 times more massive than Earth. Its wide orbit means it is moving slowly around its host star. If the suspected planet were orbiting in our solar system, it would be roughly twice Pluto's distance from the sun.

Planets are thought to form over tens of millions of years. The buildup is slow, but persistent as a budding planet picks up dust, rocks, and gas from the protoplanetary disk. A planet 7.5 billion miles from its star should take more than 200 times longer to form than Jupiter did at its distance from the sun because of its much slower orbital speed and the deficiency of material in the disk. Jupiter is 500 million miles from the sun and it formed in about 10 million years.

TW Hydrae is only 8 million years old, making it an unlikely star to host a planet, according to this theory. There has not been enough time for a planet to grow through the slow accumulation of smaller debris. Complicating the story further is that TW Hydrae is only 55 percent as massive as our sun.

"It's so intriguing to see a system like this," said John Debes of the Space Telescope Science Institute in Baltimore, Md. Debes leads a research team that identified the gap. "This is the lowest-mass star for which we've observed a gap so far out."

An alternative planet-formation theory suggests that a piece of the disk becomes gravitationally unstable and collapses on itself. In this scenario, a planet could form more quickly, in just a few thousand years.

"If we can actually confirm that there's a planet there, we can connect its characteristics to measurements of the gap properties," Debes said. "That might add to planet formation theories as to how you can actually form a planet very far out."

The TW Hydrae disk also lacks large dust grains in its outer regions. Observations from the Atacama Large Millimeter Array in Chile show dust grains roughly the size of a grain of sand are not present beyond about 5.5 billion miles from the star, just short of the gap.

"Typically, you need pebbles before you can have a planet. So, if there is a planet and there is no dust larger than a grain of sand farther out, that would be a huge challenge to traditional planet formation models," Debes said.

The team used Hubble's Near Infrared Camera and Multi-Object Spectrometer (NICMOS) to observe the star in near-infrared light. The researchers then compared the NICMOS images with archival Hubble data and optical and spectroscopic observations from Hubble's Space Telescope Imaging Spectrograph (STIS). Debes said researchers see the gap at all wavelengths, which indicates it is a structural feature and not an illusion caused by the instruments or scattered light.

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GSwift7
2.3 / 5 (4) Jun 13, 2013
Hmmm, maybe the star is already starting to blow the disk away? Perhaps the gap was wider, but dust from the inner-most part of the disk being blown out, while the outer part of the disk is shaded from this effect by the inner dust. That would also explain why we see so much fine dust, since the finest of the dust would be blown out around the top and bottom of the dense central disk?

Sorry to just throw out a random armchair theory, but it seems just as reasonable as the above suggestion that the models are completely missing something.
VendicarE
5 / 5 (3) Jun 14, 2013
Clearly this is evidence of an invasion of the planetary system by giant invisible gelatinous aliens from Mars.
visualhawk
1 / 5 (1) Jun 14, 2013
Mmmh, interesting - nature will always find a way of doing things we incompetent little humans don't understand or have not thought of. More or less in the same league as the theory that Jupiter changed the orbits of the huge outer planets but somehow decided to leave the little inner planets alone.- that one still does not fly with me .... Can the ring be a density wave of some sorts rather than a planet ?
dusanmal
1 / 5 (3) Jun 14, 2013
At that distance and estimated size (even at the largest limit), by the present definition of the planet - this is not a planet... It won't be able to "clear the neighborhood" around the orbit for the lifetime of that system. Because of the size, it also doesn't qualify for the "dwarf planet" excuse, too big. Perfect example for overturning political planet definition and replacing it by a simple straightforward science: planet is object that primarily orbits the star and is in hydrostatic equilibrium (about round shape).
Q-Star
3.2 / 5 (9) Jun 14, 2013
At that distance and estimated size (even at the largest limit), by the present definition of the planet - this is not a planet... It won't be able to "clear the neighborhood" around the orbit for the lifetime of that system. Because of the size, it also doesn't qualify for the "dwarf planet" excuse, too big. Perfect example for overturning political planet definition and replacing it by a simple straightforward science: planet is object that primarily orbits the star and is in hydrostatic equilibrium (about round shape).


Which is exactly the reason the IAU did not include exoplanets in their new system of "planets" & "dwarf planets". Not enough is known about them to specifically include or exclude them. In not extending the definitions beyond our stellar system, they hoped to avoid another situation where a thing was placed in a group before enough data was collected to justify the inclusion in a category of objects , as happened with Pluto at the time of it's discovery.

Torbjorn_Larsson_OM
5 / 5 (3) Jun 14, 2013
Seems giant formation by gravitational instability (GI) proponents have jumped on this, because the disc is young. Likely because the balance of results have slowly but steadily tipped in favor of core collapse (CC) giant formation in recent years.

While GI can't be excluded, I am fairly skeptical. The age of the disk is not stressing CC, which comes in about 1 million year formation time from other observation. And then GI has worse problem than GC, since GC can form neptunes (~20-40 Earth masses, IIRC).

Core aggregation has always had problems. More likely this shows that it is easier than we think, but that we have not the right idea on how disks goes from grains to super-Earth size planets.
Torbjorn_Larsson_OM
5 / 5 (2) Jun 14, 2013
"as happened with Pluto at the time of it's discovery."

Or Ceres before that, which also was a solar system planet for ~ 50 years.

I think it is unlikely that we will ever observe another system to the degree that classification into populations and naming will become a problem. Unless we migrate, in which case we can reuse the IAU classification & naming systems there and then.
GSwift7
5 / 5 (2) Jun 17, 2013
I think it is unlikely that we will ever observe another system to the degree that classification into populations and naming will become a problem. Unless we migrate, in which case we can reuse the IAU classification & naming systems there and then


I think that depends on what we find to be true in regard to process(es) of system formation, the abundance of various types of objects around stars, and any patterns that might exist.

As with the hertzsprung russell diagram, it was not possible to create an effective system for categorization of stars until the catalogue of observations was detailed and complete enough to allow it.

I would also say that classification systems are linked to your intent in creating them. As our knowledge of bodies surrounding stars increases, we may find reasons to classify them which we are not yet aware of.