* Astronomy

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RE: Beta Pictoris

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Detailed images of the nearby star Beta Pictoris, 63 light-years awar in the constellation Pictor, taken by the Hubble Space Telescope, confirm the existence of not one but two dust disks encircling the star. The images offer tantalising new evidence for at least one Jupiter-size planet orbiting Beta Pictoris.

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Credit NASA
Position (2000): R.A. 05h 47 m 17s.09 Dec. -51° 03' 59".5

The finding ends a decade of scientific speculation that an odd warp in the young star's debris disk may actually be another inclined disk. The recent Hubble Advanced Camera for Surveys view – the best visible-light image of Beta Pictoris – clearly shows a distinct secondary disk that is tilted by about 4 degrees from the main disk. The secondary disk is visible out to roughly 24 billion miles from the star, and probably extends even farther. This Hubble image of Beta Pictoris clearly shows a primary dust disk and a much fainter secondary dust disk. Astronomers used the Advanced Camera’s coronagraph to block out the light from the bright star.
The secondary disk is circumstantial evidence for the existence of a planet in a similarly inclined orbit. The planet may have indirectly formed the secondary disk by sweeping up smaller planetesimals – chunks of rock and/or ice – from the main disk. The planetesimals then collide, producing the dust seen in the disk.

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A comparison of the gas composition in Beta Pictoris with that of chondritic meteorites. If the gas was chondrite-like, all the elements would have abundances of 1. Deviations from 1 imply processes have enriched or depleted the gas in particular elements. Most elements have abundances (relative to Fe) that are remarkably similar to chondritic.

Carbon is the obvious exception. The line is a very simplistic model in which an energetic event vaporised chondritic material and then as the vapour cooled most (>99%) of the rock forming elements condensed out to reform dust leaving the carbon in the gas. Despite the nice fit, there are significant problems. Rock forming elements would probably not condense as one, but in a sequence according to their volatility - the least volatile first. So normally one would expect the less volatile elements to be more depleted in the gas than the more volatile ones. Sulphur is much more depleted in the Beta Pictoris gas than one would predict, and the most refractory elements (e.g. Ca and Al) are not depleted enough.
Credit: NASA

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Scientists using NASA's Far Ultraviolet Spectroscopic Explorer, or FUSE, have discovered abundant amounts of carbon gas in a dusty disk surrounding a young star named Beta Pictoris.

The star and its emerging solar system are less than 20 million years old, and planets may have already formed. The abundance of carbon gas in the remaining debris disk indicates that Beta Pictoris' planets could be carbon-rich worlds of graphite and methane, or the star's environs might resemble our own solar system in its early days.
A team led by Aki Roberge of NASA's Goddard Space Flight Center in Greenbelt, Md., presents the observation in the June 8 issue of Nature. The new measurements make Beta Pictoris the first disk of its kind whose gas has been comprehensively studied. The discovery settles a long-standing scientific mystery about how the gas has lingered in this debris disk, yet raises new questions about the development of solar systems.

"There is much, much more carbon gas than anyone expected. Could this be what our own solar system looked like when it was young? Are we seeing the formation of new types of worlds? Either prospect is fascinating" - Aki Roberge , NASA postdoctoral fellow and lead author on the Nature report.

The carbon gas detected by the spacecraft comes from unseen asteroids or comets orbiting the star that collide with each other and release material. The mere presence of gas in the Beta Pictoris disk has been a mystery. Theoretical models predict that intense light from the young star should rapidly blow the gas away. The overabundance of carbon, discovered now for the first time, explains why the disk retains so much gas. Carbon is less susceptible to expulsion than other elements, and it retards the clearing effect.

Beta Pictoris, about 60 light years away from Earth, is 1.8 times more massive than our sun. At eight to 20 million years old, it is very young. This young star's disk was discovered in 1984. Earlier observations with the Hubble Space Telescope and the Keck telescope hinted that a Jupiter-like planet may have already formed in this disk, and rocky terrestrial planets may be forming. Such planets would be too small and faint to observe with current instruments.
The terrestrial planets in our solar system -- Mercury, Venus, Earth and Mars -- formed from the collision of smaller planetary bodies such as asteroids about five billion years ago. During the few hundred million years after Earth was formed, asteroids and comets might have smashed into our planet to deliver virtually all of the water and organic material we see today. These materials are the building blocks of life on Earth.
Asteroids and comets orbiting Beta Pictoris might contain large amounts of carbon-rich material, such as graphite and methane. Planets forming from or impacted by such bodies would be very different from those in our solar system and might have methane-rich atmospheres, like Titan, a moon of Saturn.

"What we have learned in the past ten years is that our galaxy is filled with other solar systems, and each one is different from the next. Beta Pictoris may be telling us something about the variety of planets that might be out there; some might be carbon planets, very different from the Earth" - Marc Kuchner of NASA Goddard, an expert on extra-solar planets.

Alternatively, Beta Pictoris might be similar to how our solar system was long ago. While local asteroids and comets don't seem carbon-rich today, some research suggests that certain meteorites called enstatite chondrite meteorites formed in a carbon-rich environment. Some scientists also speculate that Jupiter has a carbon core.
Other co-authors on the report are Paul Feldman, Johns Hopkins University, Baltimore, and Magali Deleuil and Jean-Claude Bouret, Laboratoire d'Astrophysique de Marseille in France. The FUSE project is a NASA Explorer mission, developed in cooperation with France's Centre National d'Etudes Spatiales and the Canadian Space Agency by Johns Hopkins University in Baltimore; University of Colorado, Boulder; and University of California, Berkeley. Goddard manages the program for NASA's Science Mission Directorate.

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The degree of polarisation of the light as a function of distance to the central star for both the upper left and lower right portions of the disk.

There is a dip at a distance of 100 astronomical units at both sides. Since this dip corresponds to a decrease in brightness as well, it probably corresponds to a region where there are fewer planetesimals and therefore less dust.

Source: NAOJ News Release

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Planets form in disks of gas and dust that surround newborn stars. Such disks are called proto-planetary disks. The dust in these disks becomes rocky planets like Earth and the inner cores of giant gas planets like Saturn. This dust is also a repository of elements that form the basis of life.

Proto-planetary disks disappear as stars mature, but many stars have what are called debris disks. Astronomers hypothesize that once objects such as asteroids and comets are born from the proto-planetary disk, collisions among them can produce a secondary dust disk.

The most well-known example of such dust disks is the one surrounding the second brightest star in the constellation Pictor, meaning "painter's easel". This star, known as Beta Pictoris or Beta Pic, is a very close neighbour of the Sun, only sixty light years away, and therefore easy to study in great detail.

Beta Pic is twice as bright as the Sun, but the light from the disk is much fainter. Astronomers Smith and Terrile were the first to detect this faint light in 1984, by blocking the light from the star itself using a technique called coronagraphy. Since then, many astronomers have observed the Beta Pic disk using ever better instruments and ground and space-based telescopes to understand in detail the birth place of planets, and hence life.
A team of astronomers from the National Astronomical Observatory of Japan, Nagoya University and Hokkaido University combined several technologies for the first time to obtain an infrared polarisation image of the Beta Pic disk with better resolution and higher contrast than ever before: a large aperture telescope (the Subaru telescope, with its large 8.2 meter primary mirror), adaptive optics technology, and a coronagraphic imager capable of taking images of light with different polarisations (Subaru's Coronagraphic Imager with Adaptive Optics, CIAO).
A large aperture telescope, especially with Subaru's great imaging quality, allows faint light to be seen at high resolution. Adaptive optics technology reduces Earth's atmosphere's distorting effects on light, allowing higher resolution observations. Coronagraphy is a technique for blocking light from a bright object such as a star, to see fainter objects near it, such as planets and dust surrounding a star. By observing polarised light, reflected light can be distinguished from light coming directly from its original source. Polarization also contains information about the size, shape, and alignment of dust reflecting light.
With this combination of technologies, the team succeeded in observing Beta Pic in infrared light two micrometers in wavelength at a resolution of a fifth of an arcsecond. This resolution corresponds to being able to see an individual grain of rice from one mile away or a mustard seed from a kilometre away. Achieving this resolution represents a huge improvement over comparable previous polarimetric observations from the 1990's that had only resolutions of about one and a half arcseconds.

The new results strongly suggest that Beta Pic's disk contains planetesimals, asteroid or comet-like objects, that collide to generate dust that reflects starlight.
The polarisation of the light reflected from the disk can reveal the physical properties of the disk such as composition, size, and distribution. An image of all the two micrometer wavelength light shows the long thin structure of the disk seen nearly edge on. The polarisation of the light shows that ten percent of the two micrometer light is polarised. The pattern of polarisation indicates that the light is a reflection of light that originated from the central star.

An analysis of how the brightness of the disk changes with distance from the central shows a gradual decrease in brightness with a small oscillation. The slight oscillation in brightness corresponds to variations in the density of the disk. The most likely explanation is that denser regions correspond to where planetesimals are colliding. Similar structures have been seen closer to the star in earlier observations at longer wavelengths using Subaru's COoled Mid-Infrared Camera and Spectrograph (COMICS) and other instruments.
A similar analysis of how the amount of polarisation changes with distance from the star shows a decrease in polarisation at a distance of one hundred astronomical units (an astronomical unit is the distance between Earth and the Sun). This corresponds to a location where the brightness also decreases, suggesting that at this distance from the star there are fewer planetesimals.

As the team investigated models of the Beta Pic disk that can explain both the new and old observations, they found that the dust in Beta Pic's disk is more than ten times larger than typical grains of interstellar dust. Beta Pics dust disk is probably made of micrometer sized loose clumps of dust and ice like miniscule bacteria-size dust bunnies.

Together, these results provide very strong evidence that the disk surrounding Beta Pic is generated by the formation and collision of planetesimals. The level of detail of this new information solidifies our understanding of the environment in which planets form and develop.

"Few people have been able to study the birth place of planets by observing polarized light with a large telescope. Our results show that this is a very rewarding approach. We plan on extending our research to other disks, to get a comprehensive picture of how dust transforms into planets" - Motohide Tamura, team Leader.

These results were published in the April 20, 2006, edition of the Astrophysical Journal.