Beta Pictoris

The Star

Beta Pictoris is a bluish white main sequence dwarf star of spectral and luminosity type A5 V, but has been previously classified as A3. It is also classified as a "shell star" because it is surrounded by a shell of mostly hydrogen gas. The star may have about 1.75 times Sol's mass, 1.4 times its diameter, and 8.7 times its luminosity. The star may be as enriched than Sol with elements heavier than hydrogen ("metallicity"), based on its abundance of iron (Heap et al, 1995). It appears to be only 20 to 200 million years old at most. Useful catalogue numbers and designations for the star include: Bet Pic, HR 2020*, Gl 219, Hip 27321, HD 39060, CD-51 1620, CP(D)-51 774, and SAO 234134.

Dust Disk

A circumstellar disk of dust and gas was first detected around Beta Pictoris in 1983 (Lagrange et al, 1996). The disk may be as wide as 1,100 AUs, around 10 times the size of the known Solar System. It is probably also much more massive than the disk that the Solar System formed out of. Oddly enough, the outer disk has elliptical rings, and one side of the disk is 20 percent longer and thinner than the other, which may have been caused by the close passage of a red dwarf star (2000 press release).

Submillimetre Common-User Bolometer Array, James Clerk Maxwell Telescope, JAC
(The smaller blob at lower right may be a dusty companion planet or brown dwarf.)

This disk around Beta Pictoris is probably connected with a planetary system. The disk does not start at the star. Rather, its inner edge begins around 25 AUs away, farther than the average orbital distance of Uranus in the Solar System. Its outer edge appears to extend as far out as 550 AUs away from the star.

Interestingly, most dust grains in the disk are not agglomerating to form larger bodies; instead, they are eroding and being moved away from the star by radiation pressure when their size goes below about 2-10 microns. Theoretically, this disk should have lasted for only around 10 million years. That it has persisted for the 20 to 200 million year lifetime of Beta Pictoris may be due to the presence of large disk bodies (i.e., planets) that collide with icy Edgeworth-Kuiper Belt type objects (dormant comets) to replenish the dust (Pawel Artymowicz, 1989). Indeed the variability in the star's brightness has been tentatively attributed to its occultation (partial eclipse) by at least one orbiting planet. Indeed, years of monitoring of transient spectral absorption events have been interpreted as resulting from the transit across the line of sight from Earth of kilometer-sized (0.6 mile-sized) bodies that are evaporating as they graze Beta Pictoris. These numerous star-grazers are thought to originate from 4:1, and possibly 3:1, mean-motion resonances with a massive, Jupiter-sized planet moving between 10 and 20 AUs away from the Star in a mildly eccentric orbit (e~ 0.1) (Thébault and Beust, 2001).

Keck Observatory, S.R. HEAP/STScI, NASA -- larger image



TOP - Three disk features are labelled in this infrared image of the dust disk at 18 microns. Misalignment of dust emission intensity contours (at "A") with respect to the outer disk suggests that the inner disk is "warped."


BOTTOM - Vertically exaggeration of an image using reflected light from Beta Pictoris by the Hubble Space Telescope shows a warp in the outer disk in the opposite direction from the inner disk.


Thus, the star seems to have an inner disk with a radius of five to 30 AUs and an orbital inclination that is tilted 14 +/- 4 degrees with respect to the larger outer disk and warped in the opposite direction. "B" is the center of one of two disk lobes equidistant from the star that is consistent with a 40-AU-radius, dusk ring at the bright inner edge of the outer disk. "C" is a dust peak that may be associated with a dust ring that is even farther out, but whose counterpart on the other side of the star is not very bright in the Keck image (more discussion).

In April 2002, two teams of astronomers announced that dust confined to a Solar-System sized region close to the star orbits in a plane that is tilted by about 14 degrees relative to the outer disk. The inner "warp" in the disk is in the opposite direction from that of a larger warp previously detected in the outer disk. This double warp may be be due to the presence of one or more unseen planets.

Keck Observatory -- larger simulated image


TOP - Computer illustration shows the infrared
emission from the dust around Beta Pictoris
when viewed at 10 degrees above the disk plane.


BOTTOM - View along the actual line of sight
from Earth to the star (more discussion).

Spectroscopy was used to spread infrared emission from the dust into component wavelengths (like the way that a prism reveals the colors in visible light). Analysis of the inner disk suggests that it is composed of small particles of silicates that are hotter than expected. If a planet is warping the inner disk, it may be causing more collisions of rocks in its neighborhood that result in very small grains and tend to be hotter at the same distance from the star than larger dust grains. In contrast, dust emission from the outer part of the disk appears to come either from larger grains or from dust that is composed of something other than silicates.

On June 27, 2006, astronomers using the Hubble Space Telescope announced that an odd warp in the young star's known dust disk may actually be another fainter, inclined disk. They have imaged a distinct secondary disk that is tilted by about four degrees from the main disk, which is visible out to 260 AUs from Vega but probably extends even farther. Their best explanation for the presence of the second disk is that an unseen planet with as much as 20 times the mass of Jupiter in an orbit within the secondary disk is gravitationally sweeping up dust and icy planetesimals from the primary disk (more from hubblesite.org).

D. Golimowski, D. Ardila , J. Krist, M. Clampin , H. Ford, and G. Illingworth,
ACS Science Team, ESA, NASA -- larger image.

Beta Pictoris appears to have another fainter disk that has been created
by the gravitational tug of a Jupiter-class planet (hubblesite.org).

A Planetary System?

© John Whatmough -- larger image
(Artwork from Extrasolar Visions, used with permission)

A massive planet or brown dwarf "b" may be warping the inner
dusk disk around Beta Pictoris, as imagined by Whatmough.

The warp in the inner dust disk may be caused by the presence of a planet "b" (0.05 to 20 masses) moving along a slightly inclined orbit (3°) within 20 AU of the star (Krist et al, 1996, which should take less than 67 years to complete. Indeed, the inner edge of the dust disk is apparently being cleared by this planet.

In addition, a massive planet "c" with 10 times the mass of Jupiter may also be warping the outer regions of the dust disk. It may have an average distance of 70 AUs with a slight inclination of 2.5° from Earth's line of sight (Gorkayi et al, 2000). The orbit of such planet should take less than 450 years to complete.

On October 7, 2004, a team of Japanese astronomers led by Yoshiko Okamoto published an article (in nature.com) on their discovery of an innermost ring of silicate dust around Beta Pictoris. Their spectrographic data of relatively warm dust indicated that a belt of planetesimals or asteroids is located around 6.4 AUs from the star, within two already known outer belts at 16 and 30 AUs out. Moreover, the lack of similar dust between 6.4 and 16 AUs suggested that a planet may be orbiting Beta Pictoris at around 12 AUs out (more at astronomy.com).

ISAS / JAXA




Larger illustration.




An innermost belt of
silicate dust has been
found around 6.4 AUs
from the star (more).

On May 7, 2007, astronomers modelling the vertical thickness and dust production in Beta Pic's disk based on Hubble observations submitted a paper arguing that its disk likely contains planetary embryos as large as Pluto that are undergoing runaway growth into larger bodies (Quillen et al, 2007). Their model predicts how large the planetary bodies in a dust or debris disk must be to puff it up to a certain thickness. Although a developing star system's dust disk should thin as the system ages, more dust grains may be "knocked" into eccentric orbits that "puff out" the disk if dust agglomeration has proceeded enough for planetesimals to form increasingly large embryonic planets. In the case of Beta Pictoris, the model indicates that a Pluto-sized object may have already formed at an orbital distance of around 100 AUs from the white dwarf star.

NASA



Larger image.



A Pluto-sized planetary
embryo may have already
formed at an orbital
distance of around 100
AUs from Beta Pic (which
may look like Neptune's
icy moon, Triton).

The orbit of an Earth-like planet (with liquid water) around the star may be centered around 3 AU -- around the central orbital distance of the Main Asteroid Belt in the Solar System -- with an orbital period around 3.8 years. Even if an Earth-sized planet has already formed around young Beta Pictoris, it is unlikely to have cooled off sufficiently to have formed crustal rock. After it cools off enough for life to develop, only primitive single-cell, anaerobic bacteria is likely survive under constant bombardment by meteorites and comets as Earth was for the first billion years of existence. Since there is unlikely to be free oxygen in the atmosphere of such a planet, it probably would not have an ozone layer (O₃) although Beta Pictoris puts out a lot more hard radiation (especially ultraviolet) than Sol. Astronomers would find it very difficult to detect an Earth-sized planet of this star using present methods. (See an animation of the planetary and potentially habitable zone orbits of this system, with a table of basic orbital and physical characteristics.)