New Look at β Pictoris b

by Paul Gilster on September 17, 2015

Given the scale of our own Solar System, the system circling the star Beta Pictoris can’t help but give us pause. Imagine not only the orbiting clouds of gas, dust and debris that we would expect around a young star (8-20 million years old) with a solar system in formation, but also a gas giant planet some ten to twelve times the mass of Jupiter, in an orbit something like Saturn’s. Now factor in this: The disk in question, if translated into our own system’s terms, would extend from about the orbit of Neptune to almost 2000 AU.

Now we have a view of Beta Pictoris b as it moves through a small slice (one and a half years) of a 22 year orbital period. The work of Maxwell Millar-Blanchaer (a doctoral candidate at the University of Toronto) and colleagues, the imagery appears in a paper published yesterday by The Astrophysical Journal. Millar-Blanchaer used observations from the Gemini Planet Imager on the Gemini South telescope in Chile to image Beta Pictoris b, the work being part of the GPI Exoplanet Survey, which will examine some 600 stars in the coming three years.

“The images in the series represent the most accurate measurements of the planet’s position ever made,” says Millar-Blanchaer. “In addition, with GPI, we’re able to see both the disk and the planet at the exact same time. With our combined knowledge of the disk and the planet we’re really able to get a sense of the planetary system’s architecture and how everything interacts.”

Image: A series of images taken between November 2013 to April 2015 with the Gemini Planet Imager (GPI) on the Gemini South telescope in Chile shows the exoplanet β Pic b orbiting the star β Pictoris, which lies over 60 light-years from Earth. In the images, the star is at the center of the left-hand edge of the frame; it is hidden by the Gemini Planet Imager’s coronagraph. We are looking at the planet’s orbit almost edge-on; the planet is closer to the Earth than the star. Credit: M. Millar-Blanchaer, University of Toronto; F. Marchis, SETI Institute.

The intensively studied Beta Pictoris disk is known for a disk asymmetry (one side of the disk is longer and thinner than the other) and a ‘warp’ that has been thought to be the result of disk ‘sculpting’ by the known planet — a 1997 study argued that a planet with an inclination of between 3 and 5 degrees could account for the observed perturbation, with the subsequent discovery of Beta Pictoris b lending weight to the idea. The other possibility posed in the literature was that the disk is actually composed of two disks that appear superimposed in our view, with a roughly 3 degree difference in position angle.

The new paper refines measurements of the planet’s orbit and the circumstellar disk, showing an inner disk that is slightly offset from the main outer disk. The results also indicate that the sculpting effect cannot be accounted for purely through Beta Pictoris b. From the paper:

When considered together, the disk model and the orbital fit indicate that the dynamics of the inner edge of the disk are not consistent with sculpting by the planet β Pic b alone. This could be explained by an as-of-yet undetected planet in-between the known planet and the inner edge of the disk. Under this scenario the less massive, further out planet would dynamically influence the inner regions of disk, while the more massive β Pic b would have a greater effect at larger radii, causing the well known warp. If there is in fact another planet at this location, this will have significant consequences for our understanding of the planet formation history and dynamical evolution of this system.

Beta Pictoris, some 63 light years away in the constellation Pictor (the Painter’s Easel), is a system that is sure to see intensified investigation. In this case, we’re seeing images of the debris disk in polarized light that, as the paper notes, reach angular separations that have been inaccessible to both space- and ground-based telescopes. Learning more will require more sophisticated dust grain models that will allow researchers to further test their theories about the inner part of the disk.

The paper is Millar-Blanchaer et al., “β Pictoris’ inner disk in polarized light and new orbital parameters for β Pictoris b,” published September 16 2015 by The Astrophysical Journal (abstract / preprint). A University of Toronto news release is available.

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Ashley Baldwin September 17, 2015 at 16:08

A great teaching aid is this system. Despite having a blue A6 star it was initially observed with excessive infrared emission. This is a classic sign of a protoplanetary disk. The VLTI interferometer with an synthesised aperture of 130m providing the exquisite resolution necessary to visualise the star and accurately measure its angular diameter , an essential element along with spectroscopy and accurate measurement of its 63.4 light year distance by the Hipparchis satellite to allow the detailed classification necessary to allow classification of its planet. The fact that the system is almost inclined side on from Earth allows both mass and radius of the planet to be accurately calculated with the help of Kepler like transit photometry ( reduction of light as planet passes in front of star ) The system is somewhat old to still have a circumstellar disk , never mind two . Detailed spectroscopy has shown it has an unusually high content of carbon which helps resist the intense U..V of the nascent star’s stellar wind and in doing so allow a planet to evolve to almost brown dwarf dimensions . Given its youthfulness the planet also has much of its original internal heat of birth present that is emitted at thermal infrared wavelength . This is at a much more favourable contrast with the starlight than shorter wave visible light , with a ratio of a few million as opposed to a billion or more for visible light . This is crucial to ground based imaging as coronagraphs on telescopes like Gemini just can’t give billion times contrast but can manage a million or so , aided further by the fact that the planet is self luminous rather than merely reflecting starlight as a terrestrial planet would. The high carbon content in the disk might make for some unusual planets very unlike our own solar system and with high pressures the cores of any as yet undiscovered “Super Earths” could be entirely diamond.
This illustrates how combining available technology like the VLTI, Hipparchos,GPI , Hubble and various space infrared telescopes can classify a planet in great detail. Must not take it for granted though. To see a planet so clearly , orbiting a star 60 light years away is utterly amazing. And this is just the beginning as thanks to WFIRST in particular space based coronagraphs are progressing to the point where they will be up to 5 orders of magnitude more potent than GPI and eminently capable of imaging Earth like planets in the habitable zones of stars . Even the ground based ELTs should be able use coronagraphs with a contrast of a hundred million ( even with adaptive optics this is the performance ceiling for the foreseeable future thanks to the atmosphere ) which will potentially be just able to image large terrestrial planets in the infrared around smaller stars like nearby M dwarfs . But in the habitable zone though so more reason to look forward especially with JWST thrown into the mix.

Phil September 17, 2015 at 20:30

So, what are the chances of a transit (or I guess an occultation/eclipse)? Wouldn’t that be spectacular? Is the orbit known well enough to pin down a date/time? Will resources be tasked to this? I would have thought the rewards make it worth it

P

George W September 19, 2015 at 17:01

Based on the data we now have including refinements to uncertainties we can again look to the work done by Matthew J. Holman and Paul A. Wiegert as far as stability in dynamic systems goes.

With respecting the uncertainties of the orbital parameters of the planet (β Pic b) the limit (critical radius) for the “Inner Zone of Stability” in reference to the Star (β Pic, M=1.75) can be found with using the lower bounds on “a” and the upper bounds on “m” on the planet (m = 11 m_jup, a = 7.7 au) with an eccentricity of ~0.1 for the system; this yields a value of ~3.13 au for the “Inner Zone of Stability”. This places a boundary on where a planet interior to β Pic b could reside, this also gives us a good demarcation for where the interior edge of the inner belt is located +/- 10%.
Again when respecting the uncertainties of the planet (β Pic b) the limit (critical radius) for the “Outer Zone of Stability” in reference to the Star (β Pic, M=1.75) can be found with using the upper bounds on “a” and the upper bounds on “m” on the planet (m = 11 m_jup, a = 9.6 au) with an eccentricity of ~0.1 for the system; this yields a value of ~20.63 au for the “Outer Zone of Stability”. This places an inner boundary on where a planet exterior to β Pic b could reside.
The Hill Sphere of β Pic in relation to the galactic tide or about (2 ly) or the “Inner Zone of Stability” as relates to a nearby (a =0.6 ly) passing star of similar mass (m = 1.75; ~10,500 au) places an outer limit on where a planet exterior to β Pic b could reside in stable orbits. If β Pic b is assumed to be in a stable orbit it also further restricts the location of the orbital distance of an exterior body.

Suppositional:
It is interesting to note that in our own solar system the Hill Sphere of the Sun in relation to Jupiter +/- 5 % seems to be at the innermost edge of where TNO’s can be found and begin to increase in any sort of density of population.

Michael September 20, 2015 at 11:15

I wonder if they know the mass of the disc and whether it is still falling inwards to the star or is it on its way out as this could pull in the planets into tighter orbits or eventually pull them back out again. This system could teach us about how a planetary system first evolves and could go some way to place limits on how hot Jupiter’s form.

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