Nearby Planet Nursery

Astronomers at the University of California, Berkeley, and University of Hawaii reported in Science their discovery of the nearest and youngest star with a visible disk of dust. The relatively nearby star may be a nursery for planets.

The dim red dwarf star –called AU Mic –is a mere 33 light years away, close enough that the Hubble Space Telescope or ground-based telescopes with adaptive optics to sharpen the image should be able to see whether the dust disk contains clumps of matter that might turn into planets. The star was visible with its dust cloud extending out to where only comets reside in our solar system.

Paul Kalas, assistant research astronomer at UC Berkeley and lead author of the Science paper that first reported the discovery talked with Astrobiology Magazine about its implications for future planet-hunting.

Astrobiology Magazine (AM): The first announcement of the discovery seemed to imply that the dust from comets and asteroid collisions was imaged, and therefore planetary formation might be inferred as close as 33 light-years away. Does this refer to circumstellar dust generally, not literally these tiny objects?

Visible light image of the circumstellar disk around the nearby young star AU Mic discovered with the University of Hawaii 2.2-meter telescope. The disk is seen edge-on, revealed by starlight scattering off of small dust grains. The outer extent is about 210 Astronomical Units. The bright light from the star is blocked by an opaque mask and its supporting crosshairs, allowing detection of the very faint disk. Analysis of the radiation from AU Mic shows that the inner disk is cleared of material, indirect evidence for newly formed planets in the inner regions.
Credit:Paul Kalas

Paul Kalas (PK): The discovery image shows light from the star AU Mic reflecting off small dust grains surrounding the star. In general, dust surrounding stars in the solar neighborhood must have a source of replenishment. We therefore infer that much of the solid mass around AU Mic is locked up in larger objects such as asteroids and comets, and these objects release more dust over time.

The solar system has circumstellar dust grains – except we call them IDPs, or interplanetary dust particles. Asteroid collisions are the main source of IDPs, but the dust you see in comet tails also replenishes the solar system IDPs. The Sun’s circumstellar dust disk manifests as a phenomenon called the Zodiacal Light. If you were observing the Sun from AU Mic, you would probably first detect this dust component, because its cumulative surface area is much greater than the surface area of the parent bodies.

So far with AU Mic, we have directly detected the dust lying at 50 AU radius (1 AU=astronomical units or the Sun-Earth distance) and beyond. At these distances, the AU Mic disk is comparable to our Kuiper Belt.

AM: What is the currently favored accretion model for planet formation around a star? Is the correct sequence, dust clumping, self-gravitation, debris disk formation from collisions, and so forth?

PK: That’s about right. Also, once an object with a few Earth-masses forms, it accretes a gas envelope and can grow into a gas giant planet like Saturn or Jupiter. These massive planets then have the role of sweeping up any remaining gas, and ejecting smaller bodies into the outer solar system, eventually forming the Oort cloud of comets.

AM: What role does temperature play in the accretion model? For instance, do planetary dust rings typically form at deep space temperatures nearer to absolute zero or would this depend strongly on the distance to the parent star?

PK: The temperature dependence is often described by analogy to a mountain’s snowline – the boundary between warm weather that melts the snow and cooler air that allows snow to stay on the ground. Likewise, within a certain radius surrounding a star, ices sublimate quickly, and beyond this boundary, they are long-lived. For the Sun, the ice boundary is at about 3 AU. If you were to move Jupiter and Saturn closer to the Sun, the ice-component of the rings would quickly vanish, but we would still observe rings composed of dust grains.

AM: Does the star’s age at 12 million years old mean that the first planet in this system is perhaps a billion years in the future?

PK: Planet formation models indicate that planets like Earth or Jupiter form within 10 million years (Myr). The main reason to believe this is that gas around stars disperses in under 10 Myr. Therefore, Jupiter and the other gas giant planets must form within this time. At an age of 12 Myr, the gas and dust around AU Mic has had enough time to form some very young planet-mass objects.

Scene from a moon orbiting the extra-solar planet in orbit around the star HD70642.
Credit:David A. Hardy, (c)

AM: When will you next get telescope time on an appropriate instrument to image the infrared part of the star’s dust cloud?

PK: We have applied for Hubble Space Telescope observing time in the next period of available observing time. This period begins on July 1st every year. If our proposal is accepted, then we should have spectacular HST images of the disk by the end of the summer. However, we also have access to several ground-based telescopes equipped with both adaptive optics and coronagraphs. When AU Mic begins rising out of the East in June, we will obtain our first high-resolution ground-based images in the near-infrared.

AM: With a coronograph, are there techniques for instance with the Spitzer Space Telescope to see closer to the star than 50 AU that is blocked physically to reduce glare?

Micron-sized interstellar dust particles scatter the light around AU Mic
Credit: NASA/JPL

PK: Spitzer is an infrared telescope and it is optimized for infrared sensitivity, rather than high spatial resolution imaging. HST has three coronagraphs, two in the visible (ACS and STIS) and one in the near-infrared (NICMOS). The NICMOS coronagraph has an occulting spot 0.6 arcseconds in diameter, which corresponds to 6 AU diameter on AU Mic. Therefore, we will image the disk inward of 50 AU radius, and as close as 3 AU radius from the star.

AM: Is the absence of warm dust grains from spectra within this inner region (<17 AU) a general indicator that planetary orbits may be clearing out large regions close to the star?

PK: Yes, planet-mass objects sweep away material that lies near the paths of their orbits. Grains can also be depleted by other mechanisms that involve various drag and pressure forces that are not related to planet formation. Therefore, the existence of planets around AU Mic to explain the inner disk hole is just one working hypothesis that will be tested in the future.

The research was supported by the NASA Origins Program and the National Science Foundation’s Center for Adaptive Optics.

Related Web Pages

The University of California Planet Search Project
Astrobiology Magazine New Planets
Transit Search
Extrasolar Planets Encyclopedia
Planet Quest (JPL)
Kepler Mission
Darwin Mission
Herschel Mission
Space Interferometry Mission
Circumstellar Disk Learning Site