Hubble acquired this image of the circumstellar disk around the star Vega using the Space Telescope Imaging Spectrograph (STIS). Credit: NASA, ESA, CSA, STScI, S Wolff (University of Arizona), K Su (University of Arizona), A Gáspár (University of Arizona).

NASA’s Hubble and James Webb space telescopes have found surprisingly smooth disks around the bright star Vega.

In the 1997 movie Contact, adapted from Carl Sagan’s 1985 novel, the lead character scientist takes a space-alien-built wormhole ride to the star Vega. She emerges inside a snowstorm of debris encircling the star — but no obvious planets are visible.

It looks like the filmmakers got it right.

A team of astronomers at the University of Arizona, Tucson used NASA’s Hubble and James Webb space telescopes for an unprecedented in-depth look at the nearly 100-billion-mile-diameter debris disk encircling Vega.

The University of Arizona’s Andras Gáspár, a member of the research team, said: “Between the Hubble and Webb telescopes, you get this very clear view of Vega.

“It’s a mysterious system because it’s unlike other circumstellar disks we’ve looked at; the Vega disk is smooth, ridiculously smooth.”

The big surprise to the research team is that there is no obvious evidence for one or more large planets plowing through the face-on disk like snow tractors.

Lead author Kate Su, also from the University of Arizona, added: “It’s making us rethink the range and variety among exoplanet systems.”

Webb sees the infrared glow from a disk of particles the size of sand swirling around the sizzling blue-white star that is 40 times brighter than our Sun. Hubble captures an outer halo of this disk, with particles no bigger than the consistency of smoke that are reflecting starlight.

Webb acquired this image of the circumstellar disk around the star Vega using the Mid-Infrared Instrument (MIRI). Credit: NASA, ESA, CSA, STScI, S Wolff (University of Arizona), K Su (University of Arizona), A Gáspár (University of Arizona).

Vega debris disk

The distribution of dust in the Vega debris disk is layered because the pressure of starlight pushes out the smaller grains faster than larger grains.

Lead author of the paper presenting the Hubble findings, Schuyler Wolff of the University of Arizona team, said: “Different types of physics will locate different-sized particles at different locations.

“The fact that we’re seeing dust particle sizes sorted out can help us understand the underlying dynamics in circumstellar disks.”

The Vega disk does have a subtle gap, around 60 AU (astronomical units) from the star (twice the distance of Neptune from the Sun), but otherwise is very smooth all the way in until it is lost in the glare of the star.

This shows that there are no planets down at least to Neptune-mass circulating in large orbits, as in our solar system, say the researchers.

Su added: “We’re seeing in detail how much variety there is among circumstellar disks, and how that variety is tied into the underlying planetary systems.

“We’re finding a lot out about the planetary systems — even when we can’t see what might be hidden planets.

“There’s still a lot of unknowns in the planet-formation process, and I think these new observations of Vega are going to help constrain models of planet formation.”

Disk Diversity

Newly forming stars accrete material from a disk of dust and gas that is the flattened remnant of the cloud from which they are forming.

In the mid-1990s Hubble found disks around many newly forming stars. The disks are likely sites of planet formation, migration, and sometimes destruction. Fully matured stars like Vega have dusty disks enriched by ongoing ‘bumper car’ collisions among orbiting asteroids and debris from evaporating comets.

These are primordial bodies that can survive up to the present 450-million-year age of Vega (our Sun is approximately ten times older than Vega).

Dust within our solar system (seen as the Zodiacal light) is also replenished by minor bodies ejecting dust at a rate of about 10 tons per second. This dust is shoved around by planets.

This provides a strategy for detecting planets around other stars without seeing them directly – just by witnessing the effects they have on the dust.

Wolff said: “Vega continues to be unusual. The architecture of the Vega system is markedly different from our own solar system where giant planets like Jupiter and Saturn are keeping the dust from spreading the way it does with Vega.”

For comparison, there is a nearby star, Fomalhaut, which is about the same distance, age and temperature as Vega, but Fomalhaut’s circumstellar architecture is greatly different from Vega’s; Fomalhaut has three nested debris belts.

Puzzling

Planets are suggested as shepherding bodies around Fomalhaut that gravitationally constrict the dust into rings, though no planets have been positively identified yet.

Team member George Rieke of the University of Arizona, said: “Given the physical similarity between the stars of Vega and Fomalhaut, why does Fomalhaut seem to have been able to form planets and Vega didn’t?”

Wolff added: “What’s the difference? Did the circumstellar environment, or the star itself, create that difference? What’s puzzling is that the same physics is at work in both.”

Two papers (Wolff et al. and Su et. al.) from the Arizona team will be published in The Astrophysical Journal.