Hubble and Webb Take Closer Look at Planetary Debris Disk around Vega

There is no obvious evidence for one or more large exoplanets plowing through the face-on debris disk encircling Vega, one of the brightest stars in the night sky.

Vega is a young massive star located approximately 25 light-years away in the constellation of Lyra.

The star is classified as a type A — the name for stars that tend to be bigger, younger and much faster-spinning than the Sun.

Also known as Alpha Lyra, Gliese 721 and HD 172167, Vega is 455 million years old, and has a mass of two solar masses.

It rotates around its axis once every 16 hours — much faster than the Sun with a rotational period that clocks in at 27 Earth days.

Vega is legendary because it offered the first evidence for material orbiting a star.

This was first hypothesized by Immanuel Kant in 1775. But it took over 200 years before the first observational evidence was collected in 1984.

A puzzling excess of infrared light from warm dust was detected by NASA’s Infrared Astronomy Satellite (IRAS). It was interpreted as a shell or disk of dust extending twice the orbital radius of Pluto from the star.

In new research, astronomers analyzed images of the Vega’s debris disk taken by the NASA/ESA Hubble Space Telescope and the NASA/ESA/CSA James Webb Space Telescope.

“Vega was the first-discovered and one of the prototypical planetary debris disks,” University of Arizona’s Dr. Kate Su and her colleagues in a paper presenting the Webb findings.

“It opened a broad field of study that now is being used to identify relatively low-mass exoplanets beyond the reach of other discovery techniques, as well as to reveal detailed properties of the systems of small bodies in other planetary systems.”

“Vega continues to be unusual,” added Dr. Schuyler Wolff, an astronomer at the University of Arizona and lead author of a paper presenting the Hubble findings.

“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.”

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

“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?” said Dr. George Rieke, also form the University of Arizona.

“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,” Dr. Wolff added.

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.

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.

“Between the Hubble and Webb telescopes, you get this very clear view of Vega,” said Dr. Andras Gáspár, an astronomer at the University of Arizona and co-author of both papers.

“It’s a mysterious system because it’s unlike other circumstellar disks we’ve looked at.”

“The Vega disk is smooth, ridiculously smooth.”

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.

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

“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.”

The two papers will appear in the Astrophysical Journal.

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Kate Y.L. Su et al. 2024. Imaging of the Vega Debris System using JWST/MIRI. ApJ, in press; arXiv: 2410.23636

Schuyler G. Wolff et al. 2024. Deep Search for a scattered light dust halo around Vega with the Hubble Space Telescope. ApJ, in press; arXiv: 2410.24042