- Vega provided our first visual evidence of a debris disk, or a disk of dust and debris spinning around a star from which planets can form.
- New observations of Vega’s disk have found it’s surprisingly smooth, which indicates no large planets orbit Vega.
- Scientists are unsure why Vega’s disk is smooth, while similar stars have distinct rings in their disks.
NASA published this original story on November 1, 2024. Edits by EarthSky.
No planets for Vega?
Planets form in spinning disks of gas and dust around stars as material slowly clumps together. Scientists predicted this as early as the 18th century, but observational evidence didn’t arrive until 1984, when astronomers using the Infrared Astronomical Satellite (IRAS) spotted a strange excess of infrared light surrounding the bright star Vega. They interpreted this as a disk of planet-forming material. It was the first of many debris disks now identified around stars. Many of these disks contain ring-shaped gaps, likely carved out by exoplanets. Now, astronomers at the University of Arizona have used the Webb and Hubble space telescopes to study Vega’s legendary debris disk. Surprisingly, the disk is almost totally smooth: they found no evidence of any planets.
Andras Gáspár, a member of the research team from the University of Arizona, 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 team has written up the findings in two peer-reviewed papers – one for Hubble’s investigation, the other for Webb’s – which will be published in The Astrophysical Journal.
Disk around Vega is smooth
Newly forming stars accrete material from a disk of dust and gas around them. That disk 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 have survived up to the present 450-million-year age of Vega (our sun is approximately 10 times older than Vega).
Dust within our solar system also gets replenished by minor orbiting bodies, which eject dust at a rate of about 10 tons per second.
Planets shove around this dust. This provides a strategy for detecting planets around other stars without seeing them directly. We just have to look for the effects they have on the dust.
But the scientists didn’t see these effects around Vega. Schuyler Wolff of the University of Arizona, lead author of the paper presenting the Hubble findings, said:
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.
A smooth disk suggests no planets
For comparison, nearby star Fomalhaut is about the same distance, age and temperature as Vega. But Fomalhaut’s disk is very different from Vega’s. Fomalhaut has three distinct belts of debris, which scientists think were carved out by the gravity of orbiting planets. Though no planets have been positively identified yet around Fomalhaut, it’s thought planets must have shepherded the dust into these rings.
No such rings can be seen around Vega, apart from a subtle gap around 60 times farther from the star as Earth is from the sun. The Vega disk is very smooth all the way in, until the glare of the star obscures it. According to the researchers, this shows there are no planets down to at least Neptune’s mass (17 times the mass of Earth) circulating in large orbits around the star.
The discovery has only spawned more questions. Research 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?
And 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.
Hubble and Webb both observed Vega
Why did it take both Hubble and Webb to make these observations? Webb observed the infrared glow from a disk of particles the size of sand grains swirling around Vega. Hubble, on the other hand, captured an outer halo of this disk, where debris the size of smoke particles are reflecting starlight.
The dust in Vega’s debris disk is distributed in this order because the pressure of starlight pushes out the smaller grains faster than larger grains. Wolff explained:
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.
Together, the telescopes provided an unprecedentedly detailed view of Vega’s dust disk. Kate Su of the University of Arizona, lead author of the paper presenting the Webb findings, said:
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.
Bottom line: Scientists studying the disk of debris around bright star Vega have found it to be almost totally smooth, with no signs of any planets.