Famous Star Hasn’t Formed Planets, and We Don’t Know Why

The famous star has not created any planets, and we don’t know why

Who Jonathan O’Callaghan

The nearby star Vega has a special place in human culture. Just 25 light-years away, this bright beacon—twice the mass of the Sun and 40 times brighter—is so prominent in Earth’s sky that fascinated ancient astronomers all over the world. It was also our planet’s North Star a few thousand years ago, until Polaris took its place as the Earth’s axis wobbled. (Vega will regain the crown of the North Star in 12,000 years). Because of this, many have considered this iconic star an interesting place to look for life, rather than astronomer Carl Sagan, who in his 1985 novel depicted signs of an intelligent civilization arriving from Vega. Get in touchIn 1997 it was adapted into a very successful film.

So it was a disappointment earlier this month when astronomers announced a surprising discovery about this star. Using the Hubble Space Telescope and its next-generation cousin the James Webb Space Telescope (JWST), they observed Vega in its finest detail yet and discovered something unexpected. The star, although only about half of its billion-year life, it doesn’t seem to have formed any great world. “It was really amazing,” says Kate Su of the University of Arizona and the Space Science Institute, who led the JWST observations. Instead, it has a smooth disk of sand-like dust around the star, and while it still hides smaller planets, it doesn’t appear to have spawned larger worlds like Saturn and Jupiter. “We were really hoping to see some giant planets,” Su says. The research was originally presented in two articles published on the arXiv.org preprint server: one It was published in Journal of Astronomy, and has other will be published in Astrophysical magazine.

The more than 5,500 planets currently known outside our solar system are found around a variety of stars, from dim ones known as red dwarfs to much brighter ones like Vega. “Nowadays we are used to finding planets around many stars,” says Anders Johansen, a planet formation expert at the University of Copenhagen in Denmark, who was not involved in the research. When a star first forms, it is surrounded by a debris disk, a rotating plate of dust and gas. In the early stages of a planetary system, these debris coalesce to form increasingly larger planetesimalsthe rocky building blocks of the planets. Eventually, these slow their growth to become small worlds like Earth and Mars or continue to grow, accumulating large amounts of gas, to become giants like the planets in our outer solar system. The process is quick. “After about 10 million years, you expect most of the planet formation to be done,” says Schuyler Wolff of the University of Arizona, who led the Hubble observations of Vega.

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Vega is a 450-million-year-old A-type star. In previous studies, for example NASA’s Spitzer Space Telescope in 2005astronomers found that the star was surrounded by a large, glowing debris disk nearly 100 billion miles across, its proportions equivalent to the Kuiper Belt that stretches beyond Neptune. But it wasn’t until these latest observations that they were able to study this disc in great detail. “We are now making comparisons with the asteroid belt between Mars and Jupiter,” says Wolff. The team expected to see giant planets gouge holes through this debris disk, similar to what is thought to have happened in our solar system, but no such holes appeared. The researchers rule out the existence of planets larger than Saturn beyond 10 AU from the star—10 times the distance between Earth and the sun.

While the team can’t rule out the possibility of smaller planets, the lack of giant worlds in the disk is troubling and suggests something unusual is going on there, an unknown obstacle to understanding planet formation. “We see a fairly smooth distribution of dust,” says Wolff, for example around other A-type stars. FomalhautObservers have discovered a disk that shows clear evidence of a structure sculpted by the presence of one or more planets. “The question ‘what is different?’ becomes,” says Wolff. “Was there some chaotic event that allowed giant planets to form in one (disk) and not the other?”

Paul Kalas, a debris disk expert at the University of California, Berkeley, who was not involved in the new study, suggests one possibility: The Vega system could have been stripped of gas early in its life, preventing it from forming planets. growth of giant planets. “We don’t understand why planet formation is so unpredictable,” says Kalas. “Here, in Vegan and Fomalhaut, you have two similar stars, and yet the results are very different. Scientists don’t like surprises; one thing must follow the other. It shows that nature can surprise us.”

The lack of a giant planet around Vega is not entirely surprising to Bruce Macintosh, director of the University of California Observatory (UCO), who was not involved in the new study. Surveys have shown that up to 40 percent of stars in the Milky Way have a Jupiter-class planet. “The fact that we get a record that doesn’t have a big honking planet around it doesn’t seem that surprising,” he says. “It’s a clean record that reminds me of freshly fallen snow.” Vega is also interesting because it has been a touchstone for the study of debris disks, being the first star observed to have a disk. 40 years ago thanks to its brightness and proximity. “Vega was the archetype, the first indication that there was dust around other stars,” says Macintosh. “Now we have a beautiful image, which is nice.”

Another possibility that no planets have been observed around Vega is that the giant planets formed, but were ejected from the system or moved closer to the star to a position that we cannot now see. “There’s a lot of room to hide a planet above a star,” says Macintosh. Johansen has another suggestion: It may be the “metallicity” of a star (the abundance of elements heavier than hydrogen and helium) that dictates the existence of planets. “Vega has a relatively low metallicity, about a third of the heavy elements in our sun,” he says. “Maybe there weren’t enough planetesimals to form planets.” Heavier elements appear to be particularly crucial for the formation of giant planets, as they catalyze the rapid growth of a core up to about ten Earth masses. “If you run the simulations, you don’t get very much done” around low-metallicity stars, Johansen says. “There is not enough time to grow before the gas disk disappears.”

Such ideas can inform the search for other worlds. We might want to focus less on low-metallicity stars, perhaps assuming they don’t have many planets. Or maybe it would be the opposite. “Perhaps (a planetary system’s) ‘habitability’ stops at high metallicity, because that creates too many giant planets,” says Johansen; perhaps “a lower metallicity star is more likely to host terrestrial planets”. Whatever the answer, although we can apparently rule out giant planets in front of Vega, right now, our telescopes aren’t powerful enough to look deeper. “We don’t rule out terrestrial planets, but someone else has to make those observations,” says Wolff.