A study published today (Wednesday) reports that the mass of the Milky Way galaxy is four to five times less than previously thought, results that overturn what is known to date about the galaxy that contains the planet Earth.

As astronomer François Hamer, co-author of the study published by the journal “Astronomy and Astrophysics,” explained to Agence France-Presse, the result is “the result of the Gaia revolution.” Gaia, a satellite dedicated to mapping the Milky Way galaxy, revealed the positions and motions of 1.8 billion stars in its latest data in 2022.

It represents a small fraction of the total contents of our spiral galaxy, a disk about 100,000 light-years in diameter and consisting of four large arms, one of which includes our Solar System, all of which extend around a very luminous core. .

The authors of the study report that by studying the Gaia data they were able to calculate the rotation curve of the Milky Way with unprecedented precision. The task is to determine the speed at which celestial bodies rotate around the center of the galaxy.

Observations of spiral galaxies have previously concluded that this curve is “flattening”, meaning that once a certain distance from the center is reached, the rate of rotation is constant.

But “this is the first time we find that the curve descends outside its disk,” according to Francois Hamer, “as if there wasn’t a lot of matter” in the galaxy between 50 and 80 thousand years from the center.

As a result, “the mass of our Milky Way Galaxy has been revised to very low values,” about 200 billion times the mass of the Sun, five times less than previous estimates.

“Bold Decisions”

The study, conducted by an international team and led by astronomers from the Paris Observatory and the National Center for Scientific Research in France, has a second major conclusion, as it questions “the relationship between luminous matter and dark matter.” Astronomer.

This hypothesis is also called thematic theme because it is so far invisible and undetectable. It is thought to provide the mass necessary for the coalescence of galaxies, and represents about six times the mass of luminous matter that contains stars and gas clouds. For the Milky Way, the study calculates that this ratio is much lower, with three times as much dark matter as bright matter.

But astronomer François Combes, a colleague of François Hammer at the Paris Observatory, told Agence France-Presse that the results were “a bit bold” or “perhaps not well founded.”

This is significant because this study focuses on low galactic radii, while astronomers typically calculate galactic masses by taking greater distances into account.

However, apart from gas, globular clusters, dwarf galaxies or the Magellanic Cloud, “up to this distance we have a lot of dark matter,” notes Francois Combe, a senior expert on galaxy evolution.

But the Gombe galaxy, about 80,000 light-years away from the center of the galaxy, welcomes “very precise work that will improve our knowledge of the stars and their rotation”.

François Hammer’s team defends this work by talking about the uniqueness of our galaxy. Unlike most spiral galaxies, which saw violent collisions between galaxies six billion years ago, the Milky Way “evolved very quietly for nine billion years,” Hamer says.

Also, the difference between the Milky Way and other spiral galaxies can be justified by the observational method, which relies on stars in the former and gas clouds in the latter.

Meanwhile, Françoise Combe believes the Milky Way galaxy is “not exceptional,” but in terms of dark matter, “it’s like the others.”