A Canadian asteroid belt | La Presse

The probe New Horizons has revealed a new category of asteroids. This discovery by a Canadian could mean that there is a third asteroid belt in our solar system.

The surprise

After scrutinizing Pluto like never before, New Horizons has been studying the Kuiper Belt for several years, an area filled with celestial bodies whose existence was unknown until very recently.

The plan was then to recalibrate the probe launched in 2006 for a heliophysical mission: the study of the solar wind. The conversion was mentioned as early as 2020. But a big surprise arose.

“A couple of years ago, we started seeing an unexpected and very large number of unexpected objects,” says Alan Stern, an astrophysicist at the Southwest Research Institute (SWRI) in Texas who leads the New Horizons program.

These “objects” mentioned by Alan Stern are asteroids or comets. And as New Horizons Although there haven’t been many asteroids in recent years, astronomers thought the Kuiper Belt ended as expected at 50 astronomical units (AU) — a unit of measurement corresponding to the distance between Earth and the Sun.

But when New Horizons arrived at 60 AU, its instruments “counted” five times more asteroids than expected, says Wes Fraser, an astrophysicist at the University of Victoria in British Columbia who is the lead author of the study published earlier this month in Planetary Science Journal of the American Astronomical Society.

These objects are as far from Pluto, the best-known of the “Kuiper Belt Objects” (KBOs), as Pluto is from Earth.

This could mean that the Kuiper Belt is much larger than previously thought. Or, if the objects beyond 60 AU are different from the other KBOs, it could be a “second Kuiper Belt.” Several science media outlets, including the journal Sciencehave adopted this name. This would mean that there are three asteroid belts in the Solar System, the best known being located between Mars and Jupiter.

Is this discovery as important as that of the Kuiper belt, a little over 30 years ago?

Probably not, say Mr. Stern and Mr. Fraser. Unless these distant KBOs turn out to be of a different nature than the others, says Wes Fraser.

“In any case, we have at least demonstrated that the Kuiper belt disk is very extended, as in other solar systems that we have observed,” Stern said. “Our solar system was an anomaly with its very short Kuiper belt.”

The resonance

These new objects probably do not form a perfect circle around the Sun, because they are in “resonance” with Neptune.

Two celestial bodies in stable resonance have a linked orbit. For example, Pluto’s attraction to Neptune causes Pluto to make two orbits for every three that Neptune makes. This is called a 3:2 resonance with Neptune.

Like Pluto, a quarter of the KBOs are in a 3:2 resonance with Neptune. But this resonance is not possible everywhere in the orbit of Pluto and the Kuiper belt, which means that there are holes in the latter.

Asteroids discovered by New Horizons are in a 5:2 resonance with Neptune, meaning they make two orbits for every five that Neptune makes. “We think we’ll have regions of the orbit of this new belt where there will be objects, and other regions where there will be nothing,” says Fraser, who is also at the National Research Council of Canada’s Herzberg Research Centre for Astronomy and Astrophysics.

This explains why, unlike the asteroid belt located between Mars and Jupiter, the Kuiper belt and the one discovered by New Horizons do not form a continuous circle.

Watch a video of the 3:2 resonance between Neptune and Pluto

The verification

The New Horizons team didn’t immediately publicize its discovery. “We had to be sure it wasn’t an isolated object,” Fraser said.

A first observation by a terrestrial telescope did not find any asteroids at such a distance. This could be explained by a 5:2 resonance with Neptune.

It must be said that New Horizons had provided further evidence of the existence of this second Kuiper belt, Fraser notes: the amount of dust accumulating on its instruments had begun to increase again, a sign of collisions between asteroids.

The Japanese telescope Subaruin Hawaii, was tapped to study the regions of the sky where this 5:2 resonance predicted that asteroids from this second Kuiper belt would be found. The effort paid off, leading to the study published this month. Its results have been discussed a few times over the past year at scientific conferences.

The next step is to collect further observations, notably using the American space telescope. Nancy-Grace-Romanwhich is due to be launched in 2026.

Once enough of these distant KBOs have been observed, the proportion of their different sizes can be established. If the size profile differs from the KBOs already known, this will mean that they come from another region of the Solar System.

Three or four belts

In addition to the asteroid belt between Mars and Jupiter and the Kuiper belt(s), there is another region of the Solar System that contains several celestial bodies. The “Oort cloud” is thought to be composed mainly of long-period comets, and is located between 2,000 and 100,000 AU. This third or fourth “belt” of objects is thought to be spherical.

The Oort cloud and the Kuiper belt were predicted in the 1950s by two Dutch astronomers who gave them their names.

There are also “Trojan” asteroids, which follow the orbits of the various planets in the Solar System, and “Centaur” asteroids, located between Jupiter and Neptune. The orbits of both classes of asteroids are generally not stable.

The journey of New Horizons

The American probe took off in 2006 and studied Pluto and its moon Charron for more than six months in 2015. It passed within 12,000 km of the former ninth planet, which was downgraded to the status of “dwarf planet” in 2006. New Horizons detected volcanism on Pluto, which could mean the presence of a subsurface ocean and therefore bacterial Plutonian life. New Horizons also discovered evidence that methane is migrating from Pluto to Charron.

A previous version of this article included an incorrect figure regarding the resonance between Pluto and Neptune, as well as between newly discovered asteroids and Neptune.