The solar system may not be what we think it is

Since its feat on Pluto, which it approached within just 12,500 kilometers, the spacecraft has continued on its path, on a mission to explore the boundaries of our planetary system and the small icy worlds that exist there. “It was really an Alice in Wonderland-type story,” says Alan Stern, the mission’s principal investigator. “It was like a dream and we discovered amazing things.”

A dream that is not over yet, as New Horizons is about to give us another big surprise, a “gift” that may force us to change our ideas about the size of the solar system. But let’s see.

Unknown objects

In early 2024, one of the spacecraft’s sensors recorded an unexpected increase in the amount of dust in its path. A substance whose origin could be collisions between rock fragments, which is why astronomers are now wondering whether, beyond the Kuiper Belt, there is a large ring of debris, from the formation of the solar system, it is possible that there are a large number of objects. Because if that were the case, we would have to redraw the boundaries of our system, and even question our models of how it was formed. Stern says it’s a “detective story in progress.”

New Horizons, extended mission

In 2019, years after its visit to Pluto, the spacecraft approached a distant 35-kilometre-long asteroid called Arrokoth, from which it obtained a huge amount of data that is still being analysed. New Horizon then went into “hibernation mode”, with most of its instruments turned off. So he crossed the Kuiper Belt, and now, about 60 astronomical units away (one astronomical unit is 150 million kilometres, the distance between Earth and the sun) he is in a new and unknown region. Only NASA’s Pioneer and Voyager spacecraft flew farther, but their paths led them to leave the Kuiper Belt plane, while New Horizons remained inside.

This is how the only tool that still works, the dust detector, was able to detect something strange at the beginning of the year. In fact, between 55 and 60 AU, there was much more material than expected. A signal that reveals the presence of larger objects in the area that may collide with each other. This means that the Kuiper Belt could extend much further than we thought, and not suddenly end in the “Kuiper Escarpment” as was thought until now. Or there could be a “second belt” that we don’t know about yet.

To try to understand the situation better, New Horizons researchers used a ground-based observatory, the Subaru Telescope in Hawaii, to look at the region beyond the Kuiper Belt. Their results, presented at a conference in March 2023, show a surprising number of very distant objects beyond what was assumed to be the outer edge of the belt.

If this second belt really exists, we will be faced with a great discovery, one that will force us to “take another look” at what we think about the origin of the solar system. For example, until now, the “edge” of the Kuiper Belt (“Kuiper Slope”) has been explained by the hypothesis that another star came too close to the Sun in its youth, “sweeping away” all the objects it encountered. There was far beyond that distance. But if the Kuiper Belt doesn’t end where it was thought, it may never happen.

Scientific caution

However, astronomers are very cautious before launching their bells into the air, as the results from the Subaru telescope have not yet been reviewed, and other observations of what lies “beyond” 60 AU, made with other telescopes, have not been sufficient. Moreover, even if these data are confirmed, it is always possible that some objects in the Kuiper Belt itself have orbits that periodically take them beyond the “boundary”, which would mean that there is no second belt. To find out, more data is needed, including orbits of these objects beyond 60 AU.

Or it’s also possible that the belt is simply wider than previously thought, in the region that New Horizons now crosses.

However, if all the tests are passed and it eventually turns out that there are indeed two Kuiper Belts, this would mean that instead of a “Kuiper Escarpment,” the gap between the two belts would instead form a “Kuiper Valley.” . This, by the way, would make our solar system more like other systems than we thought. “We don’t know how unique our solar system is,” Stern says. Something that can also teach us a lot about our past. In fact, other recently formed star systems typically have gaps of around 100 AU, so if our system also has a gap, then studying these younger systems has something to teach us about our history.

Scientists aren’t entirely sure why these gaps exist. In other planetary systems, gaps in the disk are sometimes a sign of a nascent planet, sucking in dust as it rotates. But this occurs mainly in recently formed systems, and not in already mature systems, such as ours. It is also possible that the gap in the Kuiper Belt, if it exists, is caused by gravitational forces. Something similar, for example, to what happens in the asteroid belt between Mars and Jupiter, where the gap occurs because asteroids are affected by the enormous gravity of the giant planet.

However, there is still a lot to learn. New Horizons, whose mission has been extended by NASA until the end of the decade, still has a lot to contribute.

“The spacecraft – says Stern – is in excellent health, has power in its nuclear battery and a communications range that could allow it to operate until about 2050.” There are already plans to use future telescopes, such as the Vera C. Rubin Observatory and the Nancy Grace Roman Space Telescope, to discover new Kuiper Belt objects for flybys in the future.

Meanwhile, while waiting to set new targets, New Horizons is moving further and further away, entering uncharted territory.