Gembloux (Belgium): The majority of known asteroids orbit inside the main asteroid belt, which is positioned between Mars and Jupiter at an average distance of around 250 million km from Earth. Since the discovery of the first asteroid in 1801, about 750.000 asteroids have been identified, primarily in the last decade thanks to several optical surveys that examine the sky on clear nights.
Majority of asteroids
The majority of these asteroids are larger than one kilometre in diameter, with the largest, Vesta, being roughly 530 km, and millions of smaller ones are expected. While they appear to be massive figures, the total mass of all asteroids combined is less than that of the Earth's Moon.
The study of small bodies of the solar system is of high importance as they give us a unique view of the early phases of our system, even before the planets formed. Being able to observe the smallest main-belt asteroids would not only give us clues about the building blocks of planets, but also provide a unique window into the source of meteorites falling on Earth", explains Jehin, a planetary scientist at the University of Liege part to the new study led by MIT astronomers (and ULiege Alumni) Burdanov and de Wit. "But, until now, only asteroids about 1 km or larger were spotted in the main belt".
Nearby planets Mars and Jupiter
Sometimes, these asteroids are nudged into Earth's neighbourhood by the gravity of the nearby planets Mars and Jupiter. These objects are called Near-Earth asteroids and about 35000 of them have been discovered, and counting.
Many of them have slammed into Earth and the other planets in the past, playing a major role in altering the geological history of the planets and in the evolution of life on Earth. Today they are actively searched and studied in the context of the Earth's planetary defense.
De Wit and his team are primarily focused on searches and studies of exoplanets -- worlds outside the solar system that may be habitable. The researchers are part of the group led by the ULiege astronomer Gillon, which in 2016 discovered the planetary system around TRAPPIST-1, a tiny red star that is about 40 light years from Earth. Using first the ULiege Transiting Planets and Planetismals Small Telescope (TRAPPIST) in Chile, the team confirmed using many other telescopes that the star hosts seven rocky, Earth-sized planets, several of which are in the habitable zone.
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The TRAPPIST-1 system
The TRAPPIST-1 system became quickly the most observed and the best characterized exoplanetary system. In 2023, Gillon and other colleagues used the most powerful infrared observatory -- the NASA's James Webb Space Telescope (JWST) - to search for signs of atmospheres around the two inner planets of the system.
While these results are still under review, de Wit and Burdanov wondered whether the same data used to study exoplanets could be recycled and mined for serendipitous asteroids crossing by chance the TRAPPIST-1 field of view.
To do so, they looked to "shift and stack," an image processing technique that was first developed in the 1990s. The method involves shifting multiple images of the same field of view and stacking the images to see whether an otherwise faint object moving at the right speed can outshine the background noise.
This method requires significant computational resources, as it involves testing a huge number of scenarios for where an asteroid might be, but it was already tested successfully by the MIT team using state-of-the-art GPUs graphics processing units that can process an enormous amount of imaging data at high speeds.
Main belt
The team applied this approach to more than 10,000 JWST images of the TRAPPIST-1 field, which were originally obtained to search for signs of atmospheres around the system's inner planets. After processing the ima
ges, the researchers were able to spot eight known asteroids in the main belt. They then looked further and discovered 138 new asteroids, all within tens of meters in diameter -- the smallest main belt asteroids detected to date. They suspect a few asteroids are on their way to becoming near-Earth objects, while one is likely a Trojan -- an asteroid that trails Jupiter.
"We thought we would just detect a few new objects, but we detected so many more than expected, especially small ones" de Wit says. "It is a sign that we are probing a new population regime where many more small objects are formed through cascades of collisions that are very efficient at breaking down asteroids below roughly 100 meters."
"Statistics of these very small main belt asteroids are critical for modelling the asteroid population. In fact, these are the debris ejected during collisions of bigger, kilometers-sized asteroids, which are observable and often exhibit similar orbits about the Sun, so that we ca group them into 'families' of asteroids" adds Miroslav Broz, from the Prague Charles University in Czech Republic, co-author of the study, and a specialist of the various asteroid populations in the solar system.
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"We were not expecting that such impactful solar-system bonus science could be done using cutty-edge exoplanet observations!" says ULiege co-author Gillon.
JWST infrared capabilities
The success of this investigation was allowed because of the extraordinary sensitivity of the JWST which, being in space far from Earth, with its large mirror and state of the art instrumentation is particularly sensitive to infrared rather than visible light. "And as it happens, asteroids that orbit in the main asteroid belt are much brighter at infrared wavelengths than at visible wavelengths, and thus are far easier to detect with JWST's infrared capabilities", says Jehin.
Other planetary scientists are delighted by these results. "It is fantastic to see how archived JWST data can open new doors to a better understanding of the smallest asteroids, which play a crucial role in planetary defense. Our findings rely on a new and innovative technique to determine an asteroid's size from simple infrared detections without knowing the object's true orbit", explains Thomas Muller co-author from the Max-Planck-Institut in Garching, Germany and a specialist of the infrared radiation of asteroids.
"Our ability to spot thanks to the JWST these small asteroids when they are much farther away from Earth, allow us now to perform more precise orbital determinations, which are crucial for planetary defense", confirms Marco Micheli co-author from the European Space Agency (ESA) Near-Earth Object Coordination Centre, in Italy.
"This is a totally new, unexplored space we are entering, thanks to modern technologies," concludes Burdanov. "It's a good example of what we can do when we look at the data differently. Sometimes there's a big payoff, and this is one of them."
