Long before we learned to name them, strangers were passing through our neighborhood: small wanderers born around other stars, drifting in from beyond the Sun's reach. Since Oumuamua and Borisov first announced themselves, astronomers have wondered about the visitors who went unseen, and a new simulation study suggests some of them may have been caught. Jupiter's immense pull appears to be the chief gatekeeper, with the Earth and Moon offering occasional, fleeting snares. Yet the captures are impermanent, like guests who stay a while and then, over tens of thousands of years, wander off again,
Study Suggests Interstellar Objects May Be Captured in Near-Earth Orbits, Though They Don't Linger
Only 0.1 to 1 percent stay in near-Earth orbit by 10 million years
Mark
So what is this paper actually claiming?
Mimi
That interstellar objects can be captured into near-Earth orbits, mostly by Jupiter, and that some may be there now, though they don't stay long.
Mark
How long is long?
Mimi
Half-lives of about 50,000 years for Jupiter captures and 130,000 for Earth-Moon captures. Only 0.1 to 1 percent survive to 10 million years.
Luke
Worth saying plainly: this is simulation, not observation. No captured ISO has been found, and the paper isn't peer-reviewed yet.
Mark
Why does Jupiter matter so much?
Mimi
Its capture cross-section dominates by a factor of 10^4 over the Earth-Moon alone.
Luke
Though Jupiter-captured objects also die faster, so the dominance applies to capture, not to survival.
Mark
What would these objects look like?
Mimi
Mostly about one meter across. Nothing like Oumuamua or Borisov.
Luke
And the authors themselves say their initial orbits aren't a picture of today's population, since later planetary interactions reshape them. Whether any are here now is unknown.
Mimi
The Centaur point is the open thread: no known Centaur is considered interstellar, but the authors say a closer look may be merited.
Le Pouls
- A new simulation suggests interstellar wanderers are being briefly snared into orbits near Earth, and the claim has not yet faced peer review.
- Jupiter's gravity appears to outpace Earth and the Moon as a trap, catching visitors at rates far beyond what the inner planets manage alone.
- Captured objects linger for only about 50,000 to 130,000 years before drifting away, leaving astronomers racing a clock they cannot see.
- Most of these hidden travelers are thought to be about a meter across, far too small to spot easily with today's instruments.
- The Vera Rubin Observatory's coming survey may reveal them, and ESA's Comet Interceptor, slated for 2029, could one day intercept one up close.
Long before we learned to name them, strangers were passing through our neighborhood: small wanderers born around other stars, drifting in from beyond the Sun's reach. Since Oumuamua and Borisov first announced themselves, astronomers have wondered about the visitors who went unseen, and a new simulation study suggests some of them may have been caught. Jupiter's immense pull appears to be the chief gatekeeper, with the Earth and Moon offering occasional, fleeting snares. Yet the captures are impermanent, like guests who stay a while and then, over tens of thousands of years, wander off again, and the work awaits peer review and the patient eyes of new observatories.
In 2017, Oumuamua crossed the Solar System and captured the world's attention as the first interstellar object ever identified. Two years later, Comet 2I Borisov arrived as the second. Those visitors sparked wonder, and now a new study turns to the ones nobody saw, asking whether some of these wanderers were caught by the planets and held in orbits close to home.
The paper, led by Diptajyoti Mukherjee of Carnegie Mellon University with Hy Trac, Amir Siraj and Avi Loeb, has not yet been peer-reviewed. Using large suites of simulations, the team traced which incoming objects were pulled into near-Earth orbits. Their answer was clear: Jupiter dominates the capture, drawing in far more visitors than the Earth-Moon system does on its own.
But the captures do not last. Objects held by Jupiter remain near Earth for roughly 50,000 years on average, and those caught by the Earth-Moon system for about 130,000 years. Most are around a meter across, too small for present instruments to readily detect. The researchers note that their starting orbits are not a snapshot of today's captured population, since time reshapes these paths, so they ran longer calculations to see what survives.
The search now rests with new eyes. The Vera Rubin Observatory's planned ten-year survey may find a few Oumuamua-sized objects each year, and ESA's Comet Interceptor, planned for 2029, could wait near the Sun-Earth L2 point for a chance to study one closely. The authors suggest a larger hidden population may already be sitting among known near-Earth objects, waiting for someone to look more carefully.
In 2017, Oumuamua passed through the Solar System and drew attention around the world. It was the first interstellar object, or ISO, astronomers had ever identified. In August 2019 Comet 2I Borisov followed as the second. Both set off a wave of inquiry and speculation, and a new study asks what may have happened to the visitors nobody saw.
The paper is titled "Close Encounters of the Interstellar Kind: Exploring the Presence of Interstellar Objects in Near Earth Orbit." Its first author is Diptajyoti Mukherjee, a graduate student in computational astrophysics in the Physics Department at Carnegie Mellon University. His co-authors are Hy Trac, Amir Siraj and Avi Loeb. It has not yet been peer-reviewed. The authors argue that ISOs, though rare, can be captured into bound orbits by planets, and that the Solar System is old enough for many to have passed through. Earlier work looked at the Vera Rubin Observatory finding up to five ISOs a year, at the Oort Cloud possibly holding more ISOs than native objects, at cosmic rays eroding ISOs, and at Jupiter destroying them. None examined captured ISOs in near-Earth orbits specifically.
The team used numerical simulations in which each particle stands for a potential ISO arriving from outside the Solar System on its own trajectory. The particles are scattered by the Earth, the Moon, the Sun and Jupiter in various combinations. After a large suite of N-body scattering simulations, the researchers mapped the "capture cross-sections," the ranges of space and velocity that end with an object in a near-Earth orbit. One scenario included only the Sun and the Earth-Moon system, to isolate the effect of the giant planets.
Jupiter dominates. The authors report that the combined Earth-Moon and Jupiter capture cross-section outweighs that of the Earth-Moon alone by a factor of 10^4. Capture efficiency also falls as an object's excess hyperbolic velocity rises.
Comparing their results with known small bodies, the team found that a captured ISO would likely have an average distance from the Sun greater than 10 AU, which is where the Centaurs live. Centaurs are small bodies whose orbits are unstable because of the giant planets. Siraj and Loeb examined ISOs hiding among Centaurs in 2019, and no known Centaur is considered interstellar. The authors write that their study suggests "a closer examination may be merited."
They also caution that their initial orbits are not a picture of present-day captured ISOs. Objects captured long ago would have had their orbits reshaped by other planets. To test survival, the team ran long-term calculations on a subset of captured objects out to 10 million years. Only 0.1 to 1 percent stayed in near-Earth orbits by then, far shorter than the lifetimes of known near-Earth objects. Those that leave either exit the Solar System or settle into orbits with larger perihelion and semi-major axis. Objects captured by Earth survive on average two to three times as long as those captured by Jupiter.
The half-life of Jupiter-captured ISOs in near-Earth orbits is about 50,000 years, and for Earth-Moon captures about 130,000 years. The Jupiter-captured population falls to 10 percent of its original fraction in 800,000 years, and the Earth-Moon one in 2.1 million. The population is dominated by objects around one meter across. Something the size of Oumuamua or Borisov could not be captured, short of doing it artificially.
Finding any of them falls to facilities like the Vera Rubin Observatory and its Legacy Survey of Space and Time, a planned ten-year survey of the southern sky expected to begin after first light in August 2024. Some estimates say it will find a few Oumuamua-sized objects per year. The authors suggest a larger captured population could be hidden among near-Earth objects. For close study, the ESA's Comet Interceptor, partnered with JAXA and planned for launch in 2029, could wait at the Sun-Earth L2 point until an ISO is spotted, then be sent to intercept it, observe it and collect a sample from its tail.
Citations marquantes
ISOs hiding amongst the Centaurs have been examined by Siraj & Loeb (2019), but no known Centaurs are considered to have an interstellar origin. However, our study suggests a closer examination may be merited.— Mukherjee et al., 2023 preprint
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