Expert Voices

Could the solar system be teeming with interstellar objects? We'll soon find out (op-ed)

News
By
published

New telescopes will give breathtaking insight into what, or who, is visiting Earth's neighborhood.

artist's illustration of a pancake-shaped comet in deep space outgassing a whitish cloud of hydrogen
An artist's depiction of the interstellar comet 'Oumuamua, as it warmed up in its approach to the sun and outgassed hydrogen (white mist), which slightly altered its orbit. (Image credit: NASA, ESA and Joseph Olmsted and Frank Summers of STScI)

How do you tell the difference between a spaceship and a space rock?  

For astronomers like me, this question has proved tricky — so tricky that we were very nearly fooled when the first recorded interstellar object, named 'Oumuamua, was spotted passing through the inner solar system in 2017.

Astronomers once thought that any object moving under its own speed, with no visible means of propulsion, would have to be artificial technology. We knew how 'Oumuamua should behave if it were an interstellar space rock, or so we thought, but in every way it acted the exact opposite. 

Related: 'Oumuamua: The solar system's 1st interstellar visitor explained in photos

As 'Oumuamua sped through the solar system, we only had a few weeks during which we were able to monitor it with our telescopes across the world. These observations showed that the object came from outside the solar system, and was extremely elongated and tumbling. 'Oumuamua displayed no comet-like tail but still moved under its own propulsion. 

Comets also move under their own propulsion, but they display beautiful tails caused by dust and ice blown off their surfaces, which also causes a rocket-like recoil. 'Oumuamua was different. Its mysterious flight through our solar system sparked whispers of an artificial provenance. Some astronomers even called it Rama.  

The true explanation, which we determined from its motion and the amount of energy it received from the sun, is just as strange: an entirely new kind of object. We call them "dark comets." Since we spotted 'Oumuamua, we have found a population of seven dark comets hiding among our solar system's asteroids. They get their name because they move like a comet, but display no dust tail.  

Visitors from beyond the solar system — interstellar travelers — have long been predicted by science fiction writers like Arthur C. Clarke, but in reality, it's not so easy to identify what we're seeing — or just how many interstellar space rocks, or spaceships, are in Earth's neighborhood at a given time. This difficulty came to a head as we rushed to make sense of 'Oumuamua, but with new telescopes like the James Webb Space Telescope (JWST) and the forthcoming Rubin Observatory Legacy Survey of Space and Time (LSST) in the Atacama Desert in Chile, we'll have the tools we need to make sense of future interstellar travelers — and expand our understanding of the cosmos.  

If only we'd had the JWST when 'Oumuamua was discovered, we would have been able to take more detailed images of the object and get information about how it looked in different wavelengths of light. By doing this, we could have better understood what was propelling it through space. We would have seen molecules like water or carbon dioxide that only show up at certain wavelengths and don't reflect sunlight. We routinely see normal solar system comets being propelled as their surfaces heat up and water and carbon dioxide ice transform into gas. We would have identified it immediately as a dark comet-like object. The good news is, if we spot another 'Oumuamua, we already have an approved JWST program to monitor it.  

Related: James Webb Space Telescope (JWST) — A complete guide

RELATED STORIES:

— Could a spacecraft 'catch up' to interstellar visitor 'Oumuamua in just 26 years?

— 'Oumuamua origin story: How our mysterious interstellar visitor may have been born 

— The Vera C. Rubin Observatory: New view of the universe

In the future, the JWST won't be the only powerful tool in our toolkit. The LSST, set to come online in the next year or so, should detect many more interstellar objects and be able to identify dark comets in our own solar system. Located in the Atacama Desert, a prime spot for viewing the heavens, the LSST will start scanning the entire Southern Hemisphere sky almost every night. The LSST will be able to spot much fainter objects than any of our surveys that currently monitor the entire sky for rapidly moving objects. It will be able to detect mysterious objects like 'Oumuamua that are dimmer, either because they are smaller or farther away from the Earth. The LSST will also be able to find many more smaller asteroids, and will be able to spot if they are dark comets and moving under their own propulsion. 

It's possible that when the LSST makes first light, we will start detecting interstellar objects on a monthly — or even weekly — basis. Some of these may be dark like 'Oumuamua, and some may be brighter with beautiful tails, like the second interstellar comet, 2I/Borisov.   

It may turn out that the sky close to the Earth is teeming with interstellar objects even smaller than 'Oumuamua, all of which were invisible up until now. These small interstellar visitors could quite plausibly be continuously whizzing through the Earth's neighborhood within the solar system. If that ends up happening, then the LSST might spot an interstellar target close enough for a dedicated space mission.  

Close encounters with an interstellar visitor will no longer be a topic reserved for science fiction writers. 

With the LSST and JWST, we have the tools to tell the difference between an intelligent visitor and an interstellar dark comet. Let's not get fooled. 

Darryl Seligman is a research associate in the Department of Astronomy at Cornell University. His research focuses primarily on theoretical and computational planetary science and astrophysics. 

Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: community@space.com.

Darryl Seligman
Darryl Seligman
Research associate, Department of Astronomy, Cornell University

Darryl Z. Seligman completed his undergraduate degrees in Mathematics and Physics at the University of Pennsylvania in 2015. He completed his Ph.D. at Yale University in Astronomy in 2020, and was awarded the Yale University Dirk Brouwer Memorial Prize for Outstanding Ph.D. Thesis. He was the TC Chamberlin Fellow at the University of Chicago Department of the Geophysical Sciences after completing his Ph.D. He is currently a Simonyi-NSF Scholar at Cornell University, an NSF Astronomy and Astrophysics Postdoctoral Fellowship award made in recognition of significant contributions to Rubin Observatory’s Legacy Survey of Space and Time.

8 Comments Comment from the forums
  • Dave
    It has not been proven what Oumuamua actually was. Yes, we have found dark comets and yes these phenomena exist. And yet, nothing has ever behaved or looked or changed speed like Oumuamua. This is an object we no very little about. This is the fault of science it always pretends it has all the answers instead of saying the truth. The truth is there are many phenomena we no nothing about. Science does not have all the answers. We need to investigate and let science prove the facts.
    Reply
  • billslugg
    Oumuamua showed a greater than expected acceleration as it left the Solar System. This was due to invisible hydrogen gas emanating from holes in the surface. The hydrogen had been generated over the eons by the dissociation of H2O molecules by cosmic rays and held frozen by interstellar cold.

    It was never imaged as more than a point, but its brightness varied regularly by 2.5 magnitudes. Either it was unusually elongated or it had a bright patch, or both. The uncertainty in dimensions could place it within the range of objects already seen in the Solar System.
    Reply
  • Dave
    It had an unusual orbit. One perfect for observation. It does not look like a comet at all. If we wanted to observe an intelligent race at some point in our far future without revealing ourselves this would be a a successful way to do so.
    Reply
  • Unclear Engineer
    One thing that I wonder about is the certainty that the object is not really part of this solar system. I understand that it passes by the Sun at more than escape velocity for its location in our solar system. But, why could that not be the result of a gravitational "collision" (interaction without physical contact) between that object and another object originally located in the Kuiper Belt or the Ort Cloud? We know that gravitational interactions can eject even large objects from the solar system - why can't the ejection path be initially inward instead of directly outward? After all, we already have used gravitational effects ourselves to propel our Voyager and other probes to more than escape velocity.
    Reply
  • billslugg
    Oumuamua could not have obtained its velocity from any known object in the Solar System. We have a complete census of objects big enough to do it within a volume out to 21 AU. Outside that distance, no object in orbit around the Sun has sufficient velocity to do it, by at least an order of magnitude. It might have come from a passing star, though.

    https://iopscience.iop.org/article/10.3847/2515-5172/aa9f23
    Reply
  • Unclear Engineer
    Bill, Thanks for the link. Skimming it, I am wondering about the assumption that the orbits of hypothetical objects would be at their circular orbit velocities for their distances from the Sun. IF there is a "planet 9" on a highly elliptical orbit, wouldn't its velocity at perihelion be substantially more than circular velocity at that radius? If it happened to gravitationally collide with another object at perihelion, wouldn't it potentially transfer more velocity than calculated in this paper? And, then there is the potential for the smaller body to have been on a different highly elliptical orbit and having a velocity above its own V-circular at the time of collision.

    Just spit-balling, it seems to me that they needed to use the escape velocity at a specific radius for the hypothetical collision in order to logically upper-bound the velocity that something could have by the time it reaches near Earth's orbit from a collision at the far edges of the solar system.

    On the other hand, the probability of a real object experiencing an event near that upper limit seems tiny. So, if we see a substantial number of such objects zip by us, it would be very improbable that they came from such improbably gravitational interactions among bound objects, even if it is "possible".
    Reply
  • billslugg
    "IF there is a "planet 9" on a highly elliptical orbit, wouldn't its velocity at perihelion be substantially more than circular velocity at that radius?" - UE

    Yes, but the escape velocity of all objects outside of 21 AU is not sufficient, it does not matter what their orbit is. Everything massive enough within 21 AU is accounted for.

    Oumuamua must have come from another star to a near certain probability. The gap is ten times more velocity than any one Solar System object can give it. It would have had to have many, many encounters, each with a tiny probability.
    Reply
  • Dave
    It is our responsibility to know what an object is that comes from outside our star system into our own. A mission from NASA needs to be ready to investigate any phenomena we have little knowledge about. Who knows what other phenomena is out there?
    Reply