On October 19, 2017, the first interstellar object, ‘Oumuamua, was discovered by the Pan-STARRS survey. The experience was similar to having a surprise guest for dinner show up from another country. By examining this guest, we can learn about the culture of that country without the need to travel there—a good thing in this case, given that it would take us a hundred thousand years to visit even the nearest star using conventional chemical rockets.
Surprisingly, our first interstellar guest appeared to be weird and unlike anything we have seen before. By the time we realized it, the guest was already out the door with its image fading into the dark street, so we did not have a chance to get a second look at its mysterious qualities. Below is a list of six peculiarities exhibited by ‘Oumuamua:
- Assuming that other planetary systems resemble the solar system, Pan-STARRS should not have discovered this or any other interstellar rock in the first place. In a paper published a decade ago, we predicted an abundance of interstellar asteroids that is smaller by many (two to eight) orders of magnitude than needed to explain the discovery of ‘Oumuamua, assuming it’s a member of a random population of objects. Put another way, ‘Oumuamua implies that the population of interstellar objects is far greater than expected. Each star in the Milky Way needs to eject 1015 such objects during its lifetime to account for a population as large as ‘Oumuamua implies. Thus, the nurseries of ‘Oumuamua-like objects must be different from what we know based on our own solar system.
- ‘Oumuamua originated from a very special frame of reference, the so-called local standard of rest (LSR), which is defined by averaging the random motions of all the stars in the vicinity of the sun. Only one star in 500 is moving as slowly as ‘Oumuamua in that frame. The LSR is the ideal frame for camouflage, namely for hiding the origins of an object and avoiding its association with any particular star. The relative motion between ‘Oumuamua and the sun reflects the motion of the sun relative to the LSR. ‘Oumuamua is like a buoy sitting at rest on the surface of the ocean, with the solar system running into it like a fast ship. Could there be an array of buoys that serves as a network of relay stations or road posts, defining the average galactic frame of reference in interstellar space?
- Most interstellar asteroids are expected to be ripped away from their parent star when they lie in the outskirts of their birth planetary system (such as our solar system’s Oort cloud, which extends to 100,000 times the Earth-sun separation), where they are most loosely bound to the star’s gravity. At these outskirts, they can be removed with a small velocity nudge of less than a kilometer per second, in which case they will maintain the speed of their host star relative to the LSR. If ‘Oumuamua came from a typical star, it must have been ejected with an unusually large velocity kick. To make things more unusual, its kick should have been equal and opposite to the velocity of its parent star relative to the LSR, which is about 20 kilometers per second for a typical star like the sun. The dynamical origin of ‘Oumuamua is extremely rare no matter how you look at it. This is surprising, since the first foreign guest to a dinner party should be statistically common (especially given the larger than usual population inferred in the first point above).
- We do not have a photo of ‘Oumuamua, but its brightness owing to reflected sunlight varied by a factor of 10 as it rotated periodically every eight hours. This implies that ‘Oumuamua has an extreme elongated shape with its length at least five to 10 times larger than its projected width. Moreover, an analysis of its tumbling motion concluded that it would be at the highest excitation state expected from its tumultuous journey, if it has a pancake-like geometry. The inferred shape is more extreme than for all asteroids previously seen in the solar system, which have an length-to-width ratio of at most three.
- The Spitzer Space Telescope did not detect any heat in the form of infrared radiation from ‘Oumuamua. Given the surface temperature dictated by ‘Oumuamua’s trajectory near the sun, this sets an upper limit on its size of hundreds of meters. Based on this size limit, ‘Oumuamua must be unusually shiny, with a reflectance that is at least 10 times higher than exhibited by solar system asteroids.
- The trajectory of ‘Oumuamua deviated from that expected based on the sun’s gravity alone. The deviation is small (a tenth of a percent) but highly statistically significant. Comets exhibit such a behavior when ices on their surface heat up from solar illumination and evaporate, generating thrust through the rocket effect. The extra push for ‘Oumuamua could have originated by cometary outgassing if at least a tenth of its mass evaporated. But such massive evaporation would have naturally led to the appearance of a cometary tail, and none was seen. The Spitzer telescope observations also place tight limits on any carbon-based molecules or dust around ‘Oumuamua and rule out the possibility that normal cometary outgassing is at play (unless it is composed of pure water). Moreover, cometary outgassing would have changed the rotation period of ‘Oumuamua, and no such change was observed. Altogether, ‘Oumuamua does not appear to be a typical comet nor a typical asteroid, even as it represents a population that is far more abundant than expected.
The extra push exhibited by ‘Oumuamua’s orbit could not have originated from a breakup into pieces because such an event would have provided a single, impulsive kick, unlike the continuous push that was observed. If cometary outgassing is ruled out and the inferred excess force is real, only one possibility remains: an extra push due to radiation pressure from the sun. In order for this push to be effective, ‘Oumuamua needs to be less than a millimeter thick but with a size of at least 20 meters (for a perfect reflector), resembling a lightsail of artificial origin. In this case ‘Oumuamua would resemble the solar sail demonstrated by the Japanese mission IKAROS or the lightsail contemplated for the Starshot initiative. An artificial origin offers the startling possibility that we discovered “a message in a bottle” following years of failed searches for radio signals from alien civilizations. Reassuringly, such a lightsail would survive collisions with interstellar atoms and dust as it travels throughout the galaxy.
In contemplating the possibility of an artificial origin, we should keep in mind what Sherlock Holmes said: “when you have excluded the impossible, whatever remains, however improbable, must be the truth.” The Kepler satellite revealed that about a quarter of all the stars in the Milky Way have a habitable planet of the size of the Earth, with the potential to have liquid water on its surface and the chemistry of life as we know it. It is therefore conceivable that interstellar space is full of artificially made debris, either in the form of devices that serve a purpose on a reconnaissance mission or in the form of defunct equipment. However, to validate an exotic artificial origin for ‘Oumuamua, we need more data. As Carl Sagan said, “extraordinary claims require extraordinary evidence.”
In fact, the possibility of a targeted mission adds some explanatory power. It is unlikely that 1015 solar sails are launched per star to make up a random population of ‘Oumuamua-like objects. This would require the unreasonable rate of a launch every five minutes from a planetary system even if all civilizations live as long as the full lifetime of the Milky Way galaxy. Instead, the required numbers could be reduced dramatically if ‘Oumuamua-like objects do not sample all possible orbits randomly but rather follow special orbits that dive into the innermost, habitable regions of planetary systems like our solar system.
‘Oumuamua moves too fast for our chemical rockets to catch up with it now without a gravitational assist from planets. But since it would take ‘Oumuamua thousands of years to leave the solar system entirely, getting a closer look of it through a flyby remains a possibility if we were to develop new technologies for faster space travel within a decade or two. Interestingly, some interstellar objects that pass close to Jupiter can lose energy and get captured by the solar system. These are dinner guests who bumped into a wall on their way out and stayed around after dinner. The Sun-Jupiter system acts as a fishing net. If we can identify trapped interstellar objects through their unusual bound orbits with unusually high inclinations relative to the solar system plane, we could design missions to visit them and learn more about their nature.
Alternatively, we can wait for the next interstellar guest to show up. Within a few years, the Large Synoptic Survey Telescope (LSST) will become operational and be far more sensitive to the detection of ‘Oumuamua-like objects. It should therefore discover many such objects within its first year of operation. If it does not find any, we will know that ‘Oumuamua was special and that we must chase this guest down the street in order to figure out its origin.
Studying interstellar objects resembles my favorite activity when walking along the beach with my daughters. We enjoy picking up seashells that were swept ashore and learning about their different origins. Every now and then, we find a plastic bottle that indicates an artificial origin. Similarly, astronomers should examine any object that enters the solar system and study its properties. There is no doubt that the six peculiar features of ‘Oumuamua have the potential to usher in a dramatic new era in space science.