by S. Prasad Ganti

We are used to sending our spacecraft to nearby astronomical bodies like the moon, the asteroids and the planets in our solar system. What is new over the last few years is the passage of transiting astronomical objects passing through our solar system. Like an out of town visitor driving through the main street of our town and going out without any stops. The most recent one was given the name of 3I/ATLAS. The third such object since we started tracking such extraneous objects. 

3I stands for third interstellar object (dubbed as ISO). While ATLAS stands for the detection system which spotted it – Asteroid Terrestrial-impact Last Alert System. We know that it is an interstellar object by the trajectory it takes. Almost all other intra solar system objects move in circles or ellipses around the sun, however elongated the orbit may be. The picture below, courtesy NASA, shows the trajectory of 3I/ATLAS in blue which is cutting across our solar system. The speed is about 200,000 kmph. Such a fast moving almost straight trajectory would be an interstellar object. Its path is  very close to the plane of our solar system which seems to be a coincidence. 

Avi Loeb, the scientist who postulated that such objects are alien probes coming in to take a peek at our solar system, thinks that the chances of a random visitor happening to line up so neatly with our planetary plane is very low. It also has a flight path very close to Venus, Mars, and Jupiter. Such a flyby can be expected from a NASA spacecraft or the one by an alien design! However, It has frozen carbon dioxide which is becoming a gas as it gets closer to the Sun. The outgassing indicates that 3I/ATLAS is a comet. Not a spacecraft. 

Like our solar system which is confined to a plane, our galaxy the Milky Way is also a disk which is thick at the center and tapers as one goes away from the center. Our solar system is in the thin part of our galactic plane. If we move sufficient distance away from our solar system or our galaxy, we can see the essential disk structure ! 

We had two ISOs before the current one. First was 1I/Oumuamua and the second one 2I/Borisov. Investigators reveal that all the 3 ISOs originated from different regions in our galaxy Milky Way and at different times. Their ages range in age from one to  several billion years. 3I/ATLAS is the oldest of the three, with an estimated age of 4.6 billion years, and originated from the Milky Way’s thick disk (closer to the center of the galaxy). It is also the largest of the three ISOs, at about twenty km in diameter. This region of our galaxy is populated by older, lower metallicity stars. In astronomy, anything other than Hydrogen and Helium are considered as metals !

1I/Oumuamua is about 1 billion years old and originated from the thin disk where new stars are still forming. 2I/Borisov is approximately 1.7 billion years old, and originated from the thin disk as well. 

Asteroids and comets are material leftover from the formation of planetary systems. We have our own asteroid belt between Mars and Jupiter and comets at the end of our solar system. Every one of them orbiting the Sun. Presumably other planetary systems also have similar bodies. Some bodies may get ejected and travel in interstellar space and may occasionally travel through other planetary systems like ours.  

It is possible some bodies from our solar system may have broken loose, or got ejected,  and ventured into interstellar space and possibly other planetary systems. Gravitational scattering could be one reason why objects get ejected from the host planetary systems. Or other unknown dynamic processes. It is not just asteroids or comets which can get out from their parents’ home. Sometimes planets can do so. We do have single planets without a parent star. But none came to visit us so far !

Planetary systems are not the same. They vary in  terms of the sizes of the stars, number and sizes of planets, number of moons etc. Studying ISOs gives us an indication of the planetary system where they originate. Instead of us going on interstellar journeys, the objects are coming to us so that we can study them. 

While we need to keep an open mind on the possibility that any of the earlier two ISOs could have been alien spacecraft, since they could not prove to be comets, extraordinary claims need extraordinary evidence. We don’t have evidence one way or the other. More such visitors in the future could give us opportunities for further studies.