In October 2020, an international team of astronomers announced the discovery of a wandering planet the size of Earth . Dubbed the unattractive name OGLE-2016-BLG-1928 , its discovery was the result of the collaboration of two research teams: the Polish OGLE (Optical Gravitational Lensing Experiment) and the Korean KMTN (Korean Microlensing Telescope Network). Both make use of a peculiar astronomical phenomenon: gravitational microlensing.
Microlensing is a consequence of Einstein’s theory of general relativity and requires two things : a distant light source , such as a star, and a closer object with enough mass to act as a lens and distort the image of the light. that star that is received on Earth. When this happens, the light from the distant star is magnified, allowing scientists to discover information about the lensed object that might otherwise remain completely invisible . The problem is that it requires a very precise alignment, which makes it a very rare phenomenon where the measurements have quite a bit of error (the mass of the lens is too low (a planet) for the displacement of light to be easily observed ) and also requires a lot of luck, because the necessary alignment for the microlensing effect to occur is not maintained for a long time, a few seconds or perhaps years… Even so, it is a novel way of studying objects that emit little or no light, making it a powerful tool for locating dark matter . Precisely OGLE was launched in 1992 with the aim of finding clusters of dark matter and a collateral effect has been to find extrasolar planets: to date it has discovered 17.
Now, how many of these wandering planets (in English, rogue planets) can be going around the Galaxy? Giving a number is a risky game and can be done in two ways: by mathematical modeling or by extrapolating the results of gravitational microlensing observations. Using this last method, in 2011 the team of New Zealand and Japanese astronomers that make up the Microlensing Observations in Astrophysics (MOA) project published the analysis of their observations of the galactic bulge (the group of stars that form an ellipsoid in the center of spiral galaxies ): there should have been two Jupiter-like planets for every star in the Galaxy. However, in 2017 OGLE astronomers performed their own statistical analysis of more than 2,600 microlenses obtained over six years of observations, bringing that number down to one Jupiter-type planet for every four stars in the galaxy. Which of the two teams is right? The issue is not settled yet.
If we use computer models , we find a work led by Louis Strigari, from Stanford University, published in 2012 in the journal Monthly Notices of the Royal Astronomical Society. There he gave the overwhelming figure that for every star in the Galaxy there are one hundred thousand nomadic planets with sizes ranging from that of Pluto to 100 times that of Jupiter. More recently, in 2019, astronomers from Leiden University in the Netherlands published the result of a simulation of 1,500 stars located in the so-called Orion Trapezium (a cluster found inside the famous Orion Nebula). What they did was place 2,522 planets orbiting 500 stars similar to our Sun and calculated their evolution: 357 escaped the gravity of their star in their first 11 million years of life. “Of these 281 left the cluster, while the others remained within it roaming freely and only 5 were captured by another star,” commented lead researcher Simon Portegies Zwart. Interestingly, 75 of the 2,522 planets collided with their own star and 34 they did it with another planet.The most striking of the conclusions of this work is that the probability of a star losing a planet is independent of both the mass of the planet and the place where it forms, near or far from the star. From this Portegies Zwart estimates that about a quarter of the stars in the galaxy have lost one or more planets.
Be that as it may, the first of these lonely planets was discovered in 2013. Its name, PSO J318.5-22. Located 80 light-years from Earth, it is a gas giant approximately six times the mass of Jupiter, and it appeared by chance when a team of astronomers was searching for brown dwarfs. “I’ve often wondered if such rogue objects existed, and now we know they do,” said discovery team leader Michael Liu of the University of Hawaii Institute for Astronomy. To date, 21 possible errant planets have been identified, of which only 2 are confirmed. It is not a very high number but it is expected that from 2025 the situation will change because then the new Nancy Grace Roman infrared space telescope will be launched -in honor of the astronomer known as “the mother of Hubble”-. At a cost of 4,000 million dollars, it will give us a field of view 100 times greater than that of the Hubble telescope.
And our Sun? Did you also have a planet that went to live its life? There are astronomers who believe so. The reason for thinking this is the strange tilt of Uranus’ axis of rotation, practically lying on the orbital plane so that, instead of moving like a top like the rest of the planets do, it looks like a rolling ball. Of course, that anomalous movement of Uranus could also be explained by the capture of a wandering planet that passed by nearby. Obviously all eyes have been on Pluto…
In any case, to affirm that a wandering planet has passed through our cosmic neighborhood is mere speculation. What is certain is that asteroids and comets from other solar systems usually pass through here. The first of which we are aware did so two and a half years ago: astronomers from Hawaii discovered a cigar-shaped object crossing the Solar System at full speed. Baptized with the name Oumuamua (from the Hawaiian, explorer), he came from the depths of space, passed close to the Sun inside the orbit of Mercury and is now close to Neptune, heading to outer space never to return. The most fascinating thing of all is that, despite the numerous observations that have been made of it, astronomers are not clear about what it is.
Can we know if any of the asteroids we have in the solar system is actually an alien in a strange land? Loeb, together with his student Amir Siraj, has calculated the most probable orbital parameters that an object with such characteristics should present and have pointed to several asteroids as candidates: most of them belong to the Centaur group, a group of asteroids and comets located between the orbits of Jupiter and Neptune. In fact, according to his calculations there must be around 6,000 Oumuamua-type objects in our Solar System.