Planet Nine, if it exists, is somewhere between 6-10 times the mass of Earth, and it may have an orbit that takes it around the sun once every 1,500 Earth years or more.
How many planets are there in the Solar System?
We know about eight and, of course, there's poor Pluto, which has been downgraded to a 'dwarf planet' status along with several other dwarfs.
But one of the astronomers responsible for the Pluto downgrade believes there is another large planet out there, in the outer reaches of the Solar System.
When the Vera Rubin Telescope is commissioned later this year, it's hoped the Planet Nine hypothesis will be confirmed.
In 2016, Mike Brown, professor at the California Institute of Technology who is nicknamed 'Pluto-killer' for advocating the downgrade, co-authored Evidence for a distant giant planet in the solar system with his colleague Konstantin Batygin.
For years, Brown, Batygin and other astronomers had been puzzled by the movements of multiple large objects beyond the orbit of Neptune -- the so-called Trans-Neptunian Objects (TNO).
They had found at least 13 large TNOs, which didn't orbit as expected. One explanation was that the gravitational attraction of a large, invisible planet was influencing these movements.
If at all, this Planet Nine exists, it might have a very long orbit and it may be at its most distant point from the sun, which would make it hard to see.
Moreover, while astronomers have looked for it, they may not have been looking in the right places. Alternatively, it has been seen but not recognised as a planet.
The Vera Rubin Observatory in the Atacama desert of Chile is due to start operating in mid-2024.
It will run the Large Synoptic Survey Telescope (LSST), with a three-mirror system that should be able to pick close to 90 per cent of near-Earth objects and catalogue at least 10,000 TNOs.
The LSST has a very wide field of view, with unparalleled light-gathering power, and the largest digital camera ever.
The telescope will take 15-second exposures at 20-second intervals, creating a giant repository of digital images for computers to sort through for very faint, moving objects.
Planet Eight (Neptune) and Pluto were discovered by a combination of mathematically calculating orbits, and then looking for a cause for divergences between calculations and observations.
Indeed, astronomy (and other branches of physics) has always seen a fascinating interplay between data and mathematical models.
In 1781, German-British astronomer William Herschel figured out that Uranus (a green-blue giant with rings, which is just as lovely as Saturn) was a planet, not a star.
Uranus had been seen multiple times earlier, by the naked eye, as well as through primitive telescopes. It has a wide orbit (84 Earth years).
Herschel first thought it was a comet. When various astronomers calculated its orbit and noted that it did not have a tail (unlike comets), they leapt to the right conclusion and called it a planet.
It was only named decades after being plugged into the map of the Solar System.
Neptune is not visible to the naked eye though it's arguably as lovely as Uranus (similar colour and rings).
When astronomers looked closely at Uranus's orbit, they discovered observational data diverged from calculation and started looking for a culprit.
After the discovery of Neptune circa 1846, it was realised Galileo had seen it circa 1612, and so had many other astronomers over the centuries.
But its dimness and the fact that it takes 165 years to go around the Sun (so, little apparent motion in the sky) led them to assume it was a star.
Pluto was found in 1930, by a process of analysing orbital data for Neptune and looking for a reason why observation didn't fit the calculation.
From the 1990s onwards, astronomers have had the tools to look at TNOs in detail. They've found other dwarf planets and thousands of interesting objects in the far reaches of the Solar System in what's known as the Kuiper Belt.
When they started looking at these, they discovered that again, there were major anomalies between data and calculations.
Planet Nine, if it exists, is somewhere between 6 to 10 times the mass of Earth, and it may have an orbit that takes it around the sun once every 1,500 Earth years or more. It may be 10 times as far from the sun as Neptune.
Some astronomers say it probably doesn't exist, and there could be other explanations for the Kuiper anomalies. Brown says it has a 90 per cent probability of existing.
Renu Malhotra of the University of Arizona, who is acknowledged as the world's 'Pluto expert', says it's the likeliest explanation for odd TNO behaviour.
As of now, it's referred to as Planet Nine or Planet X. The honour of naming it will go to the astronomer who ultimately tracks it down -- if it does exist.
Orbital oddities
In the 17th century, Johannes Kepler crunched the data from the observed movements of Planets 1 to 6 and worked out his Three Laws of Planetary Motion.
Planets move in elliptical (egg-shaped) orbits around the sun, their speeds vary at different times, and there is a mathematical relationship between orbital time periods and the respective distances from the sun, which is the same for every planet (Kepler's Constant).
Some years later, Isaac Newton fiddled with Kepler's Laws and derived his theory of gravity to explain why planets moved the way they did.
You can think of the Sun and most of the planets as balls rolling on an egg-shaped table -- that table is an imaginary plane called the ecliptic.
But many TNOs dip below and rise above the ecliptic plane, apart from having eccentric orbits (think of very long, thin eggs).
Differences between calculations and data lead scientists to look for explanations.
Einstein's Theory of Relativity was confirmed by the weirdness in Mercury's orbit. Neptune was discovered because Uranus didn't fit the calculations.
If Planet Nine exists, the clues to its existence lie in the funny movements of TNOs.
Feature Presentation: Ashish Narsale/Rediff.com
