Planet Nine (or Planet X as NASA calls it) first emerged as a real possibility in 2016, when two astronomers from the California Institute Of Technology presented evidence of a large gravitational force far beyond Neptune.
Now, a new study has whittled a list of 13 candidates down to just one potential object which is slowly moving around our sun at approximately 46.5 billion to 65.1 billion miles away.
That makes this new planet almost 20 times farther away from the sun than Pluto is.
Pluto is less than four billion miles away from the sun, sitting in the Kuiper Belt - a region in our solar system beyond Neptune which is filled with icy objects, comets, and dwarf planets like Pluto.
Since this hidden planet may be so far away, the new study theorizes that it's likely an ice giant similar to Uranus or Neptune.
Under those circumstances, the only possibility for life there would likely make it what scientists refer to as an extremophile, which are microbes that can survive and thrive in incredibly harsh conditions where most life forms would die.
These conditions might include very high or low temperatures, high pressure, extreme acidity, or even high radiation levels.
On Earth, such life is found near underwater volcanic vents, icy glaciers at the south pole, and even in Chile's Atacama Desert.
On Planet Nine, the conditions would arguably be even more extreme. The study, still on the pre-print server arXiv as it awaits peer review, found that Planet Nine is so far away from the sun, that the temperature would likely be between -364°F and -409°F.
Based on the gravitational pull, scientists believe the distant world would have the mass of seven to 17 Earths - making it roughly the size of Uranus or Neptune.
However, Planet Nine almost certainly isn't home to liquid water, unless it is located deep under the ice, closer to the planet's core.
Also, Planet Nine is so far away from the sun that sunlight would be extremely weak, meaning lifeforms would need to find another source of energy to survive.
While life may be questionable, scientists are getting closer to determining its orbit around the sun.
The study used data from the Infrared Astronomical Satellite (IRAS), launched in 1983, and AKARI, a Japanese satellite which made space observations between 2006 and 2007.
Both scanned the entire sky to catch infrared signals from distant objects. However, the key was the 23-year gap between their observations, which allowed astronomers to locate something moving slowly across the sky.
Comparing the two datasets from these space satellites, the international team was able to hunt for distant space bodies which were traveling about three 'arcminutes' per year.
An arcminute is used by scientists to measure angles in the sky, like how far apart things appear when you look up. There are 60 arcminutes in a degree and 360 degrees in a full circle.
For comparison, the full moon is about 30 arcminutes wide in the sky. It's a way to describe tiny movements or distances between objects in space.
Based on a 2016 study which estimated that Planet Nine was sitting between 46.5 billion to 65.1 billion miles away, astronomers started to look for objects orbiting the sun within this three-arcminute range.
Three arcminutes per year would be roughly the width of a dime seen from two miles away.
Over the course of 23 years, the astronomers figured out that Planet Nine would have travelled between 42 and 69.6 arcminutes.
They combined this pattern of movement with the gravitational effect scientists believe Planet Nine is having on the Kuiper Belt to narrow down the search from 13 distant objects to just one which appears to fit the description of a distant world pulling on the solar system's icy belt beyond Neptune.
The astronomers noted that two detections (one from IRAS and one from AKARI) aren’t enough to map Planet Nine’s full orbit or confirm it’s really a planet.
While scientists are still trying to confirm Planet Nine's existence, NASA says proving that this ice giant is real would help astronomers to explain several mysteries within our solar system.
In a breakdown of Planet Nine, NASA said: 'It could also make our solar system seem a little more 'normal.'
NASA researchers added: 'Surveys of planets around other stars in our galaxy have found the most common types to be "super Earths" and their cousins - bigger than Earth, but smaller than Neptune.
'Yet none of this kind exist in our solar system. Planet Nine would help fill that gap.'
Finding a large planet at the rim of the solar system would explain why objects in the Kuiper Belt are tilted by about 20 degrees in respect to the plane that the planets sit on as they orbit the sun.
Planet Nine's gravity would be pulling on these objects over long periods of time, tilting their orbits so the entire ice belt would be out of line with the planets.
The existence of Planet Nine and its strong gravity would also explain why comets and tiny dwarf planets like Pluto cluster together and move in the same direction without floating away.
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