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Artist's illustration of Planet Nine, a world about 10 times more massive than Earth that may lie undiscovered in the far outer solar system. (Photo: Caltech)

December 15 saw the world's excitement over the search of a planet placed 336 light-years from Earth, which was claimed to be "evidence of Planet Nine". Scientists have been hunting for the hypothetical Planet Nine for years. The new characterization of this alien planet by the Hubble Space Telescope shows that worlds like the theoretical planet can exist in other solar systems.

So, there raised the question: What is the so-called Planet Nine? Why does it make such a big deal in astrophysics? Let's find out below!

What is Planet Nine?

For the past few years, the possibility of Planet Nine (the ninth planet of the solar system) - a new and big planet hanging around in the outermost regions of the solar system has tantalized scientists and the public alike. But after years of searching, astronomers have found zero new planets in that realm, according to Space.com.

Planet Nine is a theoretical, undiscovered giant planet in the mysterious far reaches of our solar system. The presence of Planet Nine has been hypothesized to explain everything from the tilt of the sun’s spin axis to the apparent clustering in the orbits of small, icy asteroids beyond Neptune. Planet Nine remains hypothetical; no one has actually seen the world. But that doesn’t mean scientists aren’t searching for it.

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Planet Nine is a theoretical, undiscovered giant planet in the mysterious far reaches of our solar system. (Photo: Axios)

Why such planet significant?

The Kuiper Belt is a collection of small, icy bodies that orbit the sun beyond Neptune, at distances larger than 30 AU (one astronomical unit or AU is the distance between the Earth and the sun). These Kuiper Belt objects (KBOs) range in size from large boulders to 1,200 miles (2,000 km) across. KBOs are leftover small bits of planetary material that were never incorporated into planets, similar to the asteroid belt.

Mathematical calculations and detailed computer simulations have shown that the orbits we see in the Kuiper Belt can only have been created if Neptune originally formed a few AU closer to the sun, and migrated outward to its present orbit. Neptune’s migration explains the pervasiveness of highly elliptical orbits in the Kuiper Belt, and can explain all the KBO orbits we’ve observed, except for a handful of KBOs on extreme orbits that always stay at least 10 AU beyond Neptune, according to EarthSky.

These extreme orbits have provided the strongest evidence for Planet Nine. Astronomers studying the Kuiper Belt have noticed some of the dwarf planets and other small, icy objects tend to follow orbits that cluster together. By analyzing these orbits, the Caltech team predicted the possibility that a large, previously undiscovered planet may be hiding far beyond Pluto.

Planet Nine could have tilted the entire solar system, researchers said in October 2016 at a joint meeting of the American Astronomical Society's Division for Planetary Sciences and European Planetary Science Congress in Pasadena, California. The eight solar system planets orbit the sun at a tilt of about 6 degrees compared to the sun's equator. Computer simulations suggest a Planet Nine-sized object could have caused the tilt over the 4.5-billion-year-old lifetime of the solar system.

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The potential orbit of Planet 9, illustrated with the existing orbit of several trans-Neptunian objects. (Photo: Caltech)

Attempts to find a "Planet Nine"

Following the discovery of Neptune in 1846, there was considerable speculation that another planet might exist beyond its orbit. The best-known of these theories predicted the existence of a distant planet that was influencing the orbits of Uranus and Neptune. After extensive calculations, Percival Lowell - an American scientist, predicted the possible orbit and location of the hypothetical trans-Neptunian planet and began an extensive search for it in 1906. He called the hypothetical object Planet X or Planet Nine.

We've only been studying the region of the solar system past the orbit of Neptune for a few decades now, and after a moment of introspection it's easy to see why: astronomy out here is kind of challenging because the objects we're trying to hunt down are a) very, very small and b) very, very far away. That makes them hard to spot.

In January 2015, Caltech astronomers Konstantin Batygin and Mike Brown announced new research that provides evidence of a giant planet tracing an unusual, elongated orbit in the outer solar system. The prediction is based on detailed mathematical modeling and computer simulations, not direct observation. This large object could explain the unique orbits of at least five smaller objects discovered in the distant Kuiper Belt.

The Caltech scientists believe Planet X may have has a mass about 10 times that of Earth and be similar in size to Uranus or Neptune. The predicted orbit is about 20 times farther from our Sun on average than Neptune (which orbits the Sun at an average distance of 2.8 billion miles). It would take this new planet between 10,000 and 20,000 years to make just one full orbit around the Sun (where Neptune completes an orbit roughly every 165 years).

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An artist's concept of accretion flares emitted from the encounter of an Oort-cloud comet and a hypothesized black hole in the outer Solar System. (Photo: Inverse)

Yet the solar system remains eight

No matter how much effort scientists put in the search for the so-called Planet Nine, opposite ideas still exist.

The inability to explain the orbit of Uranus led to the detection of Neptune, so there is some historical antecedent to the strategy. And since then, more space objects have been found in the same strange, clustered orbits. But in the years since the claim of a ninth planet made headlines, astronomers haven't snagged a picture of it. Which isn't too worrisome, at least not yet: if Planet Nine exists, it is very small (relatively) and very far away, making it hard to spot.

Some scientists argue that it's hard to square the existence of a ninth planet with the formation of the solar system as we currently understand it. Astronomers can, of course, work to fold in a ninth planet (say, by arguing that it's an ejected failed core of a planet or a captured rogue exoplanet), but the more complicated the scenario gets, the harder it is to swallow.

Scientists step closer to learning more about Planet Nine

A planet spotted 336 light-years from Earth could help scientists unravel the mystery surrounding a supposed large "Planet Nine" lurking in the outskirts of our solar system, a new study suggests. With the help of data from the Hubble Space Telescope, astronomers have been able to measure the motion of a massive Jupiter-like planet orbiting very far away from its host stars and visible debris disk.

Though the search for a Planet Nine continues, this discovery of the exoplanet — known as HD 106906 b — is evidence that such oddball orbits are possible, a news release from NASA's Goddard Space Flight Center reported. "This system draws a potentially unique comparison with our solar system," the author of the study, which appeared in The Astronomical Journal, Meiji Nguyen of the University of California, Berkeley, said in a statement to the space flight center, quoted by Newsmax. "It's very widely separated from its host stars on an eccentric and highly misaligned orbit, just like the prediction for Planet Nine. This begs the question of how these planets formed and evolved to end up in their current configuration."

The system where this gas giant resides is 15 million years old, the space flight center report noted, suggesting that Planet Nine — if it exists — could have formed very early on in the evolution of our 4.6-billion-year-old solar system. "It's as if we have a time machine for our own planetary system going back 4.6 billion years to see what may have happened when our young solar system was dynamically active and everything was being jostled around and rearranged," team member Paul Kalas said in a statement to the space flight center.

Scientists using NASA's upcoming James Webb Space Telescope plan to get data on the exoplanet to understand it in greater detail, the space flight center reported.

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