Searching For Planet Nine

In 2015, astronomers Konstantin Batygin and Mike Brown from Caltech in California made a startling announcement – the possibility of there being a ninth planet in our Solar System.

Astronomers think this mysterious planet could be over ten times the mass of the Earth and would explain anomalies in the clockwork of our Solar System. However, after countless simulations, calculations, and even a citizen science project, ‘Planet Nine’ remains undetected.

In this blog we explore the gaps in our understanding of the Solar System and why we think there’s something there.

The evidence

Astronomers are currently puzzled by two observations the Solar System: the Sun’s tilt and the clumping of icy objects on one side of the Solar System.

The Sun is tilted at an angle of about seven degrees relative to the thin plane in which its eight planets circle, but astronomers don’t know why. After the announcement of a hypothetical ninth planet, people realised that this could explain the tilt. A hidden planet that’s over ten times the Earth’s mass and circling the Sun at an even bigger angle could have tilted the orbits of the rest of the planets compared to the Sun.

Beyond the eight planets, fifty times further away from the Sun than Earth, lies a region of icy bodies known as the Kuiper Belt. Over time, the orbits of these icy bodies should become evenly spread out around the Sun. But instead, the orbits are clumped together on just one side of the Sun. The only way that astronomers can explain this is if there’s a large mass on the other side that keeps the icy objects clumped together.

These two mysteries could be explained by a large planet about ten times the mass of Earth with an year lasting between 10,000 to 20,000 Earth years. The search is on for Planet Nine!

Why haven’t we found it yet?

Hidden planets aren’t new to astronomy. Throughout history people have always been sure that there’s more to our Solar System than we think. This began in the early 1800s after William Herschel discovered the planet Uranus. There were strange effects in its orbit, so astronomers predicted the presence of a larger body further out. Along came Neptune in 1846! Since then, we’ve continued to discover more Solar System members, such as dwarf planets Pluto and Sedna.

Planet Nine, if it exists, would be even harder to find. The closest that Planet Nine would come to the Sun is 200 times greater than the distance between Earth and the Sun, or about 30 billion kilometres! This is so far away that current space telescopes such as the Hubble Space Telescope can’t resolve it. Due to its distance from the Sun, Planet Nine is also probably very cold and wouldn’t produce much radiation of its own, making it even harder to detect. However, the successor to Hubble, the James Webb Space Telescope, may be able to spot Planet Nine once it launches in a few years.

Is it even a planet?

Perhaps though, the hunt for a massive planet is premature. In June 2018, astronomers from Colorado University presented a thought-provoking argument about Planet Nine at the American Astronomical Society. Ann Marie Madigan who leads the Eccentric Dynamics team in this field, suggested that perhaps this planet is just a large group of asteroids that behave as one. Their combined gravity could be enough to account for the tilt of the Sun and the odd behaviour of the distant Kuiper Belt objects.

Distant objects like Sedna can appear as ‘detached’ from the rest of the Solar System because their orbits are so different and isolated compared to familiar planets close to the Sun – which is what led to the possibility of a ninth planet. However what Madigan and her team suggest is that instead of considering one massive object, we should perhaps consider the collective gravity of many small asteroids all working together to produce the same effects. Simulations have shown this to also work. Perhaps there is no Planet Nine after all…