Planet 9 Could Be Found on a Mission to Uranus

Astronomers and non-professional stargazers have long pondered and argued about how many planets make up our solar system. Uranus was the seventh, discovered in 1781, and Neptune the eighth, discovered in 1846. The odd orbits of these two ice giants stimulated debate about more planets beyond them whose gravitational pull could be affecting them, or whose one-time close proximity might have even resulted in a collision. Pluto, discovered in 1930 in the Kuiper belt, had its own orbital and size idiosyncrasies which eventually caused its downgrading to dwarf planet status. The area where a disruptive planet might exist is far beyond Neptune and the Kuiper belt in an area of strange, clustered space rocks known as extreme trans-Neptunian objects (ETNOs). In fact, some astronomers speculated that this Oort cloud might itself be the remnants of a Planet Nine. However, most scientists trying to justify the existence of a Planet Nine prefer to believe it is still whole or at least its core is still intact. If that’s the case, as Enrico Fermi once asked about alien intelligent life forms, then where is it? A new study proposes that a space mission to Uranus could answer that question and potentially identify the spot in space where astronomers need to look to finally discover the elusive Planet Nine.

”The recently proposed Uranus Orbiter and Probe mission by NASA's Planetary Science and Astrobiology Decadal Survey could offer the opportunity to precisely determine Planet 9's sky location and mass by carefully monitoring ranging data during the interplanetary cruise.”

In a new paper titled “Prospects for localising Planet 9 with a future Uranus mission” and presented recently at the annual EGU23 (European Geosciences Union) General Assembly held in Vienna, co-authors computational scientist Jozef Bucko, planetary scientist Deniz Soyuer and theoretical astrophysicist Lorenz Zwick build their case for why a proposed NASA mission to Uranus could be the key to discovering Planet Nine. In fact, it could do it without affecting NASA’s primary goals of the mission – to “deliver an in situ probe into Uranus’s atmosphere, then complete a multi-year orbital tour of all aspects of the Uranian system including: atmosphere, interior, magnetosphere, rings, and satellites.” That’s a hefty workload, and the three authors don’t want to add to it … they just want to use a little bit of the information the Uranus Orbiter and Probe mission is sending back to Earth. In particular, they are interested in its location, its speed and other items of ranging data used to keep the spacecraft on course on this long 10-to-15 year flight. What exactly will they be looking for?

“If there is a gravitational anomaly in the solar system, in this case, Planet 9, the trajectory of the spacecraft would be affected.”

Deniz Soyuer explains the idea in an interview with the science news magazine Eos. A planet-sized space object on the Oort cloud would be big enough to influence extremely slight changes in the speed or trajectory of the Uranus Orbiter and Probe. By comparing the new speed and path to what it should be, the researchers could determine how large this planet might be and from which direction its gravitational pull is coming from. Many Planet Nine believers believe it is about 6.3 Earth masses and 460 times farther away from the Sun than Earth is, so this measurement could also confirm or deny those estimates. However, they agree that any space body that is the size of a planet “will definitely have a nonnegligible effect on the trajectory of a spacecraft.”

Does this sound too easy? Does it sound like something that should have already been tried by previous space missions? You are right! Astronomer Mike Brown of the California Institute of Technology, who is partly responsible for Pluto being downgraded to a dwarf planet and who in 2016 with Konstantin Batygin proposed that Planet Nine might be the core of a giant planet that was ejected from its original orbit by Jupiter during the early days of the solar system, agrees with you. In fact, he says astronomers attempted to use ranging data from NASA’s Cassini mission to Saturn and its moons to look for speed and direction anomalies that might help find Planet Nine but were not able to. If Bucko, Soyuer and Zwick have any ‘pull’ over the designers of the Uranus Orbiter and Probe mission, they might be able to make sure the data is checked frequently enough to give them the ability to catch the subtle changes a Planet Nine might cause. In his comments to Eos, Brown sounds excited about this proposed plan.

“This paper gives a nice twist on the idea by tracking the spacecraft over a wide range of distances. One bit of good news is that the greater the distance [from Earth], the greater the effect of Planet 9, so making it all the way out to Uranus gives you quite a bit of leverage.”

One key area would be the last leg of the trip as the craft passes Jupiter and still has 1.2 billion miles (over 2 billion km) to get to Uranus. By that point, a planet in the Oort cloud would definitely be able to influence the craft enough to register a change. Soyuer points out that modern technology is much better than it was during the Cassini mission (which ended in 2017) and could identify a target area in the sky 20 times smaller than with the Cassini data. He believes that’s enough to achieve the team’s goal.

“You don’t really need that much of an improvement to be able to localize [Planet 9] to a place where you can convince people to point their telescopes at it.”

Will they be able to convince NASA to let them use its data to look for Planet Nine? They have plenty of time to lobby NASA officials – the mission wouldn’t begin until at least 2030. Unfortunately, the biggest challenge is budget. One of the areas where NASA has cut costs before is in the frequency the ground crews communicate with space probes. The team will need frequent samples in order to find the anomalies that could point to Planet Nine.  They have Mike Brown’s support.

“I think it is great that people are thinking creatively about different ways in which we could eventually track down this elusive planet on the edge of the solar system.”

Now all they need is for politicians in Washington to approve funding to hunt for a hypothetical planet.