Some experts believe we could be just a few years away from discovering the elusive “Planet Nine” – or ruling out the existence of the hypothetical world for good.
But if we find it, how long will it take us to send a spacecraft to the distant world? And can humans ever make the journey?
Planet Nine is the name given to a proposed planet that is supposed to be lurking nearby the edge of the solar system. If it exists, Planet Nine is likely a dark, gas or ice giant planet somewhere five to 10 times the mass of Earth orbiting the sun in a highly elliptical or elongated orbit – out of sync with the rest of the planets.
Researchers suspect Planet Nine is there because about a dozen objects beyond Neptune’s orbit move as if a large object is pulling them. However, finding this lost world has proven to be extremely difficult.
But some astronomers believe Planet Nine could be discovered within the next few years after the newest Vera C. Rubin Observatory begins observing the night sky in late 2025.
Connected: Evidence for ‘Planet Nine’ in our solar system (gallery)
If Planet Nine is ever discovered, space agencies like NASA will want to send a probe to visit the distant world. But first, they would have to plan and get approval for such a mission.
“It will take at least a decade or more,” Andreas Hein, a space systems engineer at the University of Luxembourg, told Live Science in an email. This is mainly because the missions have to go through a long and rigorous government selection process, he added.
But this process can be accelerated depending on how strange the world seems on initial readings, Manasvi Lingam, a theoretical astrobiologist at the Florida Institute of Technology, told Live Science in an email. “If Planet 9 is anomalous in some way, there may be more interest in accelerating such a mission.”
It’s also possible that a private space exploration company like SpaceX could launch missions sooner — potentially as soon as five years after discovery — because they’re not hampered by as much red tape, Hein said.
Reaching planet nine
Once a probe is sent to Planet Nine, the next question is how long will it take to arrive?
In 2022, Hein, Lingam and part-time astronomer Adam Hibberd attempted to answer this question in a theoretical paper that was posted to the database ahead of print arXiv. In the paper, which has not been peer-reviewed, the researchers estimated that it would take between 45 and 75 years for a spacecraft similar to NASA’s Voyager probes to reach Planet Nine.
These calculations were based on the assumption that Planet Nine is on average about 400 AU from the sun, or 400 times farther from the sun than Earth. This is also about 13 times farther from the sun than Neptune, the most distant known planet in the solar system.
But later studies suggest that the true average distance of Planet Nine from the sun is actually closer to 500 AU. And the most recent poll, which narrowed down the possible hiding place of the elusive worldalso hints that the planet may currently be more than 550 AU away, near its furthest point from the sun.
However, these developments do not dramatically change the 2022 timeline presented in the document, Lingam said. The team’s estimates were “conservative,” meaning the upper end of their uncertainty range “still looks stable,” he added.
Pushing the boundaries
Seventy-five years may seem like a long time for a one-way trip. But this is quite fast compared to the journeys of existing probes; for example, Voyager 1 — the farthest spacecraft from Earth — has traveled through space for 46 years and is only about 163 astronomical units from Earth, according to NASA. As a result, a Planet Nine probe would have to travel about three times as fast as Voyager 1 to reach Planet Nine in the researcher’s time frame.
That may seem impossible, but it is possible, Lingam said.
Probes traveling to the outer solar system must be gravitationally rocketed around the sun, planets and their moons to propel them through the vast distances involved. Voyager 1’s trajectory was carefully planned to allow it to fly close Jupiter, Saturn, Uranus and Neptune, as well as some of their respective moons. To do this, Voyager 1 also had to slingshot around objects to slow it down and redirect it to where it needed to go. However, a Planet Nine probe could go as directly as possible towards the mysterious planet, allowing it to pick up more speed and travel much further at the same time.
Mike Brownan astronomer at Caltech who co-proposed The Ninth Planet Hypothesis in 2016, initially estimated that a probe could potentially reach Planet Nine in just 20 years using a similar method. Still, it’s “difficult” to see how a probe could reach Planet Nine so quickly using current technology, Lingam said.
But it may be possible to reach Planet Nine sooner if we can hit on more advanced propulsion technologies, such as light sails — giant sails that capture sunlight or lasers to accelerate the spacecraft, Hein and Lingam said.
In the 2022 study, researchers suggested that a light sail could take as little as seven years to reach Planet Nine. However, it could take at least 20 years for lightweight sails to become a reality, the team estimated.
As a result, if we were to find Planet Nine tomorrow, it would be better to send an actual probe rather than wait for alternative technologies to become available, Lingam said. “Laser VLTs may be released later, if and when they become practical and cost-effective.”
As propulsion systems advance further, it may also be possible to send humans to Planet Nine. But maybe not soon.
“I’m sure that in the distant future, people will be able to visit Planet Nine,” potentially even when they go to other stars, Hein said.