Some experts believe that in a few years we could discover the elusive “Planet Nine” – or finally rule out the existence of this hypothetical world.
But if we do find it, how long would it take to send a spaceship to that distant world? And could humans ever make that journey?
Planet Nine is the name of a suspected planet located near the Edge of the solar system. If it exists, Planet Nine is probably a dark, gaseous or icy giant planet somewhere between five and ten times the mass of the Earth which orbits the Sun in a highly elliptical or elongated orbit – not synchronously with the other planets.
Researchers suspect Planet Nine is out there because about a dozen objects move beyond Neptune’s orbit as if a large object were tugging on them. However, the search for this missing world has proven extremely difficult.
However, some astronomers believe that Planet Nine could be discovered in the next few years, once the state-of-the-art Vera C. Rubin Observatory begins surveying the night sky in late 2025.
Related: Evidence of “Planet Nine” in our solar system (gallery)
If Planet Nine is ever discovered, space agencies like NASA will want to send a probe to the distant world. But first they must plan and get such a mission approved.
“This will take at least a decade or more,” Andreas Heina space systems engineer at the University of Luxembourg, told Live Science in an email. This is largely because missions must go through a lengthy and rigorous government selection process, he added.
But this process could be accelerated, depending on how strange the world appears at first readings, Manasvi Lingama theoretical astrobiologist at the Florida Institute of Technology, told Live Science in an email, “If Planet 9 is anomalous in any way, there might be greater interest in accelerating such a mission.”
In addition, it is possible that a private space exploration company like SpaceX could launch its missions earlier – perhaps as early as five years after discovery – because it would not be hampered by as much bureaucracy, Hein said.
Reaching the ninth planet
Once a probe has been sent to Planet Nine, the question arises as to how long it will take to get there.
In 2022 Hein, Lingam and part-time astronomer Adam Hibberd attempted to answer this question in a theoretical paper published in the pre-print database arXivIn that paper, which was not peer-reviewed, researchers estimated that it would likely take between 45 and 75 years for a spacecraft similar to NASA’s Voyager probes to reach Planet Nine.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 astronomical units from the Sun, which is 400 times farther from the Sun than Earth. That is also about 13 times farther from the Sun than Neptune, the most distant known planet in the Solar System.
However, later studies suggest that the actual average distance of Planet Nine from the Sun is actually closer to 500 astronomical units. And the most recent study, the potential hiding place of the elusive world is narrowed downalso suggests that the planet may currently be more than 550 astronomical units away, near its farthest point from the Sun.
However, these developments do not dramatically change the timeline laid out in the 2022 paper, Lingam said. The team’s estimates were “conservative,” meaning the upper end of their uncertainty range “still appears tenable,” he added.
Shifting boundaries
75 years may seem like a long time for a simple journey. But compared to the journeys of existing probes, that is quite fast; for example, Traveler 1 — the spacecraft farthest from Earth — has been traveling through space for 46 years and is only about 163 astronomical units from Earth, according to NASA. Consequently, a probe destined for Planet Nine would have to travel about three times faster than Voyager 1 to reach Planet Nine in the time specified by the researcher.
This may seem unlikely, but it is possible, Lingam said.
Probes traveling toward the outer solar system must be gravitationally spun around the sun, planets, and their moons to overcome the enormous distances. Voyager 1’s trajectory was carefully planned to ensure a close flight to Jupiter, Saturn, Uranus and Neptune, as well as some of their respective moons. This also required Voyager 1 to be spun around objects to slow them down and redirect them to their destination. However, a Planet Nine probe could fly as directly as possible directly to the mysterious planet, allowing it to gain more speed and travel much farther in the same amount of time.
Michael Brownan astronomer at Caltech, who co-proposed the Planet Nine Hypothesis in 2016, originally estimated that a probe using a similar method could potentially reach Planet Nine in as little as 20 years. However, it is “hard to imagine” how a probe using current technology could reach Planet Nine so quickly, Lingam said.
But it might be possible to reach Planet Nine faster if we could adopt more advanced propulsion technologies such as lightsails – giant sails that capture sunlight or lasers to accelerate spacecraft, Hein and Lingam say.
In the 2022 study, researchers suggested that a lightsail could take as little as seven years to reach Planet Nine. However, it could take at least 20 years for lightsails to become a reality, the team estimated.
So if we were to find Planet Nine tomorrow, it would be better to send a current probe than wait for alternative technologies to become available, Lingam said. “Laser sails can be launched later when they become practical and cost-effective.”
With the further development of propulsion systems, it might also be possible to send people to Planet Nine. But probably not in the near future.
“I am convinced that humans will be able to visit Planet Nine in the distant future,” perhaps even when they head to other stars, Hein said.