Remember “Planet Nine?” Everyone knows that our solar system has eight planets (sorry, Pluto), but some astronomers think there may be another one lurking at the edges.
No one has been able to find it yet (some don’t believe it exists at all), but a trio of new scientific papers shared on arVix prior to their formal publishing discuss the idea of taking a look for ourselves—by sending a fleet of 100 or more spacecraft in the hope that one of them could hit the jackpot.
What and where is ‘Planet Nine?’
Speculated to be five to 15 times larger than Earth and first mooted in 2016, a cold and dark Planet Nine is suspected by some astronomers to exist in the outer solar system, well beyond the orbit of Neptune and Pluto. It’s a region called the Kuiper Belt. Astronomers describe the scale of the solar system in astronomical units (au)—the mean Earth-Sun distance—which is about 93 million miles/150 million km.
- Mercury: 0.4 au
- Venus: 0.7 au
- Earth: 1 au
- Mars: 1.5 au
- Jupiter: 5.2 au
- Saturn: 9.5 au
- Uranus: 19.2 au
- Neptune: 30 au
- Pluto (dwarf planet): 39.5 au
- Planet Nine (suspected planet): 500 au
So if there is a Planet Nine, it’s incredibly far away. For context, NASA probes Voyager 1 and Voyager 2, which both left Earth in 1977, are currently 148 au and 123 au distant, respectively.
What’s the evidence for Planet Nine?
Astronomers are puzzled by what they find in the Kuiper Belt. The more they look, the more they find Kuiper Belt objects (KBOs) or trans-Neptunian objects (TNOs)—minor planets and dwarf planets, of which Pluto is one. The thing is, there’s a rather odd cluster of them in physical space. Their orbital planes are aligned, too. The cause is the gravitational influence of … something. It’s suggested the presence of a slow-moving Planet Nine, which is on the same orbital plane as these objects, but in the outer reaches of the solar system, could be the culprit. Or it could be a black hole (yes, in our solar system).
How a spacecraft could find Planet Nine
Direct searches for Planet Nine have so far yielded nothing. In the new papers, “The Brute-Force Search for Planet Nine” by Scott Lawrence and Zeeve Rogoszinski at the University of Maryland, and “Searching for a Black Hole in the Outer Solar System” by Edward Witten at the Institute for Advanced Study, the authors discuss sending a spacecraft to the Kuiper belt to search for it. Such a mission would require:
- A fleet of hundreds of “near-relativistic” spacecraft traveling, if not at light speed, then at a significant fraction of it (perhaps hundreds of kilometers per second), to search a wide area of the outer solar system from around a decade after launch.
- Some of the hundreds of spacecraft would speed-up as they passed through any Planet Nine’s gravitational field, thereby revealing its location, as detected by high-precision atomic clocks on board each one. The spacecraft would need to be sent in different directions so that one flyby Planet 9 within dozens of au, rather than hundreds of au.
Why 100 spacecraft may not find Planet Nine
There are, sadly, a few technological problems with such a mission, namely:
- We can’t make spacecraft that go fast enough.
The authors suggest a speed of 670,617 mph would be needed; NASA’s New Horizons—the fastest spacecraft ever—topped-out at 52,000 mph. So speeds would have to increase by an order of magnitude to make any Planet Nine mission possible.
- We can’t make high-precision clocks small enough.
Engineers could go for a Breakthrough Starshot-style super-fast, ultra-light interstellar spacecraft design weighing just a few grams. This project suggests using powerful lasers to accelerate miniature spacecraft. However, in that case, we have no suitable atomic clocks that are anywhere near small enough.
Why we should send 100 spacecraft to look for Planet Nine
OK, so the chances are pretty low that we can send spacecraft that are; (a) fast enough to get to Planet Nine, and (b) carry the right equipment to detect Planet Nine. That’s alright, say Lawrence and Rogoszinski—let’s just send the spacecraft anyway and use radio waves to precision-measure their exact position in the same way we can track near-Earth satellites. That way we can figure-out if, and by how much, the spacecraft’s trajectories are altered—and pinpoint Planet Nine’s location. Space-based telescopes could even be used to increase the precision.
A third paper on Planet Nine this month, “Can Planet Nine Be Detected Gravitationally by a Sub-Relativistic Spacecraft?” by Thiem Hoang at the Korea Astronomy and Space Science Institute and Harvard University’s Avi Loeb, suggests that Planet Nine’s gravitational signal would be drowned-out by turbulence. However, even if this multi-decade mission finds no trace of Planet Nine, the 100 spacecraft can help astronomers to map the region’s gravitational disorder, claim Lawrence and Rogoszinski.
Then at least we can identify what the hell is going on in the Kuiper Belt.
If further study of the Kuiper belt strengthens the case for existence of Planet 9, but it’s not seen by any telescopes, then “a direct search by a fleet of miniature spacecraft may become compelling,” writes Witten. “Once Planet Nine is found by this method, subsequent searches by the same method could pin down its location far more precisely and perhaps eventually make possible a close-up study of this object.”
Planet Nine, it seems, is going to take a monumental effort to find—or forget.
Wishing you clear skies and wide eyes.