A theory developed to replace dark matter may mean bad news for Planet Nine, a hypothetical as-yet-unseen world in the outer solar system. The odd orbital alignments of icy objects in this region that were initially considered a smoking-gun signature for the putative planet’s gravitational influence could instead be caused by gravity itself behaving against expectations, some researchers say.
In 2016 astronomers proposed Planet Nine’s existence as a way of explaining a strange sort of clustering observed in several small bodies found in and beyond the Kuiper Belt, a distant region of the solar system containing Pluto and many other objects. But fervent searches for the planet have so far proved fruitless, leading some scientists to doubt Planet Nine’s existence.
So what else might explain the curious orbital patterns? One suggestion is modified Newtonian dynamics, or MOND. Originally developed to explain the surprisingly speedy rotations of galaxies without resorting to dark matter, today MOND is a collective term given to theories that assume the force of gravity does not always (as Isaac Newton and Albert Einstein insisted) follow the inverse square law. Variations of this law pop up everywhere in physics; for gravity, it would dictate that the gravitational force between two objects is inversely proportional to the square of the distance between them. MOND’s success as a dark matter alternative has been limited, however, because most astrophysicists remain convinced that the universe we see around us is better accounted for by theories that incorporate the mysterious invisible substance.
MOND has typically been used to explain wide-scale galactic phenomena. But when theoretical physicists Katherine Brown of Hamilton College and Harsh Mathur of Case Western Reserve University realized that its effects should be measurable at the outskirts of the solar system, they decided to see how it could affect the proposed Planet Nine. Their new study was published on September 22 in the Astronomical Journal.
“We thought we would probably disrupt Planet Nine’s predictions a little bit through MOND,” Mathur says. “It took us completely by surprise when we found that, in fact, we could explain the whole alignment that is seen by MOND, and so it becomes an alternative to Planet Nine.”
A Dark Matter Alternative
Classical physics recognizes the law of universal gravitational proposed by Newton in 1687. This law works on a level that is familiar to our everyday experience and can be measured by grade school students—think of apples falling from trees, balls rolling down ramps, and so on. When Einstein proposed his general theory of relativity in 1915, among other tweaks, it modified the Newtonian understanding of gravity in cases of extreme acceleration—think of starships throttling up to approach the speed of light or objects plunging into black holes.
Proponents of MOND suggest that something similar is at play—that scientists only understand the mathematics of the universe under certain circumscribed conditions. The equations that govern MOND work best for scenarios in which mass is highly distributed and gravitational forces are lower—such as when stars are spread out at the edges of a galaxy.
“The one really basic assumption that seems extraordinarily reasonable is that we know how gravity looks,” says MOND researcher Stacy McGaugh, also at Case Western. “What MOND says is ‘Jeez, maybe we don’t.’”
But critics of MOND are plentiful, and they maintain that it falls short of standard dark-matter-based approaches in accounting for a host of well-known cosmic phenomena. “It doesn’t explain all the data,” says Kathryn Zurek, a cosmologist at the California Institute of Technology. “All of these cosmological datasets, combined, give us a consistent picture [that] only works in the presence of dark matter.”
MOND in the Solar System
While MOND is capable of describing the cosmic motions of galaxies, it struggles with smaller, more proximate concerns such as the objects in our solar system. The theory is developed to work in a weak gravitational regime, but the sun’s mass acts as a thumb on our scales. Its gravitational influence overpowers the far more subtle effects MOND might have on the movements of comets, asteroids and other small bodies. The theory’s greatest “local” hopes, then, might be found in far-distant regions of the solar system such as in the Oort cloud, a vast assemblage of long-period comets extending out perhaps a light-year from the sun, where our star’s gravitational grip is most tenuous.
While working on a previous MOND study, Brown and Mathur realized that MOND’s effects could potentially be seen in the orbits of objects that, while much closer in than the Oort cloud, still loop out to extreme distances from the sun. Six such bodies, dubbed extreme trans-Neptunian objects (eTNOs), were the original basis for the Planet Nine hypothesis, as proposed in 2016 by California Institute of Technology astronomers Mike Brown (who is not related to Katherine Brown) and Konstantin Batygin. The two researchers showed how these eTNOs could have been nudged into their orbit by a hidden world, which they dubbed Planet Nine. Subsequent searches have revealed other objects that support their theory.
According to MOND, the push and pull of gravity at the outskirts of the solar system should result in a gravitational well, a region where objects will naturally gather in a manner akin to raindrops flowing downhill to form a puddle. The MOND-sculpted orbits of these bits of ice and rock should line up with the center of the Milky Way as the galaxy’s gravitational field—again, in presumptive accordance with MOND—drags them into alignment. And that’s what the new study claims to see.
Batygin, who is intrigued by the recent results, isn’t completely convinced about the alignment. “You could reasonably make the argument that the alignment is suggestive,” he says. “It might be a coincidence.”
Outer solar system modeler David Nesvorny of the Southwest Research Institute says that “it’s not very convincing that there is a strong alignment,” though he doesn’t dismiss it completely.
“It’s an interesting idea,” says Aurélien Hees, a physicist at the Paris Observatory, who studies and tests gravitational theories. In 2014 Hees used measurements from NASA’s Cassini spacecraft to surmise whether MOND has had any discernible effect on Saturn. During its mission, Cassini’s telemetry was so precise that researchers could confidently calculate Saturn’s location to less than one meter. Despite these exquisite measurements, though, Hees saw no impact of MOND on the ringed planet.
“There is no evidence for deviation from general relativity,” Hees says. “It doesn’t mean that MOND is excluded by Cassini. It just means that if it’s there, it has to be very small, at least on the trajectory of Saturn.”
Nesvorny says that what would make the new study more convincing is large-scale modeling of orbital motions of all the relevant known eTNOs, something that Katherine Brown and Mathur say they are working on. They hope to have this follow-up work finished before a deadline of sorts that has been imposed by the upcoming operations of the Vera C. Rubin Observatory, which will begin later this decade.
Located in northern Chile, Rubin will make a large-scale survey of the sky that should reveal any other eTNOs whose orbits were affected by either MOND or Planet Nine. It will also resolve concerns of whether the supposedly anomalous orbital arrangements cited for Planet Nine and MOND alike are in fact merely products of observational bias and small-number statistics. This key uncertainty—that predictions of undiscovered worlds or quirky fundamental forces hinge on only a handful of data points—remains a worry for many scientists following the debate.
“The question of whether the orbits are clustered or not will be answered,” Batygin says. “It will in fact be answered even if Vera Rubin discovers zero additional objects.” With luck, Rubin could even spot Planet Nine itself, Mike Brown says, though he admits it’s possible that the elusive world may prove too faint for the new telescope.
According to Mathur and Katherine Brown, additional objects should be drawn in gravitationally, so they are also hoping that Rubin identifies more eTNOs in extreme orbits.
“What we really want is to have 100 high-quality objects similar to the six objects,” Mathur says. “Then we can say one way or the other what’s happening for sure.”