Lily Dorstewitz, Contributing Photographer

Yale astronomers are joining the scientific effort to search for Planet Nine — a hypothesized planet in the outskirts of the solar system — despite some skepticism in the field about its existence.

Malena Rice GRD ’23, a graduate student in the Yale astronomy department, and Gregory Laughlin, a professor in the department, are attempting to map the path of the hypothetical planet using a “shifting and stacking” technique, which involves a detailed survey of the sky. Rice, who is the first author of the study, presented the research alongside Laughlin, the study’s senior author, at the annual meeting of the Astronomical Society’s Division for Planetary Sciences.

“We are trying to get all of the light from that object in one place by just following its movement across the sky,” Rice said. “To do this, we shift our images along an object’s orbital path and stack them together, summing together all of the light collected from that object over time to get one strong aggregate signal.” 

This method has previously been used to discover some of the moons around Neptune and Uranus and it is often used to track trans-Neptunian objects — or TNO’s — which are objects beyond Neptune’s orbit in the distant solar system. Rice and Laughlin were able to extend this method and use data from NASA’s Transition Exoplanet Survey Satellite, a space telescope designed to search for exoplanets, to survey the sky and map the orbits of possible candidates for Planet Nine.

The evidence for Planet Nine so far exists in the curious alignment of the orbits of small, icy objects in the distant reaches of our solar system. But in an email to the News, Laughlin explained that because these objects are only about 1,000 km across, they are not massive enough to exert a gravitational influence strong enough to create the actual configuration of orbits observed by astronomers.

While concrete details about the size and other features of Planet Nine have not yet been determined, astronomers have been able to make approximations on some of its characteristics.

“Among the planets in our Solar System, it would probably appear most similar to Neptune — a nearly featureless blue orb,” Laughlin wrote. However, he noted that due to Planet Nine’s distance from the sun, the sunlight that reaches it would be hundreds of thousands of times fainter than the one that reaches Earth, making Planet Nine incredibly faint to observers. Planet Nine would be approximately 12 to 13 times more distant from Earth than Pluto is.

If it exists, Planet Nine would be a super-Earth, having five to 10 times the mass of Earth, Laughlin wrote, and it would be roughly 500 times further from the Sun than Earth. However, he noted that this estimate is fairly uncertain, and that Planet Nine could be 250 times closer or 1000 times farther from the Sun than the Earth.

Even if findings hint at the existence of Planet Nine, many astronomers are skeptical of its presence. Some, including Amir Siraj, a Harvard undergraduate student, and Avi Loeb, a professor in the Harvard astronomy department, believe that the curious alignment of orbits can be explained by a cluster of asteroids beyond the Kuiper belt or a primordial black hole, a remnant of the Big Bang.

In a joint study, Siraj and Loeb explored the possibility of a primordial black hole in the distant solar system as an alternative to Planet Nine. Siraj and Loeb proposed that a failure to detect light from Planet Nine would work in favor of the black hole model. But they say that in either case, it is too early to come to a definitive conclusion.

“In astronomy, and science in general, evidence should lead the way,” Loeb said in an interview with the News. “We should be open-minded.” 

Although Rice and Laughlin both agree that it is too early to fully support the idea of Planet Nine over that of a cluster of asteroids or a primordial black hole, they argue that a planet is the most plausible explanation at the moment. 

Rice noted that a cluster of asteroids large enough to produce this alignment of orbits has never been recorded, so the possibility of this being the case now is not too high. A similar conclusion applies to primordial black holes, as astronomers have only hypothesized their existence. Therefore she thinks that at the moment, a planet seems like the mostly likely explanation. 

While there are debates regarding the nature of the orbits’ clustering, these astronomers can agree on one thing: without enough evidence, these are all just theories. 

But if the existence of Planet Nine is confirmed, it could revolutionize astronomers’ understanding of the solar system. 

“The current prevailing model for how the solar system formed … is based on minor planets in the solar system,” Rice said. “And so, if we found Planet Nine that would be a crazy wrench in our current model of the solar system because we really do not have a good explanation for how the planet would have gotten there.”

Even if astronomers are not able to find Planet Nine, Laughlin noted that by surveying the outermost parts of the solar system, they would be able to see pristine material dating back to its initial formation. According to Laughlin, this could give the researchers clues about the kind of environment in which the Sun formed, as well as how the solar system came to be what it is today. 

Planet Nine was first speculated in 1846, following the discovery of Neptune.


Nicole Rodriguez | nicole.rodriguez.nr444@yale.edu