It was Robert Browning who said “Ah, but a man’s reach should exceed his grasp, or what’s a heaven for?” A rousing thought, but we don’t always know where we should reach. In terms of space exploration, a distant but feasible target is the solar gravitational lens distance beginning around 550 AU. So far the SGL definitely exceeds our grasp, but solid work in mission design by Slava Turyshev and team is ongoing at the Jet Propulsion Laboratory.

Targets need to tantalize, and maybe a target that we hadn’t previously considered is now emerging. Planet Nine, the hypothesized world that may lurk somewhere in our Solar System’s outer reaches, would be such an extraordinary discovery that it would tempt future mission designers in the same way.

This is interesting because right now our deep space targets need to be well defined. I love the idea of Interstellar Probe, the craft designed at JHU/APL, but it’s hard to excite the public with the idea of looking back at the heliosphere from the outside (although the science return would be fabulous). Pluto was hard enough to sell to the powers that be, but Allen Stern and team got the job done because they had a whole world that had never been seen up close. Will Planet Nine, if found, turn out to be the destination that some budget-strapped team finds a way to explore?

We have a lot to learn about planetary demographics given that our planet-finding technologies work best for larger worlds closer in to their stars. But a recent microlensing study suggests that as many as one out of every three stars in our galaxy should host a super-Earth in a Jupiter-like orbit (citation below). Microlensing is helpful because it can reveal planets at large distances from their parent stars. Super-Earths appear to be abundant. If it exists, Planet Nine isn’t in a Jovian orbit, but it’s probably smaller than Neptune.

In so many ways this planet makes sense. We’ve found planets like it in innumerable stellar systems. We also know that mechanisms of gravitational slingshotting can push a planet either out of its system entirely or into an entirely new orbit, explaining our prior lack of detection. We’re talking about a planet on the order of a super-Earth or a mini-Neptune, the former larger than our planet but still rocky, the latter smaller than Neptune but gaseous.

Image: An illustration shows what Planet Nine might look like orbiting far from our Sun. We have found many such worlds around other stars, but finding one in our own backyard is taking time because of its distance and an orbit that is probably well off the ecliptic. Assuming a planet is there at all. Image credit: NASA/Caltech.

The evidence in the orbits of a number of outer system objects points to something perturbing their paths, and seems to implicate something big. Now we have, after years of evidence gathering and debate, at least a possible candidate for this world. In process at Publications of the Astronomical Society of Australia (PASA), although not yet published, the paper digs deep into data from the Infrared Astronomical Satellite as well as AKARI, a Japanese satellite somewhat more sensitive than IRAS and launched in 2006.

The distant Sedna and other similar bodies with unusual orbital characteristics tell us that something has to account for their high eccentricity and inclination, while a number of Kuiper Belt objects seem to be similarly affected, as shown in the orientation of their orbits (described through what is known as the argument of perihelion). Computations thus far indicate mass on the order of something equal to or greater than 10 Earth masses, with a semi-major axis of about 700 AU.

Image: The six most distant known objects in the solar system with orbits exclusively beyond Neptune (magenta) all mysteriously line up in a single direction. Moreover, when viewed in 3-D, the orbits of all these icy little objects are tilted in the same direction, away from the plane of the solar system. PL-Caltech/R. Hurt.

A huge amount of work has gone into this analysis, all ably summarized in the paper, which comes from Terry Long Phan (National Tsing Hua University, Taiwan) and colleagues. Optical surveys have heretofore failed to find this object, but 700 AU is fully 23 times the distance of Neptune from the Sun, far enough out that visible wavelengths need to give way to infrared to find it.

Phan’s team looked for candidate objects in the range of 500 to 700 AU using the two far-infrared surveys mentioned above, which are both all-sky in range but separated by 23 years, making for useful comparisons. The idea was to find an object that moved from an IRAS position to an AKARI position after those 23 years. Assuming the kind of mass and distance they were looking for, they had to remove stars and noisy sources toward galactic center. Narrowing the field to 13 pairings, Phan and his doctoral advisor Tomotsugu Goto found only one that matched in color and brightness, indicating they were the same object.

The paper cites the result this way:

After the rigorous selection including the visual image inspection, we found one good candidate pair, in which the IRAS source was not detected at the same position in the AKARI image and vice versa, with the expected angular separation of 42′ – 69.6′.

The AKARI detection probability map indicated that the AKARI source of our candidate pair satisfied the requirements for a slow-moving object with two detections on one date and no detection on the date of 6 months before.

Image: This is Figure 5 from the paper. Caption: Comparison between IRAS (left) and AKARI (right) cutout images of our good candidate pair. The green circle indicates the location of IRAS source, while the white circle indicates the location of AKARI source. The size of each circle is 25′′. The yellow arrow with a number in arcminute shows the angular separation between IRAS and AKARI sources. The colour bar represents the pixel intensity in each image in the unit of MJy/sr. The AKARI source in the right panel is not visible as a real physical source due to the characteristics of AKARI-MUSL, which include moving sources without monthly confirmation. Credit: Phan et al.

This could be considered tantalizing but little more, because it is impossible from these two observations to determine an orbit for this object. The authors say that the 570 megapixel Dark Energy Camera (DECam) may be useful for follow-up observations. But I also noted an article in Science by Hannah Richter. The author quotes Caltech astronomer Mike Brown, who came up with the original Planet Nine hypothesis in 2016.

Brown argues that this object is not likely to be Planet Nine because its orbit would be far more tilted than what is predicted for the undiscovered world. In other words, a planet in this position would not have the observed effects on the Solar System. In fact, a planet in this orbit would make the calculated Planet Nine orbit itself unstable, which would eliminate Planet Nine altogether.

Is there an entirely different planet out there? Future observations will have to sort this out. It surprises me that it has taken so long for this kind of search to occur. Discoveries are made when seasoned observers can prompt up-and-coming scientists to consider avenues hitherto unexplored. I think we can applaud Phan’s doctoral adviser, Tomotsugu Goto, for the insight to suggest this direction of study for a young astronomer who will bear watching.

For now, returning to the thoughts with which I began this post, there are a few implications even for missions in their current planning stages. From the paper:

Several recent studies proposed and evaluated the prospects of future planetary and deep-space missions for the Planet Nine search, including a dedicated mission to measure modifications of gravity out to 100 AU (Buscaino et al. 2015), the Uranus Orbiter and Probe mission (Bucko, Soyuer, and Zwick 2023), and the Elliptical Uranian Relativity Orbiter mission (Iorio, Girija, and Durante 2023).

We’ll be going deeper into these missions in the future. For now, I find the interest in Planet Nine heartening, because even in budget-barren times, we can be doing useful science by way of exploration with existing instruments and considering designs for missions we will one day be able to send. On that score, Planet Nine – or perhaps even a different world equally distant from the Sun – is an incentive for propulsion science and a driver for the imagination.

The paper is Phan et al., “A Search for Planet Nine with IRAS and AKARI Data,” accepted for publication in Publications of the Astronomical Society of Australia (PASA). Preprint. The paper on super-Earths is Weicheng Zang et al., “Microlensing events indicate that super-Earth exoplanets are common in Jupiter-like orbits,” Science Vol 388, Issue 6745 (24 April 2025), 400-404 (abstract).