In a radio interview from DC a few hours ago, the interviewer asked: “Is Pluto a planet? I am discussing an astronomy book with my 3.5-year-old son and we are confused by conflicting arguments raised by astronomers over the years on whether to call it a planet. What is your take?” My reply: “It does not matter. Pluto is one of many Solar system objects assembled from rocks, ice and gas within the debris disk left over from the formation of the Sun. Astronomers classify some of these objects as planets, but nature produced a broad spectrum of objects with no class system in mind. We should consider the diverse splendor of all these objects with equal attention, as if they were family members, each with its own unique characteristics and special qualities.” The interviewer was surprised and noted: “I never heard this before. Thank you. I will let my young astronomer know.”
The debate about naming Pluto a planet stems from the 1992 discovery by Jane Luu and David Jewitt, followed by Mike Brown’s findings, of many more objects like Pluto beyond Neptune’s orbit at 30 au, the so-called Kuiper belt of trans-Neptunian objects, roughly 40–50 times farther than the Earth is from the Sun (with the Earth-Sun separation defined as an `astronomical unit’ and abbreviated as `au’). The largest trans-Neptunian objects are: Pluto, Eris, Haumea and Makemake.
In August 2006 the International Astronomical Union (IAU) downgraded the status of Pluto, discovered by Clyde Tombaugh in 1930, to that of a “dwarf planet.” Only the rocky worlds of the inner Solar System and the gas giants of the outer system were designated as planets. The inner Solar system within Jupiter’s orbit contains the asteroid belt as well as the terrestrial planets, Mercury, Venus, Earth, and Mars. The “gas giants” include Jupiter, Saturn, Neptune, and Uranus. This classification counts eight Solar system planets instead of the nine before the IAU decision.
A “dwarf planet,” as defined by the IAU, is a celestial body in orbit around the Sun that is massive enough to have its (nearly spherical) shape controlled by gravity rather than mechanical forces (which allows small objects to be unusually shaped), but has not cleared its neighboring region of other objects. The three criteria of the IAU for a full-sized planet are:
- It is in orbit around a star.
- It has sufficient mass to maintain hydrostatic equilibrium with a spheroidal shape.
- It has cleared the neighborhood around its orbit.
My point is simple. Even though the IAU elevated planets to a special class, these objects should not receive any priority relative to their smaller family members. After all, they are much less massive than their host star, which is by itself a trillionth of the mass of the Milky Way galaxy, which is one out of a trillion galaxies in the observable volume of the universe, not to speak about the volume of spacetime beyond what we observe. Moreover, the smaller objects are more numerous and may represent the most abundant habitats for life under their icy surface.
It is also refreshing to recall the statement made by William Shakespeare in the play Romeo and Juliet: “What’s in a name? That which we call a rose. By any other name would smell as sweet.” In other words, Kuiper belt objects are still fascinating after Pluto’s demotion.
Low-mass objects in the outskirts of the Solar system are found from their reflection of sunlight. Since the flux of sunlight impinging on their surface declines inversely with distance squared and the flux that gets returned to our telescope has an extra dependence of inverse distance squared, a receding Kuiper belt object is expected to fade away inversely with distance to the fourth power. However, if an object were to produce its own light, it would fade away like the lights of a boat sailing away at sea, inversely with distance squared. In a 2012 paper with Ed Turner, I showed that the deep image from the Webb telescope, celebrated on July 11, 2022 at the White House, could identify a light source as faint as the city of Tokyo at the distance of Pluto.
When Mike Brown visited my office at Harvard seven years ago, I asked him whether he ever checked if the brightness of Kuiper belt objects varies inversely with distance to the second or fourth power as they change their distance. He replied promptly: “Why would I check? Their brightness must change with distance to the fourth power.” I quietly ruminated: “If you never imagine unusual things, you would never discover them.”
The following year, Mike Brown suggested with Konstantin Batygin, his colleague at Caltech (and a former postdoc with me at Harvard), that there may be a new planet to replace Pluto in the outer Solar system. The possible existence of this “Planet Nine”, was inferred from the clustering of extreme trans-Neptunian objects. Batygin and Brown calculated Planet Nine to have 5–8 times the mass of the Earth and orbit the Sun at a distance of 3–5 hundred au. So far, the search for light from this hypothetical planet came empty handed.
Could this object be dark because it is a black hole? In a recent paper with my student, Amir Siraj, I considered the possibility that Planet Nine is a primordial black hole with a planetary mass and a corresponding horizon size of a grapefruit. We showed that icy rocks in the outer Solar system should arrive close enough to such a black hole for them to be disrupted by its strong gravitational tide. The resulting flare from accretion onto the “mouth” of the black hole could be detected at a reasonable rate with the Legacy Survey of Space and Time (LSST) of the upcoming Vera C. Rubin Observatory.
Here’s hoping that the 3.2-billion-pixel LSST video of the Southern sky every 4 days will inform us of new objects in the outskirts of the Solar system. Whether they are a new planet, new objects like the demoted Pluto, smaller rocks reflecting sunlight, primordial black holes disrupting icy rocks, or artificial lights from a city or a spaceship, would be as exciting to me as they would be to the 3.5-year-old son of the interviewer who will carry the torch of astronomy to new frontiers, irrespective of any classification system generated by senior members of the IAU.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He chairs the advisory board for the Breakthrough Starshot project, and is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021.