Does 3I/ATLAS Generate its Own Light?
upstart writes:
Does 3I/ATLAS Generate Its Own Light?:
The best image we have so far of the new interstellar object, 3I/ATLAS, was obtained by the Hubble Space Telescope on July 21, 2025. The image shows a glow of light, likely from a coma, ahead of the motion of 3I/ATLAS towards the Sun. There is no evidence for a bright cometary tail in the opposite direction. This glow was interpreted as evaporation of dust from the Sun-facing side of 3I/ATLAS.
Figure 3 of the analysis paper (accessible here) [PDF] shows a steep surface brightness profile of the glow with a projected power-law slope of -3, which implies a three-dimensional emissivity profile with a radial power-law slope of -4. Such a slope is steeper than observed in solar system comets. Together with my brilliant colleague, Eric Keto, we realized that the observed slope of -4 is consistent with an alternative model in which the dust outflow around 3I/ATLAS is illuminated by a central source. This model naturally accounts for the steep brightness profile, since the outflow density slope of -2 is accompanied by the radial decline of the illuminating radiation flux with an additional declining slope of -2.
If 3I/ATLAS generates its own light, then it could be much smaller than expected from a model in which it reflects sunlight. The reflection model requires a diameter of up to 20 kilometers, which is untenable given that the limited reservoir of rocky material in interstellar space can only deliver such a giant rock once per 10,000 years or longer (see the calculation in my paper here).
Last night, we held the annual soccer cup match between the faculty and the students at Harvard's Institute for Theory & Computation, for which I serve as director. Although I scored 2 goals for the faculty team, the students won 3 to 2. Disappointed by the outcome, I focused on 3I/ATLAS as soon as I woke up the following morning.
First, I calculated that the luminosity of 3I/ATLAS needs to be of order 10 gigawatt. Second, I realized that the steep brightness profile around 3I/ATLAS implies that the nucleus dominates the observed light. This must hold irrespective of the origin of the light. In other words, the nucleus dominates over the emission from the glow around it.
The illumination by sunlight cannot explain the steep 1/R profile of scattered light, where R is the radial distance from the nucleus. This is because a steady dust outflow develops a 1/R^2 profile which scatters sunlight within the same emissivity profile. Sunlight would dominate the illumination in this model because a rocky nucleus would reflect only a small fraction of the solar intensity from a much smaller area than the 10,000-kilometer region resolved in the Hubble Space Telescope image. Another possibility for the steep brightness profile is that the scattering halo is made of icy particles that get evaporated as they move towards the Sun from the warm Sun-facing side of 3I/ATLAS. This would explain why there is no tail of these scattering particles. The required evaporation time must be of order 10 minutes but it is unclear whether this would lead to the observed 1/R brightness profile.
The simplest interpretation is that the nucleus of 3I/ATLAS produces most of the light. I calculated that the nucleus cannot be a thermal emitter with an effective surface temperature below 1000 degrees Kelvin or else its peak emission wavelength would have been longer than 3 micrometers with an exponential cutoff at shorter wavelengths, incompatible with the data. At higher effective temperatures, the required luminosity of 3I/ATLAS can be obtained from a source diameter smaller than 100 meters. A compact bright emitter would make 3I/ATLAS of comparable size to the previous interstellar objects 1I/`Oumuamua or 2I/Borisov, making more sense than the 20-kilometer size inferred in the model where it reflects sunlight.
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