Astronomy
Using Stellar Occultations to Probe Outer Solar System Bodies
Quick fact
The occultation of a star by Pluto in 1988 revealed that Pluto has a tenuous atmosphere, surprising scientists who thought it might be an airless ice ball.
Why this is interesting
Imagine trying to measure a city's shape from miles away using its silhouette against the sun. Now replace the sun with a distant star and the city with a tiny world billions of kilometers away.
Read the full explanation
Understanding Using Stellar Occultations to Probe Outer Solar System Bodies
A stellar occultation happens when a body like a planet, moon, or asteroid passes directly in front of a background star. As the body moves, it casts a shadow that sweeps across Earth. Observers in the shadow's path see the star's light disappear and reappear. By recording the exact times of disappearance and reappearance, we can measure the body's size and shape. If the body has an atmosphere, the star's light is progressively dimmed and bent (refracted) as it passes through the atmosphere, encoding atmospheric properties. This technique works because the geometry is precise: the star is effectively a point source, and the body's motion is well known. For outer solar system bodies, which are small and far away, occultations provide a high-resolution snapshot that even the best telescopes cannot match directly.
A deeper explanation
The light curve from an occultation is a record of stellar flux over time. For a body without an atmosphere, the light curve shows a sharp drop to zero and a sharp rise back. The duration of the occultation gives the chord length across the body, and when combined with the known velocity of the shadow across Earth, yields the body's diameter. For an atmosphere, the light curve is more complex. As the star approaches the limb, its light passes through the uppermost atmosphere, where the refractive index is slightly greater than the vacuum. The starlight is refracted away from the observer, causing a gradual dimming. The amount of bending depends on the density gradient of the atmosphere, which is related to the temperature and pressure through the ideal gas law and hydrostatic equilibrium. By modeling the light curve, scientists can extract the atmospheric temperature and pressure as a function of altitude, and thus the density profile. This technique has been used for bodies like Pluto, Triton, and even the Kuiper Belt object (486958) Arrokoth, revealing that even these small worlds can have atmospheres or that their shapes can be measured with unprecedented accuracy. The method is powerful because it uses a natural source, works from Earth, and provides data that would otherwise require a spacecraft to gather.