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Explain xkcd: It's 'cause you're dumb.
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Ruling Out
We were able to replicate and confirm prior authors' detection of a moon orbiting the Earth with high confidence.
Title text: We were able to replicate and confirm prior authors' detection of a moon orbiting the Earth with high confidence.


Ambox notice.png This explanation may be incomplete or incorrect: Created by a TECTONICALLY-ACTIVE BOT WITH SUBSURFACE OCEANS. Do NOT delete this tag too soon.

Most science studies are intended to discover new things. In astronomy, the goal is often to find different types of objects in space, or learn how astronomical objects are formed and behave. But often from studying things that exist, we also learn about limits in the kinds of things that 'can' exist; when this happens, we say that we've ruled out these other phenomena.

Cueball lists a number of obviously impossible[citation needed] objects.

  • "Earthlike stars": A play on "Earth-like planets" which scientists are very interested in finding. The Earth is not a star, hence stars cannot be Earthlike.
  • "Exoplanets in our solar system": Exoplanets are by definition not in our solar system.
  • "Habitable zone quasars": while not certain by any means, habitable zones around some quasars have absolutely not been ruled out.[1][2] Perhaps Cueball is referring to quasars in the habitable zones of stars, which are also feasible depending on the criteria for classifying a black hole with an active accretion disk as a quasar.[3]
  • "Stars with subsurface oceans": Because the temperatures inside stars are higher than that which can support the existence of liquids as we understand them, stars cannot have subsurface oceans. After many billions of years, a white dwarf will cool to the point where it no longer emits significant heat or light, becoming a black dwarf, and eventually cooling to the point where it might develop subsurface liquids.[actual citation needed] However, the universe is not old enough for any black dwarfs to exist yet,[4] and sufficiently cool black dwarfs might not even be considered stars according to the conventional definition.
  • "Tectonically active black holes": Black holes do not have tectonic plates, so they cannot be tectonically active. There are theories that neutron stars can exhibit tectonic-like movements, but the physics of the 'inside' of a black hole is thought to be so much different.

The joke is that you don't actually have to study anything to come to these almost patently obvious conclusions. The counter-proposals would need far more effort to even justify them as valid theories, by common understanding, and greater still to try to observe any supporting proof.

Some studies are also done to confirm the results of previous studies, to ensure that the conclusions were not mistaken or a fluke. The title text describes a study that was done to confirm the existence of a moon orbiting Earth, although the existence of the moon has been known for at least as long as humanity has existed, and the fact that it orbits the Earth has been assumed or known for upwards of 3000 years.[actual citation needed] The ancient Greeks and Babylonians, for example, thought that the Moon orbited the Earth, though they lacked a detailed physical understanding of the system. Anaxagoras (c. 500–428 BC) is credited with the correct explanation of lunar eclipses, and reportedly was the first to explain that the Moon shines due to reflected light from the Sun. However, it was not until the work of Nicolaus Copernicus in the 16th century that a detailed and accurate model of the Moon's orbit around the Earth was developed. Regardless, at this stage, a study to confirm the validity of Copernican orbits would contribute nothing to the scientific process, much less a study confirming the mere existence of the Moon.


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