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Detecting Planet X

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Social media and internet searches show a plethora of articles on “Planet X”, a vaguely worded term for some supposedly mysterious planet of apparently great importance.There are also conspiracy theories about the government or NASA supposedly hiding “Planet X” for some nefarious reason.

In science, we generally never use this phrase. When a scientist does say “Planet X” he/she merely means “any undetected planet in our solar system”.

Planets beyond Pluto

Scientists never quite said “Pluto isn’t a planet anymore.” That’s a misleading statement which muddies the waters. Here’s what really is going on.

Old view

Solar system is made of one star, several planets, comets, meteors, and gas & dust particles.

More recent, yet now outdated view

Solar system is made of one star, several planets, comets, meteors, and gas & dust particles.
The planets are either terrestrial (“Earth like”) or gas giants.

New view

Solar system is made of one star, several planets, comets, meteors, and gas & dust particles.
The planets are now in categories:
terrestrial, gas giants, ice giants, or dwarf planets.

So all that really happened is that Pluto was moved from one general group, into a more specific group (dwarf planets.)

Here are some of the planets beyond Pluto, in our own solar system, already discovered. For size comparison they are shown as if they are near each other.

Ceres, Charon, Eris, Dysnomia, Pluto, Haumea, Makemake,


This picture shows the sizes of the original three dwarf planets (Eris, Ceres, and Pluto) as compared to Earth. It also shows Pluto’s large moon Charon (and its two small moons Nix and Hydra) and Eris’s moon Dysnomia to scale. The image also shows Earth’s Moon (Luna) and the planet Mars for comparison. None of the distances between objects in this image are to scale. Images courtesy of NASA, ESA, JPL, and A. Feild (STScI).

also see

Dwarf Planets Pluto Makemake Haumea Eris

Credit: Konkoly Observatory/András Pál, Hungarian Astronomical Association/Iván Éder, NASA/JHUAPL/SwRI

Why is it difficult to find new worlds?

Out there, space gets dark alarmingly fast. Planets twice as far away look 16 times dimmer: The intensity of the sunlight weakens by a factor of four going out and then four times again coming back.

At an orbital distance of 600 astronomical units (an AU is the distance between Earth and the sun), Planet Nine would be 160,000 times dimmer than Neptune is at 30 AU.

At 1,000 AU, it would appear more than 1 million times weaker.

“There’s really a brick wall, basically, at 1,000 AU,” said Kevin Luhman, an astronomer at Pennsylvania State University.” That’s partly why laying eyes on the planet has proven so tough.

Why Can’t We Find Planet Nine? Quanta Magazine

Possible large planet orbiting beyond Pluto


Planet X detection new planets

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No Need for Planet Nine? Small Objects’ Gravity Could Explain Weird Orbits

A New Study Could Explain Away Some Evidence for Planet Nine

Goodbye, Planet Nine! New and better data disfavors the existence of a giant world beyond Neptune.

General resources



Science Bulletins: The Hunt for Planet X. American Museum for Natural History.

Astronomers find evidence of a ninth planet in the solar system – Caltech, Robert Hunt, Reuters



Learning Standards

Next Generation Science Standards
Connections to Nature of Science: Science Models, Laws, Mechanisms, and Theories Explain Natural Phenomena.
A scientific theory is a substantiated explanation of some aspect of the natural world, based on a body of facts that have been repeatedly confirmed through observation and experiment, and the science community validates each theory before it is accepted. If new evidence is discovered that the theory does not accommodate, then the theory is generally modified in light of this new evidence. (HS-ESS1-2),(HS-ESS1-6)

A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas (2012)

Some objects in the solar system can be seen with the naked eye. Planets in the night sky change positions and are not always visible from Earth as they orbit the sun. Stars appear in patterns called constellations, which can be used for navigation and appear to move together across the sky because of Earth’s rotation…. The solar system consists of the sun and a collection of objects, including planets, their moons, and asteroids that are held in orbit around the sun by its gravitational pull on them. This model of the solar system can explain tides, eclipses of the sun and the moon, and the motion of the planets in the sky relative to the stars.

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