When you search for within the night time sky, our moon is unmissable.Just like Earth, it is spherical and has a stable, rocky floor. But is our moon truly a planet?
The reply is technically no — at the least in response to NASA and the International Astronomical Union (IAU), an essential international authority in astronomy.
An object wants to fulfill 4 standards to be a planet, per the IAU. It should be sufficiently big that gravity pulls it into a pleasant, spherical form and sufficiently big to have cleared out all of the asteroids and different junk from its orbit. It additionally must orbit the solar, and — importantly for the moon — a planet by definition isn’t a satellite tv for pc of one other object.
The IAU’s definition of a planet was finalized in 2006. But that hasn’t at all times been the consensus. The historical Greeks, as an illustration, thought the moon was a planet. (They did additionally assume the Earth was the middle of the universe, although, so that they weren’t fairly proper about a number of issues.)
Even immediately, the IAU definition is a bit controversial. “Whether or not the moon is a planet truly depends upon who you ask,” Skylar Grayson, an astronomer at Arizona State University, advised Live Science. The 2006 IAU definition “attracts a reasonably arbitrary line between planets and dwarf planets that (in)famously led to the demotion of Pluto,” she added. “Some planetary scientists thus elect to think about totally different definitions of planets, a few of which embody Pluto and the opposite dwarf planets, and others that even embody the moon.”
Related: Why is Pluto not thought-about a planet?
One alternate definition comes from geophysics: A planet (or planetary physique) is something huge sufficient to be spherical however sufficiently small that it could not generate power from nuclear fusion at any level in its life. The first half of this definition divides planets from smaller bits of the photo voltaic system like asteroids, and the second half attracts a line between planets and stars. By this definition, the Earth could be a planet — and so would the moon and Pluto.
Most individuals consider a moon as one thing that is smaller than a planet, particularly the planet it orbits. But this will get difficult, too. Our moon is surprisingly huge in comparison with our planet — a whopping one-fourth the dimensions of Earth. And Pluto’s largest “moon,” Charon, is definitely half the dimensions of Pluto — sufficiently big that it is sometimes called a binary planet as a substitute of a moon. Looking past the photo voltaic system, astronomers have even discovered a moon as huge as Neptune orbiting a planet the dimensions of Jupiter about 8,000 light-years from Earth.
The IAU definition is essentially the most broadly accepted one for planethood, and it clearly states our moon isn’t a planet. However, “The vagueness round what’s and is not a planet factors to how arduous it may be to outline phrases in astronomy,” Grayson stated. “Most issues within the universe exist not in neat containers however quite on a spectrum, and we’ve got to determine the place we need to draw the strains between classes. Sometimes these strains are agreed upon, and generally they are not.”
