How many planets are there in the universe?

How many planets are there in the universe? The answer is very simple, though unexpected. It's all about definitions

The answer is much simpler than one might think: there are only eight planets in the universe, namely Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. There are no others, at least for the time being.

Before we understand why, it is necessary to clarify what is meant by the term “planet”. Starting from August 24, 2006, when the IAU (International Astronomical Union) convened to precisely define the classification of celestial bodies in the Solar System. A planet, to be defined as such, must:

  1. Orbit the Sun.
  2. Have enough mass to assume a nearly spherical shape, in other words, be almost spherical.
  3. Have cleared its orbit of other celestial bodies, meaning that within its orbit, it is not subject to the gravitational influence of other celestial bodies.

Based on these three principles, there are only eight planets in the universe.

Why was Pluto demoted?

Until the 20th century, Pluto was always considered the ninth planet in the Solar System. The first doubts began to arise when it was discovered to be the largest body in the Kuiper Belt. On July 29, 2005, Eris, a new trans-Neptunian object with the same size as Pluto, was discovered. That marked the final blow to Pluto’s status as a planet. Although initially Eris was dubbed the tenth planet, the astronomical community was strongly divided on their classification and, consequently, the subsequent reclassification of Pluto.

In August 2006, based on what was established by the IAU, Pluto was reclassified with the asteroidal designation (134340) Pluto. This is because Pluto does not meet the third requirement regarding gravitational dominance, as its mass is 0.07 times that of all other objects that orbit in its vicinity. For example, Earth has a mass nearly 1.6 million times greater than all other objects in its orbit. However, Pluto’s demotion did not sit well with a significant portion of the astronomical community.

What is an exoplanet?

An exoplanet is an extrasolar planet, meaning any planet that orbits a star other than the Sun and is not part of the Solar System. The presence of exoplanets has always been the basis for numerous scientific speculations, dating back to 1713 when Isaac Newton formulated the first hypothesis of the existence of these celestial bodies. However, it was only in 1995 that their existence was confirmed. On October 5 of that year, astronomers Geoff Marcy and Robert Butler confirmed the existence of a Jupiter-mass planet around the star 51 Pegasi, a yellow dwarf in the Pegasus constellation similar to the Sun.

Map of all exoplanets discovered until April 2022. Credit: NASA
Map of all exoplanets discovered until April 2022. Credit: NASA

Its proximity to the star suggested it should be a rocky planet, so it was immediately hypothesized as a super-Earth based on our knowledge of the Solar System’s structure (all rocky planets are near the Sun, while gas and ice giants are at the outer edges). However, further studies later revealed it to be a gaseous exoplanet, termed a hot Jupiter, revolutionizing our understanding of planetary formation. In the subsequent years, hundreds of gas giants were confirmed. It wasn’t until 2005 that the first super-Earth was discovered: Gliese 876d, a rocky planet orbiting the red dwarf Gliese876 at about 15 light-years away, in the Aquarius constellation.

To date, there are more than 5,000 exoplanets discovered (ref.) out of an estimated hundreds of billions in our galaxy alone, all in just three decades of research and thanks to highly advanced tools and knowledge. The number continues to rise and likely will never end. The future generations’ next challenges will certainly be finding not only potentially habitable planets in the universe but even habitable ones.

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