Planets Throughout History
We think we will know a planet when we see one. That is not true. To an uninitiated observer, a planet looks like a bright star. Ancient people have divided stars into two kinds: fixed stars and wandering stars. Fixed stars don’t change their position relative to other stars. So if you see one pattern made by stars that looks like a box or another that looks like a triangle they will look the same the entire night, tomorrow next month every year the same stars won’t leave their position and move close to some other stars. These patterns were called constellations. But some bright ones were like wandering nomads today that are seen in one constellation and in a few days, they move and are found in another. These moving stars were called wandering stars or planets. There were five such wandering stars, Mercury, Venus, Mars, Jupiter and Saturn. Sun and Moon were also observed to wander from one constellation to another. So in Pre-Copernican astronomy, there were 7 ‘Planets’. In India, Astrologers added two more imaginary planets Rahu and Ketu responsible for eclipses and causing people to have bad and good times.
Copernicus and Galileo made the Sun into a star around which the whole solar system revolved along with the Earth. So we removed the sun and added Earth to the list of planets. Moon is the only solar system body that revolves around the Earth and is classified as Earth’s natural satellite. So the number of planets in the solar system becomes 6. Hershel’s discovery of the Uranus and discovery of Neptune by urban Le Verrier and Adam’s. Pluto joined the list of planets and was then removed before it could complete even one orbit around the Sun as it was discovered that it has a pretty large moon Cheron and its orbit is not cleared and settled. So finally we have only eight planets. Jupiter has 95 moons out of that 4 are large galilean moons. If Ganymede, Calisto, Europa and IO were directly orbiting they would have been considered planets. Saturn also has 146 Moons out of these Titan is the second largest moon with a dense nitrogen-rich earth-like atmosphere. So it is also a ‘planet’ but in the wrong orbit. We have a large spherical body that failed to get the status of a planet in the asteroid belt, Ceres. When scientists were desperate to find a planet between Mars and Jupiter, it was discovered and deemed a planet. But later when more bodies were found in that part it was reclassified as an asteroid. Interestingly when poor Pluto was ‘demoted’ from the class of planets Ceres was kind of promoted from an asteroid to a dwarf planet. From time to time there were rumours of an imaginary planet Vulcun inside the orbit of Mercury and a Planet X out in the Kuiper belt ‘Nemesis’. The first is fully ruled out but the second one can not be fully ruled out yet.
There is a large gang of exoplanets, almost 6,660 confirmed and this number is soon going to explode as the James Webb space telescope is expected to discover more planets. Most of the planets discovered initially were Jupiter-type gas giants because it is easier to pick up a large object than a small one but as technology has been refined astronomers are capable of finding earth-size planets too. Some time I will talk about methods of discovering extrasolar planets.
