Thanks to generations of astronomers, now backed up by space exploration, the solar system has an ever-growing list of dwarf planets, moons, asteroids, comets, as well as smaller objects. But there are only eight major planets in our solar system. In order of distance from the sun, they are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
As Universe Today explained, these eight worlds are distinguished from all other objects orbiting the sun — including the (in)famous dwarf planet Pluto — by the way their massive gravitational fields “sweep out” the region of space around their orbits. Thus, apart from their own moon systems, they are all but devoid of near neighbors in space.
We tend to imagine the planets as having been discovered by astronomers with telescopes, but this has only happened twice, with Uranus and Neptune. Of the other six, five of them — Mercury, Venus, Mars, Jupiter and Saturn — are among the brightest visible “stars” in the night sky.
Early astronomers discovered that these five planets gradually drift across the night sky, shifting their position from night to night compared to neighboring stars. The University of Oregon noted that ancient reports of these planetary movements can be traced back at least 4,000 years.
Another major planet is the one under our feet, Earth: We didn’t have to discover it, either, though recognizing that it is a planet was an amazing achievement.
Enter the Telescope
Of the planets orbiting the sun, the seventh, Uranus, was, per Sky & Telescope, the first to be discovered by an astronomer with a telescope: William Herschel in 1779. It happens that Uranus is in fact visible to the naked eye, though just barely. Because of its faintness — and because no one was on the lookout for a faint outer planet — it eluded discovery though the centuries.
Herschel wasn’t looking for a planet, either, he was looking for double stars. Uranus had a slightly odd appearance though his telescope; it caught his attention and led to his unexpected discovery.
Enter Sir Isaac Newton
Proof that Uranus was, indeed, one of the planets orbiting the sun was a triumph for Sir Isaac Newton’s law of universal gravitation (though he had died long before Uranus was discovered). But there was more to come, and the human history of the solar system was poised to take a new turn, driven by mathematics.
As astronomers tracked Uranus during the decades after its discovery, said Sky & Telescope, they found tiny discrepancies between its actual orbit and the orbit predicted by Newton’s theory. These discrepancies were not random. They suggested to mathematicians that there must be another planet nearby, tugging on Uranus, which could account for its straying motion.
As Space.com reported, two astronomers, Urbain Le Verrier and John Couch Adams, both calculated a position for the eighth planet. In 1846, astronomer Johann Gottfried Galle examined the region of sky their calculations pointed to, and discovered Neptune.
But there is a twist. As it turns out, none other than Galileo had previously seen Neptune. Galileo recorded it in his notebook while observing Jupiter and its moons, which he had recently discovered with his pioneering telescope, but it’s believed he mistook the planet for a star.
A Ninth Planet?
In the decades after the discovery of Neptune, history seemed poised to repeat itself, said the American Astronomical Society. Measurements hinted that another planet was influencing Uranus’ orbital motion, and astronomer Percival Lowell launched an effort to find what he dubbed Planet X.
In 1930, this effort seemed successful when Clyde Tombaugh, working on Lowell’s project, discovered a faint, distant object near the predicted location for Planet X. This object was named Pluto; it seemed much smaller than predicted, but was regarded as the ninth planet for decades. Lowell’s Planet X predictions were eventually dismissed when more precise orbital data from the 1982 Voyager 2 space mission showed that Neptune was big enough to influence Uranus’ orbit by itself.
Only in the early 21st century did astronomers discover other similar-sized bodies, such as Eris, orbiting the sun far beyond Neptune. Thus, said Universe Today, Pluto is now classed as one of a growing list of dwarf planets, larger than asteroids but smaller than the major planets.
The possible existence of another major planet far beyond Neptune remains a matter of dispute. Some astronomers continue to examine the orbits of known outer-system objects, looking for hints of orbital perturbations by massive undiscovered objects.
Others have turned to planetary exploration beyond the solar system, where the list of discovered planets now runs into the thousands. In contrast, says Robert Lockwood of the TESS mission operations team at Northrop Grumman, there are probably “only eight planets in our solar system. There are legions of other interesting objects in our solar system, asteroids, moons, comets, etc., but I don’t know of anyone referring to them as planets.”
The history of the solar system has entered a new era with the discovery of more dwarf planets. Future deep space probes as well as orbiting observatories such as the upcoming James Webb Space Telescope (JWST) will be central to our exploration of this vast region of space.