Top 10 Greatest Astronomy Discoveries

In 1610, one of the most influential astronomers in history, Galileo Galilei, was able to see four strange objects that were circling the planet Jupiter millions of miles away through his telescope. These objects, as it turned out, were Jupiter’s four largest moons, Ganymede, Callisto, Io, and Europa, from biggest to smallest. Aside from Earth’s moon, this quartet became the first moons ever discovered in the solar system, thanks in large part to their absolutely huge sizes; Ganymede and Callisto are both as big as planets. Today, Jupiter’s moons are still some of the most intriguing bodies in the solar system, and one of them, Europa, has subsurface oceans that even have the potential to host alien life.

It’s strange to imagine now that there was a time before we knew about the eight planets in the solar system, but two of those planets – Uranus and Neptune – are difficult or impossible to see with the naked eye. Humans thousands of years ago could see Mercury, Venus, Mars, Jupiter, and Saturn in the night sky, but it wasn’t until 1781 that Frederick Wilhelm Herschel spotted Uranus, using a telescope he’d built himself. It took another sixty-five years for Neptune to be discovered as well and then, in 1930, Clyde Tombaugh observed Pluto. Although it was thought to be a planet at the time, it was later reclassified as a dwarf planet and the first-ever “trans-Neptunian object” – which is exciting in and of itself.

What makes stars shine? Stars are made, in part, of the lightest element in the periodic table, hydrogen. Through the process of nuclear fusion, stars fuse together hydrogen atoms into the next-lightest element, helium, and then so on, eventually producing heavier elements. This process is why stars burn and produce so much energy and, when stars run out of hydrogen to fuse, they’ll die. The theory was first suggested by the physicist Arthur Eddington in 1920 and, later, additional research developed the theory, proving that he was correct. Stellar nucleosynthesis is why all life on Earth is made of stardust.

The Milky Way is huge. It’s estimated to be more than 100,000 lightyears from one side to the other, and is full of billions of stars and planets just like ours. But it wasn’t until the mid 1920s that we had confirmation of another. That “other”, of course, is the Andromeda Galaxy (or Messier 31). It was first described as a “nebula” by Abd al-Rahman al-Sufi, a Persian astronomer who lived during the Islamic Golden Age. Almost a thousand years later, the astronomer Edwin Hubble would determine that Andromeda is not a nebula at all, but rather an entire galaxy, making it the first galaxy other than our own to be conclusively identified. Since then, we’ve gone on to find evidence for roughly two billion galaxies in the observable universe and confirmed over four thousand exoplanets beyond our star system.

In 1916, Albert Einstein published the final form of his Theory of Relativity. Those many equations, which still form the basis of much of modern physics, predicted the existence of probably the most terrifying phenomenon in the universe: the black hole. Karl Schwarzschild found the solution to Einstein’s proposed theory that same year, but it would take another fifty years for the first black hole, which was thought by many to be impossible, to be observed. That black hole was Cygnus X-1. Many scientists contributed to its discovery, as well as developing this area of science – Stephen Hawking chief among them. Then, in 2019, the Event Horizon Telescope was able to capture the first image of a black hole.

Although dark matter is still technically hypothetical, there’s really no other explanation for the way galaxies exist and function. Galaxies, the way we observe them, don’t have enough mass to maintain their large shapes and sizes... Unless, of course, it turns out that there’s some other type of mass that we’re unable to detect. Enter dark matter. It’s thought to make up about 85% of the matter in the universe and is needed in order to make sense of the gravitational effects accepted today. Dark matter shouldn’t be confused with dark energy however. Dark energy is still quite the mystery, but it’s theorized to be a force that balances out gravity and drives the universe’s expansion.

One of the big reasons we know the universe is expanding at all is the existence of redshift. As large objects in outer space move further away, the wavelength of the light is stretched toward the red part of the electromagnetic spectrum. This means the faster an object moves away, the redder it looks, and we can use this information to estimate how quickly it’s traveling. There’s also the opposite phenomenon, blueshift, which happens when a celestial object is coming towards us. One such famous example is the Andromeda Galaxy, which will someday collide with the Milky Way.

In 1912, astronomer Vesto Slipher discovered that distant galaxies are redshifted. Hubble drew on this data in 1929 to demonstrate that the universe is expanding, and that the rate of expansion is speeding up! In essence, Hubble’s law states that galaxies are moving away from Earth at speeds proportionate to their distance: the further away they are, the faster they’re moving. It’s also known as the Hubble–Lemaître law, since Belgian ​​astronomer Georges Lemaître derived the idea from General Relativity two years earlier. This knowledge also gave way to the Hubble constant: the rate of cosmic expansion. Using the Hubble constant, we’ve been able to get extremely accurate measurements of the age of the universe, which is around 13.77 billion years old.

The Big Bang was an explosion of incomprehensible scale and force. We can actually still detect the radiation left behind by the Big Bang today, which is called “cosmic microwave background radiation” (or CMBR). It was first predicted by Ralph Alpher and Robert Herman in the late 40s, but was actually found by complete chance in the 1960s by Arno Penzias and Robert Woodrow Wilson, who were experimenting with satellite communication. Their instrument indicated the presence of CMBR, which physicists had been after for twenty years. CMBR quickly became a cornerstone of cosmology, winning the duo a Nobel Prize.


Before we unveil our top pick, here are a few Honorable Mentions:

Kepler’s Laws
In the 17th Century Johannes Kepler Wrote the Laws of Planetary Motion
Johannes Kepler https://youtu.be/ELysowMKYLA?t=20

Gravitational Waves
In 2016, We Finally Made Our First Observation of Gravitational Waves

Radio Astronomy
Pioneered by Karl Jansky, Radio Astronomy Is Our Best Tool for Studying the Cosmos
Karl Jansky https://forvo.com/word/karl_jansky/#en

Water on Mars
In the 2010s, NASA Announced Evidence of Liquid Water on Mars, Although Not All Researchers Agree

Organic Molecules on Comets
This 2015 Discovery Suggests Life on Earth Could Have Originated from Outer Space

We’ve actually known the Earth was a globe for thousands of years, but the fact our planet orbits the Sun rather than vice versa is another story. Greek astronomer Aristarchus of Samos proposed the idea back in the 3rd century BC. But it didn’t gain traction until the 16th century, when Renaissance mathematician Nicolaus Copernicus argued that the Sun, not the Earth, is the center of the solar system. It’s tough to come to terms with the fact you’re not the center of the universe, and the Catholic Church condemned the idea as heretical when Galileo’s observations supported it. Copernicus was, of course, correct. Today, this model is called the “heliocentric model”. Without it, we wouldn’t have been able to advance astrophysics at all.