We Are Not Alone

This illustration shows a stage in the predicted merger between our Milky Way galaxy
and the neighboring Andromeda galaxy, as it will unfold over the next several billion years.
In this image, representing Earth's night sky in 3.75 billion years, Andromeda (left) fills the field of view
 and begins to distort the Milky Way with tidal pull.
(Credit: NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas; and A. Mellinger)

Astronomer Edwin Hubble announced the discovery of other galaxies beyond the Milky Way in December 1924. Before he made his discovery, everyone thought that our Milky Way galaxy was the only galaxy in the universe. The thought was that outside the Milky Way was only the Magellanic Clouds. Those are visible by the naked eye if you're in a dark place in the Southern Hemisphere. It was thought that they were clouds of gas or dust. 

After he served in World War II, Hubble took a job at the Mount Wilson observatory in California where they had the new 100-inch Hooker telescope. Hubble had written his doctoral dissertation on “Photographic Investigations of Faint Nebulae" but with older or smaller telescopes, nebulae just looked like clouds of glowing gas. Using the Hooker telescope, Hubble was able to see that there were actually stars within the nebula.

Now we know that the Magellanic Clouds are dwarf galaxies. He renamed the Andromeda Nebula the “Andromeda galaxy,” and he went on to discover 23 more separate galaxies. 

Within a few years of Hubble’s discovery, most astronomers came to agree that our galaxy is just one of millions. Methods to measure astronomical distance have gotten more precise, and it’s now estimated that the Andromeda galaxy is 2 million light-years from Earth.

Hubble Space Telescope

Hubble died in 1953, but we know his name today because we hear about the Hubble telescope named for him as it moves through space. The Hubble Deep Field, an extremely long exposure of a relatively empty part of the sky, provided evidence that there are about 125 billion galaxies in the observable universe.

The numbers are staggeringly hard to grasp. So far, astronomers have found more than 500 solar systems and are discovering new ones every year. Given how many they have found in our own neighborhood of the Milky Way galaxy, scientists estimate that there may be tens of billions of solar systems in our galaxy, perhaps even as many as 100 billion.

But how many of those many galaxies might be Earth-like and support life? Astronomers reported in 2013 that based on Kepler space mission data, there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs within the Milky Way.

Are we alone? Doubtful.