This Hubble Space Telescope image shows thousands of galaxies whose light took billions of years to reach Earth.NASA, ESA, and J. Lotz (STScI)

The earliest estimates of the number of galaxies in the Universe were very small. For centuries, astronomers thought there might be just one—our own. Most recent estimates built off observations from 1995, when NASA’s Hubble Space Telescope stared at a dark patch of sky for hours and returned a picture of thousands of glittering galaxies no one had ever seen before. Further measurements led astronomers to believe there are between 100 billion and 200 billion galaxies in the observable Universe that human-made technology can detect.  

And that was the working estimate for the next two decades, until this week. Astronomers at the University of Nottingham now say the number of galaxies in the observable Universe is 2 trillion, more than 10 times as many as previously thought.

To reach this figure, researchers studied decades of images of galaxies—clusters of millions or billions stars, gas, and dust—taken by Hubble and other powerful telescopes. Their research, announced Thursday, will be published in the Astrophysical Journal.

There’s only one way to count galaxies with existing technology: Point a telescope at a small chunk of sky, tally up the number you see, and then extrapolate that across the whole sky. But when the Nottingham researchers examined the masses of the galaxies in those patches of sky, they realized there must be missing galaxies that are “too faint and too far away” to be imaged by modern technology, even the most powerful telescopes in the world.

The light from distant galaxies takes billions of years to reach us; the most distant galaxy Hubble has ever imaged left 13.4 billion years ago, about 400 million years after the Big Bang.

“It boggles the mind that over 90 percent of the galaxies in the [observable] Universe have yet to be studied,” said Christopher Conselice, the astrophysics professor at Nottingham who led the research. “Who knows what interesting properties we will find when we discover these galaxies with future generations of telescopes?”

Mind-boggling, indeed. Two trillion is, to put it in scientific terms, a lot.

And it won’t take another 20 years for astronomers to come up with a new estimate. The next decade will see a host of new observatories come online. The Hubble’s successor, the James Webb Space Telescope, will be launched into orbit in 2018. One hundred times more powerful than Hubble, the Webb will be able to peer further in the history of the Universe, to see the earliest stars and galaxies. In 2020, the European Space Agency will launch the Euclid probe into space, where it will map the shapes, positions, and movements of 2 billion galaxies to study dark matter and energy. In 2022, the Large Synoptic Survey Telescope, a ground-based telescope currently under construction in northern Chile, will spend 10 years photographing the sky every night, detecting billions of stars and galaxies. In 2024, the European Extremely Large Telescope, another ground-based telescope in Chile, will begin surveying the sky, looking for pretty much everything.

As humans see further, and with greater clarity, billions more sparkling galaxies may yet come into view. For now, the best prediction is probably like the one Ed Churchwell, an astronomy professor at the University of Wisconsin-Madison, gave in an interview with Universe Today last year. “We don’t know,” he said. “We know it’s a very large number.”

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