We know so very, very little about the universe, as a paper published in the Astrophysical Journal in October reminds us. We thought the observable universe contained billions of galaxies. But it seems there are at least 2 trillion galaxies out there — 10 times greater than astronomers’ previous estimates.
Examining decades of data from the Hubble Space Telescopes and other observatories around the world, an international team of scientists now believe that 90 percent of the galaxies in the universe are “too faint and too far away to be seen.”
But they are out there.
Let’s pause for a moment to let this all sink in.
A galaxy can house hundreds of billions of stars, and each star system can contain dozens of planets. Astronomers have estimated there may be a billion Earth-like planets rocky planets orbiting in a star’s “habitable zone” in our galaxy alone. And now, we learn, there are around 2 trillion (12 zeros!) galaxies. Whoa. As Forbes points out, that’s 200 galaxies for each human on Earth. (And to be clear, the discovery doesn’t mean there are more stars or matter in the universe than previously thought. As Phil Plait explains at Slate “So instead of being in a smaller number of big galaxies, stars are divvied up into a bigger number of smaller ones.”)
"It boggles the mind that over 90 percent of the galaxies in the universe have yet to be studied,” Christopher Conselice, who led the study at the University of Nottingham, said in a press release. “Who knows what interesting properties we will find when we discover these galaxies with future generations of telescopes?”
How do astronomers count the number of galaxies?
Previous estimates of the number of galaxies in the universe come from a series of Hubble photos. The first one was “Hubble Deep Field,” which was captured in 1995.
Vox’s Joss Fong and Dion Lee tell the story of Deep Field in the following video, which you should watch. But in summary: Hubble scientists pointed the space telescope at a tiny, unassuming spot in the night sky where there were no known celestial objects. They saw thousands of previously unknown galaxies.
That so many galaxies could be found in such a tiny stretch of the sky allowed astronomers to begin to estimate the population of galaxies in the universe. The Deep Field observation (and its follow-up, the Ultra Deep Field, in 2004) led them to put the number of galaxies at around 100 billion.
This is the original Deep Field image, taken of a portion of the night sky “a little larger than a pinhead at arm's length,” as Fong explains in the video.
And this is the Ultra Deep Field image. It looks even farther back in time and space. There are 10,000 galaxies in this image.
The latest Astrophysical Journal paper builds on the estimate from the Hubble Deep and Ultra Deep Fields with new data.
It complies decades of Hubble observations with observations from other telescopes around the world. The team then created a three-dimensional map of galaxies from the data (in space, the further away a galaxy is, the older it is). And then they used that map, and a mathematical model, to answer this question: How many very faint, very old galaxies did we miss in our previous assessments?
That’s how they came up with the conclusion that 90 percent of the galaxies in the universe are out of our sights.
Hubble’s days are numbered. But it is still awesome.
The Hubble Telescope is 26 years old and nearing the end of its run, so it’s remarkable that it keeps churning out new discoveries. Just in the past few months, data from Hubble has been used to determine that there are plumes of water vapor erupting from Europa, Jupiter’s icy moon, and that the universe may be expanding faster than we previously thought. The Hubble has broken its own record of observing the farthest galaxy ever seen, it still discovers massive black holes, and it’s taken pretty pictures of Mars and Jupiter.
The James Webb Space Telescope is set to launch in 2018, and astronomers are excited. With its larger mirror and sensor, it will be able to see even deeper into the universe than the Hubble can.
Who knows what it will find. But here’s a good bet: We’re going to learn much, much more about the universe. And perhaps we’ll be adding a lot more zeros to our estimates of the galaxies out there.