Our galaxy is lonelier than we thought, New Horizons data shows

When you look up into the sky, all the stars you see are part of our galaxy, the Milky Way. And we know our galaxy is just one among many in the universe.

It was previously thought that there were trillions of galaxies out there in the vastness of space, but new research using data from NASA’s New Horizons mission questions that figure. Now, it seems that galaxies may number in the hundreds of billions, rather than the trillions.

“It’s an important number to know — how many galaxies are there?” Marc Postman of the Space Telescope Science Institute in Baltimore, Maryland, a lead author on the study, said in a statement. “We simply don’t see the light from two trillion galaxies.”

To figure out how many galaxies are out there, it’s not possible to count them because there are so many and they are so far away from us. So researchers instead estimate the total number of galaxies by looking at how much faint background light suffuses the darkness of space. That glow was measured using data from the New Horizons mission to Pluto, which was able to put an estimated cap on the number of faint galaxies that exist beyond the bounds of our ability to see using tools like the Hubble Space Telescope.

“Take all the galaxies Hubble can see, double that number, and that’s what we see — but nothing more,” said Tod Lauer of NSF’s NOIRLab, a lead author on the study.

This photo shows a phenomenon known as zodiacal light. At lower left, a glowing patch extends to the upper right in the direction of Jupiter, the bright object left of center. Zodiacal light is caused by sunlight reflecting off tiny dust particles in the inner solar system—the disintegrated remains of comets and asteroids. Attempts to measure how dark space is using telescopes like Hubble have been thwarted by this ambient glow. Z. Levay / STScI

The reason that measuring the background glow of distant galaxies is hard is that it is hampered by a phenomenon called zodiacal light, where dust particles in our solar system reflect sunlight and create a glow in the sky.

The inner solar system where our planet is located is full of light from these particles which limit Hubble’s ability to see these faint galaxies. As New Horizons traveled to the outer solar system, it was able to look further out and investigate the background glow more clearly.

“These kinds of measurements are exceedingly difficult. A lot of people have tried to do this for a long time,” said Lauer. “New Horizons provided us with a vantage point to measure the cosmic optical background better than anyone has been able to do it.”