Remember the stunning close-up photo of Pluto that lit up the internet in 2015? The spacecraft taking that picture, NASA’s New Horizons probe, just made another groundbreaking discovery, this time about the deeper truth of our universe: that there are likely much fewer galaxies in space than we initially thought, meaning that our chance to Earth-like planets and extraterrestrial life, too, has dimmed.
Scientists have long believed that there are as many as two trillion galaxies in the observable universe, based on data from the Hubble Space Telescope. But new data returned by the New Horizons probe, which was four billion miles away from Earth when observations were taken, suggest that the upper limit of the number of galaxies falls in the range of hundreds of billions, according to a study presented to the American Astronomical Society this week.
“It’s an important number to know—how many galaxies are there? We simply don’t see the light from two trillion galaxies,” the study’s lead author, Marc Postman of the Space Telescope Science Institute in Baltimore, Maryland, said in a statement.
In order to estimate the number of galaxies in the universe, scientists first count everything visible in a Hubble deep field and multiply them by the total area of the sky. Then, to account for distant galaxies that are too faint to be seen by a telescope, astronomers study the weak background glow they cast in space. To measure that glow, spacecraft need to be sent beyond the inner solar system to avoid light pollution caused by sunlight reflecting off dust.
The New Horizons is the first space probe that was able to measure the brightness of this cosmic optical background. From the outskirts of the solar system, the probe could observe an ambient sky ten times darker than the darkest sky accessible to Hubble.
“These kinds of measurements are exceedingly difficult. A lot of people have tried to do this for a long time. New Horizons provided us with a vantage point to measure the cosmic optical background better than anyone has been able to do it,” said Tod Lauer of NSF’s NOIRLab, another lead author on the study. “Take all the galaxies Hubble can see, double that number, and that’s what we see—but nothing more.”
NASA launched the New Horizons mission in 2006 with two main goals: to study Pluto and the Kuiper Belt. The probe successfully flew by Pluto in 2015 and captured a high-resolution photo of the dwarf planet for the first time. In January 2019, it discovered a Kuiper Belt object called Arrokoth.
The New Horizons spacecraft is now 4.4 billion miles from Earth.