A massive galaxy surrounding our own appears seemingly out of nowhere. And now, scientists have discovered a completely new moon lurking on the boundaries of our own Solar System.

The moon, which is roughly 161 km (100 miles) in diameter, has been detected around the second brightest frozen minor planet, Makemake, way out past Pluto in the Kuiper belt. So, what exactly is going on? And why have we waited so long to notice these significant objects in our own cosmic backyard?

The newly discovered moon, which has been temporarily called ‘S/2015 (136472) 1’ or the more friendlier ‘MK 2’ for short, was able to remain hidden for so long because it’s so small.

The moon, like the newly discovered galaxy earlier this month, reflects such a tiny quantity of light that we’ve struggled to see it in the glare of Makemake.

It’s more than 1,300 times fainter than its host planet, and it’s so dim that Makemake was previously thought to be the only officially recognized distant dwarf planet without a satellite… a title it’s now lost.

The moon was not seen until astronomers decided to point the Hubble Space Telescope at Makemake for more than two hours in April of last year. While reviewing the data, astronomer Alex Parker of the Southwest Research Institute in Texas saw a faint point of light moving around Makemake at a range of around 20,900 km (13,000 miles) away.

“I was sure someone had seen it already,” Parker told National Geographic. He asked fellow researcher Marc Buie about it, who responded with: “There’s a moon in the Makemake data?”

“Everything got exciting and kicked into high gear at that point,” Parker said.

The researchers now want to use Hubble to further study and figure out the orbit of MK2 in the hopes of finding out more about the composition and density of the icy dwarf planet.

“Makemake is in the class of rare Pluto-like objects, so finding a companion is important,” said Parker. “The discovery of this moon has given us an opportunity to study Makemake in far greater detail than we ever would have been able to without the companion.”

The Kuiper belt is a massive reservoir of frozen material left over from our Solar System’s formation roughly 4.5 billion years ago, including many dwarf planets. But scientists still don’t understand much about these icy worlds.

One of Makemake’s mysteries is why it looked to have different patches of dark and brilliant, reflective material across it. But the planet spins every 7.7 hours, so if that was the case, the planet’s brightness should change – which it doesn’t.

However, as Parker stated on Twitter, this could be due to the previous infrared data picking up MK2, not only Makemake:

The new discovery also increases the similarities between Pluto and Makemake – Pluto’s mass also wasn’t known until the discovery of its moon Charon in 1978. “That’s the kind of transformative measurement that having a satellite can enable,” said Parker.

In addition to Hubble observations, he is now hoping that the NASA probe New Horizons will pass by Makemake on its way out of the Solar System.

It’s amazing to know that there’s still a lot of space out there that we haven’t discovered yet. Who knows, this might also be the year we finally discover Planet Nine. Eyes to the skies, people.

The research has been submitted for peer review, and is published on the pre-print site arXiv.org so others in the industry can add their thoughts.