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In this illustration, several dust rings circle the sun. These rings form when planets’ gravities tug dust grains into orbit around the sun. 
Mary Pat Hrybyk-Keith/NASA’s Goddard Space Flight Center

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7 solar system mysteries scientists haven’t solved yet

Why is our moon so weird? Was there ever life on Mars? Big cosmic questions lurk in our celestial backyard.

Brian Resnick is Vox’s science and health editor, and is the co-creator of Unexplainable, Vox's podcast about unanswered questions in science. Previously, Brian was a reporter at Vox and at National Journal.

The next time you look up at a bright full moon, think about this: No one knows, precisely, where the moon came from.

“We have no idea why the moon is here,” science writer Rebecca Boyle says on Unexplainable — Vox’s podcast that explores big mysteries, unanswered questions, and all the things we learn by diving into the unknown. “I think for a lot of people [the moon] is taken for granted, it’s this sort of humdrum thing, and galaxies and nebula and stars and planets are more intriguing.”

It’s true that some of the most epic questions in science are found in the farthest reaches of space — how and when did the first galaxies form, what happens inside a black hole — but equally epic questions exist right here in our celestial neighborhood, in our own solar system.

To explore our own solar system — the moons and planets in it — is to better understand what’s possible in the farthest reaches of the universe. Anything we find or discover in our own cosmic backyard will help us understand what’s possible in the broader universe. If evidence of ancient life is found on a hostile world like Mars, we might better understand how common life might be in other solar systems. If we understand how a possibly once-vibrant world like Venus fell into ruin, we might understand how often similar planets around other stars die in an apocalypse.

The most provocative solar system mysteries help us understand why we are here, how long we might have left, and what we might leave behind. Here are some of the solar system mysteries we’ve encountered on Unexplainable.

For more mysteries, listen to and follow Unexplainable wherever you listen to podcasts.

What killed Venus?

The clouds of Venus captured in 1974 by NASA’s Mariner 10 spacecraft.

“Hellscape” is the most appropriate word to describe the surface of Venus, the second planet from the sun. At 900 degrees Fahrenheit, it’s the hottest planet in the solar system, thanks to an atmosphere that’s almost entirely made up of carbon dioxide, which generates a really strong greenhouse effect. Clouds made of highly corrosive sulfuric acid are draped over a volcanic landscape of razor-sharp volcanic rock. The pressure on the surface of Venus is about 92 times what you’d feel at sea level on Earth.

Yet some scientists suspect Venus was once much like Earth, with a liquid water ocean like the ones that support life on our planet. This prompts an existential question for life on Earth.

“Venus and Earth are planetary siblings,” says Robin George Andrews, volcanologist and author of Super Volcanoes: What They Reveal about Earth and the Worlds Beyond. “They were made at the same time and made of the same stuff, yet Venus is apocalyptic and awful in every possible way. Earth is a paradise. So why do we have a paradise next to a paradise lost?”

There are two leading hypotheses. One is that the sun cooked Venus to death. The other is that volcanoes did.

Further reading: Venus could have been a paradise but turned into a hellscape. Earthlings, pay attention.

Where the heck did the moon come from?

This view from the Apollo 11 spacecraft shows the Earth rising above the moon’s horizon.
HUM Images/Universal Images Group

Before the moon landings, scientists thought they knew how the moon formed. The prevailing theory was that it formed a lot like the planets did: bits of material left over from the formation of the sun, lumping together. But then, Apollo astronauts brought samples back from the lunar surface, and those rocks told a totally different story.

“Geologists had found that the moon was covered in a special kind of rock called anorthosite,” Unexplainable senior producer Meradith Hoddinott explains on the show. “Glittery, bright, and reflective, this is the rock that makes the moon shine white in the night sky. And at the time, it was thought, this rock can only be formed in a very specific way. Magma.”

But magma means the moon must have formed in some sort of epic cataclysm. “Something that poured so much energy into the moon that it literally melted,” Hoddinott says. Scientists aren’t precisely sure how it all played out. But each scenario is a cinematic story of fiery apocalyptic proportions.

Further reading: How Apollo moon rocks reveal the epic history of the cosmos

Is there anything alive in the human poop left on the moon?

A bag of astronaut detritus left on the moon in 1969.

During the Apollo moon missions, astronauts went to the moon and, to save weight for returning to Earth, they dumped their waste behind. Across all the Apollo missions, astronauts left 96 bags of human waste on the moon, and they pose a fascinating astrobiological question.

Human waste — and in particular, feces — is teeming with microbial life. With the Apollo moon landings, we took microbial life on Earth to the most extreme environment it has ever been in. Which means the waste on the moon represents a natural, though unintended, experiment.

The question the experiment could answer: How resilient is life in the face of the brutal environment of the moon? And for that matter, if microbes can survive on the moon, can they survive interplanetary or interstellar travel? If they can survive, then maybe it’s possible that life can spread from planet to planet, riding on the backs of asteroids or other such space debris.

Further reading: Apollo astronauts left their poop on the moon. We gotta go back for that shit.

Was there an advanced civilization on Earth before humans?

Illustration of the supercontinent Gondwana, a landmass that was fully formed by around 550 million years ago and began to break up about 180 million years ago.
Science Photo Libra/Getty Images

Many scientists have long wondered: Is there intelligent life out in the deep reaches of space? But climate scientist Gavin Schmidt and astrophysicist Adam Frank have a different question: Was there intelligent life in the deep reaches of Earth’s history? Could we find evidence of an advanced non-human civilization that lived perhaps hundreds of millions of years ago, buried in Earth’s crust?

This is not strictly a “solar system” mystery, but it is cosmic in scope. At the heart of it, Schmidt and Frank are asking: How likely is an intelligent life form on any planet — here or in the deepest reaches of space — to leave a mark, a sign that they existed? And for that matter: Hundreds of millions of years from now, will some alien explorers landing on Earth be able to find traces of humans if we’re long, long gone?

Further reading: The Silurian hypothesis: would it be possible to detect an industrial civilization in the geological record?

Can we nudge an asteroid out of a collision course with Earth?

What if?
Tobias Roetsch/Future Publishing/Getty Images

Many disasters — volcanic eruptions, earthquakes, hurricanes, tornadoes — are unavoidable. Scientists talk about when, not whether, they’ll strike. Though humans make some calamities worse, natural disasters have been happening since long before we were here. They’re a fact of life on Earth. But one kind of disaster need not be inevitable: a collision between an asteroid or comet and the Earth.

The problem is: We’ve never tried to deflect an asteroid, and don’t know if a plan to do so would work.

To help answer this question, last year, NASA launched the Double Asteroid Redirection Test (DART), which is a car-size box outfitted with solar panels. It’s currently on its way to a 160-meter asteroid called Dimorphos. In the fall, DART will crash into Dimorphos at 24,000 kilometers an hour (about 15,000 miles per hour) in pursuit of a big question: Can the collision nudge the asteroid into a slightly different orbit?

Further reading: The quest to avert an asteroid apocalypse is going surprisingly well

Was there ever life on Mars?

The Perseverance Rover takes a selfie on Mars.

Mars today is a desert, devoid of any obvious signs of life. But over the years, scientists have uncovered evidence of a lost Mars, long ago, that might have looked a lot more like Earth.

“Mars is a very different place today than it was 4 billion years ago, but you can see evidence of what it was like,” says NASA astrobiologist Lindsay Hays. “You see things like the remnants of a huge river delta, which indicates not only did you have water flowing, but you probably had lots of water flowing over a long period of time that continued to deposit sediments.”

And where there was water, there could have been life. Last year, a new rover landed on Mars, and it is our best shot at answering the question of “was there ever life on Mars?” If the answer is “yes,” it could change our understanding of how common life is in the universe.

Further reading: Why scientists really, really want to find life on Mars

Is there a true ninth planet lurking in the darkness?

pluto july 13
Sorry, Pluto, there may be a new ninth planet.

In 2006, the International Astronomical Union voted to change the definition of what constitutes a planet, and Pluto didn’t make the cut. No longer were there nine official planets in the solar system, but eight.

But then “we started getting these hints that there really is something else out there — and a real giant planet that we think is still now lurking well beyond Neptune, waiting to be found,” astronomer Mike Brown says on Unexplainable. Astronomers have yet to detect this planet, but they suspect it is there: Other objects far out in the solar system seem to be impacted by its gravity.

Could these hints lead us to a true, new ninth planet? Maybe. But it will be hard to find.

”It‘s kind of like taking a little black grain of sand and throwing it on the beach,” Brown says of the search. “That’d be a little hard to find that one in the sea of all the rest of them. And that’s the problem with Planet Nine.”

Further reading: The hunt for planet 9

If you have ideas for topics for future shows, send us an email at


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