by Paul M. Sutter
May 13, 2025
from PopularMechanics Website








A revolutionary new telescope

is poised to unveil the truth...

Earth is the only planet in the Solar System with liquid water on its surface.

But we weren't born with all these oceans - it had to come from somewhere else. And new research suggests that mysterious "dark comets" may be the source.

They aren't like the typical comets most of us know, trailing an icy coma, but are much sneakier, slipping through interplanetary space with nothing but a whisper.

Besides the eight planets, the Solar System is full of small bodies.

Among these are the comets and asteroids, which are usually easy enough to tell apart.

Comets tend to live in the outer reaches of the system, where the temperatures are cold enough for water, carbon dioxide, nitrogen, and other elements to freeze.

 

Asteroids usually live much closer to the Sun, where it's so warm that all of these ices sublimate into gases. This makes asteroids far rockier than their cometary cousins.

But the telltale signature of a comet is its coma...

Occasionally, a comet will tumble its way into the inner Solar System. As it approaches the Sun, the ices sublimate, carrying with them a train of dust. We can spot this trail, which can stretch for millions of miles, as the comet's coma.

So in short,

comets have tails, and asteroids don't...

Note that these cometary tails introduce a curious bit of motion in a comet's path.

While the movement of a comet is dominated by the gravity of the Sun, the release of the dust and gas acts as a little exhaust, giving the comet a source of acceleration not due to gravity alone.

That was all well and good, until 2016, when astronomer Davide Farnocchia found an asteroid, named 2003 RM, that was not moving under pure gravity.

Despite looking like any other normal asteroid, it moved like a comet - except that it didn't have a visible coma...

After groups around the world discovered a dozen more of these enigmatic creatures, another astronomer, Darryl Seligman, gave them a fittingly enigmatic name:

dark comets...

It's becoming clear that comets and asteroids aren't binary choices, but rather two ends of a wide spectrum.

There are plenty of asteroids that occasionally sport comas, and now we're seeing comets that don't. However, there is no universally accepted explanation for what causes dark comets.

They might be comets that passed by the Sun multiple times without breaking apart, leaving them depleted of dust... but still enough ice to give them an extra push.

This means that they are still releasing some small amount of gas, but not enough for us to detect it from Earth.

There seem to be two populations:

• large "outies," up to a kilometer across, that live in the outer Solar System

• "innies," which are only a few tens of meters across, that inhabit the inner Solar System

However, we don't know how many of them there are.

The question of their population numbers weighs heavily on another outstanding question in astronomy:

how Earth got its water...

Certainly some water was carried along with all the other dust and debris that assembled to form our planet 4.5 billion years ago, but much of that was lost as evaporation due to the heat of our formation and the intense glare of the Sun.

After the Earth began solidifying, some trapped water managed to squeeze out of the surface, forming shallow pools... but not the gigantic oceans that cover our planet where Earth life presumably began.

That means something - or a lot of things - had to deliver water to our planet after it formed.

A giant collision with another planet isn't going to work; the intense heat of that kind of impact would vaporize any water that could have been along for a ride.

That leaves lots of smaller impacts, and the only objects left are the asteroids and comets.

But asteroids, by definition, don't have a lot of water:

they're too close to the Sun to keep it trapped in ice.

And comets, typically around 50-80 percent pure water ice, spend almost all their lives in the outer Solar System.

It's not a great place to be if you're trying to carry water to an inner world like the Earth.

So perhaps dark comets are the key.

They live in the inner Solar System, but have more water than the average asteroid.

 

And while they may not be numerous today, perhaps they were far more common in the early days of the Solar System.

It's an intriguing hypothesis, but right now is merely a guess.

There are barely more than a dozen known dark comets, and we barely know anything about them except for the fact that they're difficult to classify and contain some fraction of molecules like water.