Otherwise, they'd be ferociously unstable.

For the past few years, the possibility of a new (and big!) planet hanging around in the far outer solar system has tantalized scientists and the public alike. Is "Planet Nine" out there or not?

A physicist proposes that the moon is a great place to build a particle collider and conduct high-energy physics experiments.

Physicists are figuring out how close you can get to a black hole before you are unlikely to escape. That threshold is called the innermost stable circular orbit (ISCO).

String theorists are shifting focus to solve some rather sticky problems in physics.

After 60 years, this beautiful theory hasn't produced many tangible results.

Magnetars — highly magnetized, rapidly rotating super-dense stars — are among the most enigmatic creatures to inhabit the cosmos and their origins are shrouded in mystery.

Is space-time ultimately smooth at the tiniest of scales, or something else? It seems impossible to measure, but researchers are beginning to look down. Deep down.

The Big Bounce theory was once thought impossible. But two physicists have just resurrected it.

Although simulations suggest that black holes should grow quickly in the early universe, when astronomers look back in time they simply cannot find many such structures.

String theory hopes to be a literal theory of everything — a powerful idea that could completely revolutionize our understanding of the physical world. But it has never been directly tested.

The answer to the dark-matter mystery may be under our feet.

The sun is a roiling, boiling ball of plasma, constantly churning and turning itself inside out.

Double sunsets may be just as common in our galaxy as the solitary kind that we know on Earth, and this has big implications for our search for life outside the solar system.

The five main string-theory candidates may all just be pieces of a larger, cohesive whole — and M-theory could bring them together.

A new research paper has found a new potential type of abode for life: a rocky planet orbiting just past the event horizon of a rapidly spinning supermassive black hole.

As far as cosmologists can tell, the mysterious force behind the accelerated expansion of the universe, a force that we call dark energy, remains constant. But it may not have done so in the past.

All four outer planets in our solar system sport at least a few rings, but so far, we haven't observed any such features around exoplanets. That's confusing.

Earth is safe from such a catastrophe — almost.

String theory is a purported theory of everything that physicists hope will one day explain … everything.

A dark web ties the universe together. Now, we can see it.

But also super-mysterious.

Life for the 19th-century astronomer was kind of monotonous, 200 years after Galileo's revolutionary work with the telescope.

As far as we can tell, we're alone. But it could be that we're looking in the wrong places.