Our solar system has eight major planets, and nearly 200 moons. Though astronomers have to date found nearly 4,000 planets orbiting other stars, no moons have yet been found. That hasn't been for any lack of looking, it’s just that moons are smaller than planets and therefore harder to detect.

The Hubble and Kepler space telescopes found evidence for what could be a giant moon accompanying a gas-giant planet that orbits the star Kepler-1625, located 8,000 light-years away in the constellation Cygnus. The moon may be as big as Neptune and it orbits a planet several times more massive than... Read more

Imagine crushing more than 50,000 aircraft carriers into the size of a baseball. This describes neutron stars. They are among the strangest objects in the universe. Neutron stars are a case of extreme physics produced by the unforgiving force of gravity. The entire core of an exploded star has been squeezed into a solid ball of neutrons with the density of an atom’s nucleus. Neutron stars spin as fast as a blender on puree. Some spit out death-star beams of intense radiation — like interstellar lighthouses. These are called pulsars.

These beams are normally seen in X-rays, gamma-rays, and radio... Read more

The universe is so huge that it's estimated that a star explodes as a supernova once every second. Astronomers capture a small fraction of these detonations because they are comparatively short-lived, like fireflies flickering on a summer evening. After skyrocketing to a sudden peak in brightness, a supernova can take weeks to slowly fade away.

For the past decade astronomers have been befuddled by a more curious "flash-in-the-pan" that pops up and then disappears in just a few days, not weeks. It's called a Fast-Evolving Luminous Transient (FELT). Only a few FELTs have been seen in telescopic sky... Read more

When Galileo discovered Jupiter's moon Europa in 1610, along with three other satellites whirling around the giant planet, he could have barely imagined it was such a world of wonder.

This revelation didn't happen until 1979, when NASA's Voyager 1 and 2 flew by Jupiter and found evidence that Europa's interior, encapsulated under a crust of ice, has been kept warm over billions of years. The warmer temperature is due to gravitational tidal forces that flex the moon's interior — like squeezing a rubber ball — keeping it warm. At the time, one mission scientist even speculated that the Voyagers... Read more

In the 1400s, two power struggles were taking place quadrillions of miles apart. In England, two rival branches of the royal House of Plantagenet were battling each other for control of the country's throne. And, in a nebula far, far away, a cluster of stars was waging a real-life star wars, with the stellar members battling each other for supremacy in the Orion Nebula. The gravitational tussle ended with the system breaking apart and at least three stars being ejected in different directions.

Astronomers spotted two of the speedy, wayward stars over the past few decades. They traced both stars... Read more

Eerie mysteries in the universe can be betrayed by simple shadows. The wonder of a solar eclipse is produced by the moon's shadow, and over 1,000 planets around other stars have been cataloged by the shadow they cast when passing in front of their parent star. Astronomers were surprised to see a huge shadow sweeping across a disk of dust and gas encircling a nearby, young star. They have a bird's-eye view of the disk, because it is tilted face-on to Earth, and the shadow sweeps around the disk like the hands moving around a clock. But, unlike the hands of a clock, the shadow takes 16 years to make... Read more

Interstellar forecast for a nearby star: Raining comets! The comets are plunging into the star HD 172555, which resides 95 light-years from Earth. The comets were not seen directly around the star. Astronomers inferred their presence when they used NASA's Hubble Space Telescope to detect gas that is likely the vaporized remnants of their icy nuclei.

The presence of these doomed comets provides circumstantial evidence for "gravitational stirring" by an unseen Jupiter-size planet, where comets deflected by the massive object's gravity are catapulted into the star. These events also provide new... Read more

In 1977, NASA's Voyager 1 and 2 spacecraft began their pioneering journey across the solar system to visit the giant outer planets. Now, the Voyagers are hurtling through unexplored territory on their road trip beyond our solar system. Along the way, they are measuring the interstellar medium, the mysterious environment between stars that is filled with the debris from long-dead stars. NASA's Hubble Space Telescope is providing the road map, by measuring the material along the probes' trajectories as they move through space. Hubble finds a rich, complex interstellar ecology, containing multiple... Read more

Great balls of fire! The Hubble Space Telescope has detected superhot blobs of gas, each twice as massive as the planet Mars, being ejected near a dying star. The plasma balls are zooming so fast through space that they would travel from Earth to the moon in 30 minutes. This stellar "cannon fire" has continued once every 8.5 years for at least the past 400 years, astronomers estimate. The fireballs present a puzzle to astronomers because the ejected material could not have been shot out by the host star, called V Hydrae. The star is a bloated red giant, residing 1,200 light-years away, which has... Read more

New findings from NASA's Hubble Space Telescope show suspected water plumes erupting from Jupiter's icy moon Europa. These observations bolster earlier Hubble work suggesting that Europa is venting water vapor. A team of astronomers, led by William Sparks of the Space Telescope Science Institute in Baltimore, Maryland, observed these finger-like projections while viewing Europa's limb as the moon passed in front of Jupiter. The team was inspired to use this observing method by studies of atmospheres of planets orbiting other stars.

The plumes are estimated to rise about 125 miles before,... Read more