SETI researchers have directed their search toward interesting star systems where extraterrestrial civilizations could send signals toward Earth. According to a new paper, sufficiently advanced civilizations will communicate across the galaxy, sending signals between stars, rogue planets, and interstellar objects. They'll probably be using pulsars as a navigation system to coordinate their activities at a vast scale. To confirm their existence, we just have to listen in.

Astronomers continue to find more and more evidence of black holes, but there's an alternative theory that could explain the observations: gravastars. First proposed in 2001, these objects are one possible solution to Einstein's field equations, having the same impact on the Universe. But they won't have an event horizon or singularity. Instead, they'd contain thin shells of matter surrounding an exotic dark energy that pushes back against the star's gravitational force.

One of the largest reentries in recent years, ESA's ERS-2 satellite is coming down this week.