Although we live in relative quiet within the cosmos, going about our lives and seeing the stars as a distant backdrop, we are very much part of the universe that surrounds us. Dangers lurk in space, as any glance at the Moon’s cratered surface confirms.
Not only must our planet avoid collisions with Earth-crossing asteroids, but more remote threats exist. If a nearby star went supernova, a gamma-ray burst erupted nearby, or a black hole or stream of antimatter somehow wandered into our neighborhood, it could spell disaster. While astronomers say those events are unlikely, another dark, distant interloper could create havoc on Earth by its mere presence.
Where could such trouble come from? The Sun hasn’t always been a solitary star. It was born in a group of suns, as all stars are, and its native companions have been scattered by the gravitational tug created by orbiting the galaxy’s center.
Yet some 5 billion years after the Sun’s birth, a few of its associates still linger near the old neighborhood. Among them are the Sun-like star Alpha (α) Centauri, the yellowish dwarf Tau (τ) Ceti, and the cool red dwarf Wolf 359. Is the Sun truly single, or could a cool, dark companion loom in the background, periodically nudging comets toward Earth?
The discovery of Sedna, a trans-Neptunian object found in 2003, and the subsequent discovery of Eris, fueled the idea that large, dark bodies float in the solar system’s distant reaches. Those bodies exist apart from the numerous comets that populate the Oort Cloud. Close passages of well-known stars will occur far on down the line: For example, in less than a million and a half years, Gliese 710, a red dwarf now 60 light-years away, will slide within a light-year of the Sun. This will unleash a torrent of comets from the Oort Cloud into orbits that could intersect Earth’s, and they will arrive near our planet within a liberal span of about 2 million years.
But bombings from comet nuclei could result from other sources, too. A number of astronomers suggest the Sun may have a hidden, dark companion that periodically sends comets sunward, raining them down on the inner solar system.