Nemesis

Nemesis is a star that may never have existed: a dim companion of the Sun, proposed in 1984 to explain why mass extinctions on Earth seemed to come in a regular rhythm. Its orbit would carry it about 1.5 light-years out, far beyond the planets, and every 26 million years it would pass through the Oort cloud of comets, knocking some of them inward. A few of those comets would hit Earth.

The extinction clock

In 1984 the palaeontologists Raup and Sepkoski looked at the fossil record of marine life and found 12 extinction events over the past 250 million years, spaced on average 26 million years apart. That regularity needed a cause. Soon after, two teams proposed one in the same issue of Nature. Davis, Hut and Muller, and separately Whitmire and Jackson, suggested that the Sun has an unseen companion on a wide orbit, and that its passages through the Oort cloud set off comet showers.

The search

Muller thought Nemesis was most likely a red dwarf, a small, faint star of magnitude 7 to 12 as seen from Earth: too faint for the eye, but bright enough to sit unnoticed among thousands of catalogued stars. His group set out to examine 3 000 candidate stars. Whitmire and Jackson favoured a brown dwarf, a failed star even dimmer than a red dwarf.

The infrared surveys that followed found nothing. The most thorough was NASA’s WISE, which mapped the whole sky in infrared light. In 2014 Luhman reported that it ruled out a Jupiter-mass brown dwarf out to 26 000 AU, and a planet like Jupiter out to 82 000 AU. “The outer solar system probably does not contain a large gas giant planet, or a small, companion star,” he said. A red dwarf Nemesis is now ruled out by full-sky surveys.

The clock itself

The extinction rhythm that Nemesis was invented for is now disputed in its own right. In 2010 Melott and Bambach found a 27-million-year cycle in the fossil record, but so regular over half a billion years that a star like Nemesis, whose orbit would be disturbed by passing stars, could not be its cause. In 2011 Bailer-Jones found no periodic pattern in the ages of impact craters at all. In 2020 Rampino and colleagues found a 27.5-million-year cycle in the extinctions of land animals. The debate goes on without Nemesis.

Not Planet Nine

Nemesis is often confused with Planet Nine, but they are different ideas. Planet Nine is a proposed planet of about 5 to 10 Earth masses, 400 to 800 AU from the Sun, invoked to explain the clustered orbits of distant icy bodies. Nemesis would be a star more than a hundred times farther out.

There may have been a real Nemesis once. In 2017 Sadavoy and Stahler concluded from young stars in the Perseus cloud that most stars are born in wide pairs that break apart within a million years. “Yes, there probably was a Nemesis, a long time ago,” Stahler said. If so, the Sun’s twin drifted away into the galaxy billions of years ago.