As star observations improve, Earth's history and future become more uncertain

No matter what stock market analysts, political pollsters and astrologers may say, we cannot predict the future. In fact, we can't even predict the past.

This is what Pierre-Simon Laplace, the French mathematician, philosopher, and king of determinism offered. In 1814, Laplace declared that if it were possible to know at a given instant the speed and position of every particle in the universe—and all the forces acting on it—“then to such a mind nothing would be uncertain, and the future would be uncertain.” “Just like the past, the present will be his.”

Laplace's dream remains unfulfilled because we cannot measure things with extreme precision, so small errors spread and accumulate over time, creating more uncertainty. As a result, in the 1980s, astronomers, including Jacques Laskar of the Paris Observatory, concluded that computer simulations of planetary motions Can't be trusted When applied more than 100 million years in the past or future. For comparison, the universe is 14 billion years old, and the solar system is about five billion years old.

“You can't accurately predict your zodiac sign for a dinosaur,” Scott Tremaine, an expert in orbital dynamics at the Institute for Advanced Study in Princeton, New Jersey, recently commented in an email.

The old astrological chart is now more blurry. A new set of computer simulations, which take into account the effects of stars moving through our solar system, has reduced scientists' ability to look back or forward by another 10 million years. Previous simulations had considered the solar system to be an isolated system, a clockwork universe, and the main perturbations in the planets' orbits were internal, caused by asteroids.

“Stars matter,” said Nathan Cape, a senior scientist at the Planetary Science Institute in Tucson, Arizona, who he and Sean Raymond of the Astrophysical Laboratory in Bordeaux in France have published their findings. In Astrophysical Journal Letters In late February.

Researchers have discovered that a Sun-like star called HD 7977, which currently lies 247 light-years away in the constellation Cassiopeia, could have passed close to the Sun about 2.8 million years ago to jolt the largest planets into their orbits.

This additional uncertainty makes it difficult for astronomers to forecast more than 50 million years in the past, to correlate temperature anomalies in the geological record with possible changes in Earth's orbit. This knowledge will be useful as we try to understand the climate changes taking place today. Dr. Cape said that about 56 million years ago, the Earth clearly went through the Paleocene and Eocene thermal maximum, a period lasting more than 100,000 years during which average global temperatures rose by as much as 8 degrees Celsius.

Was this warm wave caused by some change in the Earth's orbit around the Sun? We may never know.

“So I'm not an expert, but I think this is the warmest period in the last 100 million years,” Dr. Cape said. “It's almost certainly not the Earth's orbit itself. But we know that long-term climate fluctuations are linked to Earth's orbital fluctuations. So if you want to detect climate anomalies, it helps to be confident about what the Earth's orbit is doing.”

“The simulations were done carefully, and I think the conclusion is correct,” Dr. Tremaine noted. “This is a relatively minor change in our understanding of the history of Earth's orbit, but it is conceptually important,” he added.

The really interesting story, he said, is how chaos in Earth's orbit can leave a mark in the ancient climate record.

The ability to track the movements of stars outside the solar system has been greatly improved by the European Space Agency's Gaia spacecraft, which has mapped the positions, motions and other properties of two billion stars since its launch in 2013.

“For the first time we can actually see individual stars, project them backwards or forwards, and see which stars are close to the Sun and which aren't, which is really cool,” Dr. Cape said. “.

According to his calculations, about 20 stars come within one parsec (about 3.26 light-years) of the Sun every million years. HD 7977 could have come as close as 400 billion miles to the Sun — roughly the same distance to the Oort Cloud, a huge reservoir of frozen comets at the edge of the solar system — or stayed 1,000 times farther away from it. The gravitational effects of the closest encounter could have shaken the orbits of the outer giant planets, which in turn could have shaken the inner planets like Earth.

“This is potentially powerful enough to change simulations' predictions about what Earth's orbit was like approximately 50 million years ago,” Dr. Cape said.

As a result, he said that almost anything is statistically possible if you look far enough ahead. “So you find, for example, if you move forward billions of years, not all the planets are necessarily stable. There's actually a 1% chance that Mercury will collide with the Sun or Venus over the next five billion years.”

Whatever happens, we probably won't be around to see it. We are stranded at present, not knowing for sure where we have come from or where we are going; The future and the past give way to myth and hope. And yet we move forward, trying to transcend our horizons in time and space. As F. wrote F. Scott Fitzgerald in “The Great Gatsby”: “And so we go on boating against the current, endlessly returning to the past.”