Oort cloud

And our fifth and final fun fact for today, the edge of our solar system has a massive shell of icy objects stretching nearly a light year into space.

Most people think Neptune or Pluto marks the edge of our solar system, but gravitational influence extends much further to the Oort cloud.

The Oort cloud is a spherical bubble of billions, possibly trillions, of icy bodies left over from the formation of the planets.

It begins about 2,000 astronomical units from the Sun.

Now, an astronomical unit is the distance from the Earth to the Sun, about 93 million miles or 150 million kilometers.

Its outer boundary stretches between 100,000 and 200,000 astronomical units away, which is halfway to the nearest neighboring star system, Alpha Centauri.

The Oort Cloud is named for the Dutch astronomer Jan Oort, who proposed the existence of this region in 1950 to explain where long-period comets came from.

These icy bodies take thousands or even millions of Earth years to complete an orbit around the Sun.

But even that is not the border.

Far beyond it, astronomers infer the existence of the Oort Cloud, a titanic shell of frozen, icy bodies, the dormant seeds of comets wrapped around the entire solar system like a faint spherical fog.

We have never seen it directly, Its existence is deduced from where long-period comets fall in from, so it belongs, honestly, to the realm of well-founded inference rather than direct sight.

But if it is there, as nearly all astronomers believe, it is staggeringly large.

On one hand, it means the truly empty no-man's land between our star and the next is narrower than the raw distance suggests the Sun's own faint outer territory.

and perhaps a matching shell around Alpha Centauri, may reach toward one another across the dark, leaving a smaller, strictly empty gap in between.

On the other hand, it makes the human scale of the crossing worse, not better, because Voyager 1, our fastest and farthest traveller, will take roughly 300 years merely to reach the inner edge of the Oort Cloud, and something like 30,000 years to pass all the way through it, and leave the sun's domain behind at last.

30,000 years just to escape the outskirts of our own star's territory, and only then would the real journey to Proxima even begin.

The comets we observe generally start out as icy, dirty snowballs of frozen gases like water and frozen carbon dioxide in the Oort cloud.

This halo of potential comets extends from two to 100,000 times further from the Sun than we are.

Its members take from tens of thousands to millions of years to orbit the Sun The European Space Agency's Gaia mission discovered that approximately 2.5 million years ago, the star HD 7977, now in the constellation of Cassiopeia, passed through the outer Oort cloud at some 4,000 to 25,000 times our distance from the Sun.

In a recent study of the orbits of long-period comets, Dr. Nathan Kayeb of the Planetary Science Institute and Dr. Sean Raymond of the University of Bordeaux have found evidence suggesting that HD 7977 passed between 6 and 10,000 times our distance from the Sun, producing a shower of comets to enter the inner solar system that persists to this day.

They arrived at this result by noting that over billions of years the gravity of the more distant stars in the Milky Way changed an initial disk of material into a roughly spherical Oort cloud we see today.

These scientists analyzed the orbits of newly discovered long-period comets and found that the passage of HD 7977 had perturbed the Oort cloud making some of its members much more likely to pay us a visit.

We can check their prediction as the new Vera Rubin Observatory discovers large numbers of comets in the next decade.

If the analysis of these researchers is correct, we are now witnessing a parade of comets produced by the passage of a star when our first ancestors began to roam the Earth in Africa.

Stars are not packed together the way they appear from Earth.

They are separated by voids so immense that our entire solar system, including all eight planets and the enormous sphere of the Oort Cloud extending perhaps one to two light years from the Sun, fits within the space between neighbouring stars with room to spare.

The solar system is not a large structure by interstellar standards.

It is a small island in an ocean whose nearest neighbouring island lies at a distance of four light years on average.