A world of extremes

Hal Clement, a well-known science fiction writer, had a very particular story theme.

He imagined bizarre worlds with extreme conditions, such as gravitational attractions hundreds or thousands of times stronger than ours.

Some of his worlds were very cold or very hot. The stories involved living creatures on those worlds. In 1957 he wrote the book Cycle of Fire, in which he described a world with temperature variations of hundreds of degrees. It orbited a red star, which in turn orbited a bright, blue star.

The orbit around the blue star was very eccentric, so the distance from that star varied immensely over the orbital period of 100 years or so. This drove the huge temperature variations.

The living creatures on this world handled the temperature variations by alternating generations of "hot ones" and "cold ones.

Clement put together this unlikely scenario because our understanding of how planetary systems form is that the planets end up in tidy, concentric, almost circular orbits around their star.

Our solar system is a good example. No such planets would experience the huge temperature changes needed for the story, so some other recipe was needed. At least that is what we thought until very recently.

Now, a planet custom made for Clement's story has been found. Moreover, it is orbiting just a single star. No extra star is needed.

One theory for how this planet came about is just a small variation of the standard mechanism as to how planets form.

This new and exceptional planet orbits the star HR 5183, and has been named, with typical astronomical creativity, HR 5183b. Its orbit is amazingly eccentric. If it were transplanted to our solar system, its orbit would take it from as far out as Neptune and almost as close to the sun as Mars.

We don't know yet whether this planet has an atmosphere, but if it does, over its "year" it would range from so cold some of the atmospheric gases would liquefy and water would be hard frozen, to a coolish temperate environment with liquid water lakes, rivers and oceans.

How could such a planet form?

We now know of thousands of exoplanets (planets orbiting stars other than the sun) and until this recent discovery, all have been more or less planets of the usual kind, moving around their stars in almost circular orbits.

What could have happened here?

Stars and planetary systems form from the collapse of a huge cloud of gas and dust. The dust forms a disc. The core of the disc becomes a star and the other disc material forms planets.

Dust grains collide and stick together, forming larger lumps, which stick together to make bigger ones. Usually this leads to single large lumps orbiting where the future planets will be, sweeping up the other stuff, becoming bigger and bigger.

It is unlikely but possible that instead of one big lump and a lot of small stuff we could end up with two big lumps. Sharing the same orbit will inevitably lead to a collision or a close interaction.

n that interaction one body will be accelerated and thrown outwards, while the other decelerated and thrown inwards. One would be our bizarre planet, in a very eccentric orbit, with the other body either falling into its sun or thrown right out of the system.

It is unlikely there are any "conventional" planets in that system, because something moving in and out, crossing their orbits will eventually result in a collision.

Maybe this odd orbit change happened very recently, and the collision has yet to occur.

There are objects in our solar system that have highly eccentric orbits, crossing the orbits of the planets, but these are small bodies: asteroids and comets. Hitting one of them would not change our Earth's orbit, but would still be a disaster.

• At 02:50 EST on the 23rd, or 23:50 PST on the 22nd, the sun will cross the equator heading south, marking the fall equinox. From here we will have more hours of darkness than we have of daylight.
• Jupiter and Saturn lie low in the southern sky after dark.
• The moon will reach Last Quarter on the 21st.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.