Tilted

Incidentally, talking about Martian winters brings up a good point that is perhaps not immediately obvious:

Planets do not HAVE to have seasons. In fact, we have three planets in our system that do not: Mercury, Venus, and Jupiter.

Whether or not a planet has seasons is determined by its tilt, if any, relative to the sun. Planets that are not captured (ie, planets that are formed "in situ" within their solar systems) generally start out with no relative tilt, solidly spinning on an axis which is aligned to their star, and in the same plane (called the plane of the ecliptic).

But planets are always at risk of being hit by various bodies from space - meteors, comets, asteroids, etc. And some of these objects are very large. When a planet is unlucky enough to be hit by a really big one, say a large comet, the planet can be knocked off kilter and will start to precess (spin about a wobbly axis like a top). From that point on that planet will have seasons, as the tilt affects how much sunlight hits it and at what angle, throughout the year.

Big impacts like this are uncommon in the short term, but almost inevitable if given enough time. We know that the Earth has been hit multiple times in the last few hundred million years by objects large enough to create planet-wide ecosystem collapses and mass extinctions - these were impacts caused by meteors about 5-10km in diameter. It would take a much larger collision to knock the Earth into precession, and our season-causing event (SCE, I just coined that lol) probably happened early on in the Earth's history, when it was still very hot and had no life. In those early days when the solar system was forming, there were many more objects soaring around these parts and the chance of a really big impact would have been much greater.

The impact that tilted the Earth would have been the most violent event in its history, and probably also liquefied a great deal of its crust sending molten material into our orbit and beyond. Some scientists think our moon formed from that molten material - in other words, the moon may have condensed out of material ejected from Earth during this collision. One confirmation of this is that rocks originating on the Earth (yes, they can tell) have been found on the moon. That's funny when you think about it - we sent astronauts to the moon to go bring back a few little pieces of the Earth that we lost.

The Earth now has a tilt of about 23.5 degrees, which gives us well defined seasons in most areas. Planets that have no seasons generally just lucked out and missed being hit.

Mars, Saturn, and Neptune all have tilts about the same as the Earth and they all have seasons of about the same severity.

And then there is Uranus. Poor Uranus. Uranus was the first planet discovered by telescope, in 1781. Today we know that something very, very traumatic happened to Uranus a long time ago, and that poor gas giant will never be the same.

Uranus, you see, has been whacked all the way over. It has a tilt of 98 degrees, and it's north and south poles are in the orientation of most planets equators. So it rotates perpendicular to the plane of the ecliptic. The effect of this is that Uranus has very strange seasons:

"Other planets can be visualized to rotate like tilted spinning tops on the plane of the Solar System, while Uranus rotates more like a tilted rolling ball. Near the time of Uranian solstices, one pole faces the Sun continuously while the other pole faces away. Only a narrow strip around the equator experiences a rapid day-night cycle, but with the Sun very low over the horizon as in the Earth's polar regions. At the other side of Uranus's orbit the orientation of the poles towards the Sun is reversed. Each pole gets around 42 years of continuous sunlight, followed by 42 years of darkness. Near the time of the equinoxes, the Sun faces the equator of Uranus giving a period of day-night cycles similar to those seen on most of the other planets."

Uranus has huge storms, propelled by the dramatic temperature gradients, with wind speeds that often break the sound barrier (we complain about 35 mph winds, imagine 560 mph winds!)

Being knocked down flat is pretty embarrassing, but Venus actually got it worse than Uranus. You may note that above it says that Venus does not have seasons, and that is true. But that's only because when Venus' time came to pay the piper it was knocked all the way around, head over heels. Crazy as it sounds, Venus is upside down. Its North pole is on the bottom and it rotates in the opposite direction as the other planets. That's why the graphic above lists its tilt at 177 degrees. So I guess Venus lucked out to have been hit so hard and still not get seasonality.

As with most properties of our Goldilocks Earth, we have seasons that are in between "none" and "severe". Just right.