We take it for granted that during one part of the year it is hot and during another it is cold . In Spain it is hot between May and September and cold between November and March, but in Argentina it is the other way around. All this of course has an astrophysical origin, although probably not the one you imagine. So if the seasons are the result of astrophysical factors, could they be on other planets?
Let’s go by parts. Let’s start by seeing what exactly causes these variations in the temperature of one point or another on the earth’s surface. The seasons arise, solely and exclusively, from the tilt of the Earth’s axis of rotation with respect to the plane that describes its orbit around the Sun. That is, the distance between planet and star has no influence at all. It of course influences the total amount of sunlight the Earth receives, but not the difference in temperature between Helsinki and Johannesburg on any given day. In fact, the Earth reaches its perihelion, the closest point to the Sun in its orbit, around January 4 of each year. This means that at the time when the Earth is closest to the Sun, Spain is in the middle of winter , with snow covering its mountains and beaches empty of tourists.
Therefore the inclination of the axis of rotation determines the change of season. For the Earth this tilt is just over 23º. In its translational movement around the Sun, the direction in which this axis of rotation points does not change. Lie, it does change, but it does so with a period of about 26,000 years, so we can ignore this effect for what we are discussing. This axis points north towards the North Star and south towards the Southern Cross. The main consequence of all this is that, for half of the year, the northern hemisphere will be pointing towards the Sun and the south in the opposite direction. During the other half the opposite will happen. The half of the year in which the hemisphere points towards the Sun (or points more than the other hemisphere) corresponds to spring and summer. The other half, to autumn and winter.
This tilt will cause the Sun to stay above the horizon longer in summer and shorter in winter when viewed from the planet’s surface. This is nothing more than what you already know from your own life experience: in summer the days lengthen and in winter they shorten. Also, since the Sun is higher above the horizon in summer, its rays need to pass through less of the atmosphere to reach the surface . By doing so they lose less energy along the way and are able to heat the environment more efficiently. So it seems that having an atmosphere can influence the seasons.
On an Earth without an atmosphere, the main variation in temperature would be due to the cycle of day and night and not the seasons . On the Moon, for example, the difference between the temperatures reached during the day and at night can reach more than 200 ºC. However, the Moon’s axis is hardly tilted, so it is more difficult for us to study how these temperatures would vary in a tilted body with no atmosphere.
Now that we have all this clear we can face our question: can there be winters and summers on other planets? And the answer is yes, of course, as long as their tilt axis is slightly tilted and as long as they have an atmosphere capable of retaining heat from the surface. Venus’s axial tilt is less than 3 degrees , so it has no noticeable seasons . In addition, its thick atmosphere is very effective at trapping the heat that reaches the surface, making the entire surface incredibly hot, be it summer, winter, day or night.
Mars has a very similar tilt to Earth, just over 25º, so it has seasons quite similar to ours, with two differences. The first is that Mars, as a whole, is colder than Earth . Because it is further from the Sun and because it has an atmosphere so thin that it barely retains the little heat that reaches it. The second is that, since the Martian orbit is considerably longer than the Earth’s, lasting about 687 Earth days, its seasons will also be. Seasons on Mars last about 6 months. Similarly, on planets even further from the Sun, this effect is exaggerated. The seasons on Saturn last about 7 years each, while on Neptune they exceed 40 years in length.
On planets like Pluto, the rules of the game are different. The dwarf planet’s orbit has a high eccentricity , so it is not as circular as the Earth’s or the rest of the planets. This means that at its closest point to the Sun (perihelion) it will be almost half as far away as it is at its furthest point in its orbit (aphelion). In addition, its axis has an inclination of about 60º. Therefore, although it will always be cold on Pluto (it has an average temperature of about -230 ºC), different seasons can be perceived, influenced both by the tilt of the axis and by the distance from the star.
As far as exoplanets are concerned, our technology has not been able to measure the tilt of the rotation axis of the detected planets, although it has been able to measure the eccentricity of their orbit. However, we have no doubt that many of the nearly 5,000 planets detected orbiting distant stars will have seasons, we just don’t know exactly what they are like.
