Caoimhín's Content

In a recent video, I discussed the differences between the gas giants like Jupiter and Saturn and the ice giants like Neptune and Uranus. These two types of planets both contrast with the rocky planets in our solar system. This gives us three types of planets, though the gas and ice giants are commonly lumped in together. The rocky planets, are always smaller, the Earth is the biggest rocky planet in our solar system, though rocky planets around other stars may be bigger. Venus is very very close to the Earth in size, though it is hard to see at the moment as it is close to the Sun from our perspective. Mercury is very small, as small as some moons in our system, both Ganymede around Jupiter and Titan around Saturn are bigger than the planet Mercury. Like the other rocky planets, Mercury is a solid planet made of rock and stone, and it’s the same with Mars and even Venus. Venus does have a thick, opaque atmosphere, but it still has far less than the gas or ice giants. The giants planets are mostly made of gas, or gas under enough pressure to act liquid or solid, but the majority of Venus is a ball of rock, rather than the gas that is around it. In our solar system, the rocky planets are closer to the Sun and the gas and ice giants are further outside. This often leads to people conflating the “inner planets” with rocky planets in general, and the “outer planets”, with gas or ice giants that, but this is not always the case. There are many exoplanets, planets orbiting other stars, a topic that I have discussed a few times before, and some of them break the trends that we see in our solar system.

For some reason, I didn’t think to use the star Tuirean and planet Bran, formerly HAT-P-26 and HAT-P-26b, as an example, although it is one. Instead, I take a look towards the region of the sky studied by the Kepler Space Telescope. I have discussed this region of the sky previously, right now all that matters is that we know of a lot of exoplanets in this direction. Most importantly, there are a number of planets very similar to the giant planets in our solar system, some even bigger than Jupiter, that orbit very close to their star. This is a kind of planet has been discovered quite often, so often that it has been given a special name, a Hot Jupiter. In fact, most of the planets that we know of outside our solar system are very big planets, and usually they are planets that are orbiting their star very very closely. This is because that combination is a lot easier to spot, compared to a very small planet far from its star. A big planet will block more light from its star, and this drop will happen more frequently if the planet is orbiting close to the star.

When talking about other planets, their size is often compared to either the Earth or Jupiter. In the same way that different distances in the solar system are measured using the distance between the Earth and the Sun, an AU or Astronomical Unit, planets are said to be however many Earth masses or Jupiter masses in size. These values are often indicated using symbols, 🜨 (a circle around a cross or plus sign) for the Earth and ♃, which looks almost like a cursive 4, for Jupiter. The symbol for Jupiter comes from a Z for Zeus, but it does look quite different today. These symbols are pretty distinct, however the symbol for the Sun is ☉, a circle around a dot, and as well as being commonly used to indicate the mass of other stars in solar masses, it’s easy to mistake for the symbol for Earth masses on small screen, as they both look like partly filled circles. Ignoring the mass for now, we can see that the orbital period of a particular exoplanet here is 9.73 days. These days are the days we use here on Earth, the rotational period or local day for exoplanets is usually unknown. This planets whole year is just a little less than 9 and three-quarters of an Earth day. This is very fast, which means you only need to look at the star it orbits for about 20 days to see the planet come back around. This makes it easier to spot the drop in light and confirm that it is happening repeatedly and at regular intervals. This is how this planet was discovered in 2016, this method is known as the transit method. Looking at another exoplanet, it seemed to say that the planet had a mass of 0.02 somethings, and I concluded that the something must be Jupiter. It would be very hard to spot a planet that is only 2% of the Earths mass, where as 2% of Jupiter is still big enough, that would still be about 6 times more massive than the Earth. In review, I am pretty sure that it was indicating the star in solar masses, and a star just 2% of the Sun is still over 20 times the mass of Jupiter, a small red dwarf but still much bigger than any planet. The planet in question orbiting this star has and orbital period, or year, of over a thousand Earth days. This isn’t particularly frequent, but this planet was not found by the transit method, but by the radial velocity method. The radial velocity measures the amount that a star moves from side to side, due to the influence of a planet orbiting it. If there is a big planet such as Jupiter or larger going around a star, the star and the planet will really be co-orbiting a common barycenter. This point is usually close to the surface of the star, rather than right in the middle between the planet and the star, the planet is still much smaller than the star of course. Technically, this happens even with smaller planets, but the influence is much less noticeable. If a star is orbiting a barycenter significantly different from it’s own center, then it will noticeably move form side to side as it does so, and detecting this is another way exoplanets are discovered.

Thanks to the large size and close orbits of Hot Jupiters, they are particularly well represented in exoplanets discovered by the transit method, which is itself one of the most productive methods for exoplanet discovery. However, I managed to click mostly on small planets or planets discovered by the radial velocity method. This led to plenty of bad examples, though many of them at least had short years, and short years are what tell us that the planet orbits close to its star, and it is that closeness that makes these Jupiters hot. For an even shorter example, a year of just 7.99 Earth days then it must be the case that that planet is very close to its parent star. If the planet is as big as Jupiter or bigger then it is most likely a gas planet, and it may be a gas planet even if it is a bit smaller than Jupiter, as Saturn is. However, if it is smaller than Jupiter there is also a chance that it is actually an ice giants like Neptune or Uranus. We have found planets that are likely ice giants close to their star as well, these may be termed Hot Neptunes.

All of this lets us know, that despite the gas and ice giants being further from the Sun in our solar system, it isn’t always the case in other solar systems. Up until the discovery of exoplanets, the only giant planets we knew existed far from their stars, leading many to be believe that this was the default state of solar systems. We now know that this is not the case, and this is the sort of knowledge that we are finding out thanks to the study of exoplanets. We only had one solar system to study in order to learn about how various systems might work, it’s much better to have more samples to learn from. This is the reason why there are a telescope like the Kepler Space Telescope, to learn about these strange planets. One planet I selected near the end of the video was orbiting its star every 0.63 days, every 0.63 Earth days, giving it a almost two years every 24 hours. Beyond this, there are also dwarf planets like Pluto out there as well, but there are more types, though these will be especially hard to detect around other stars. There may be even more strange types of planets waiting to be discovered.

If you enjoyed this piece you can improve your chances of seeing more by subscribing to my website or YouTube channel. There will be another Irish language video at some stage next month, and as usual there will be two videos a week in English along the way, so I hope you come back to check out those as well.