Podcaster: Avivah Yamani

Title: Space Stories: The Planetary Family Tree

Organization: Planetary Science Institute; langitselatan

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Description:  So we have rocky planets, gas giants, and ice giants in Solar System. What are their differences? And how about exoplanet. Are they the same? This is the story of planetary types in Solar System and Extrasolar Planet.

Bio: Avivah Yamani is a an astronomy communicator from Indonesia.

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Planets are born from the particles of gas and dust in the leftover material from star formation. This material formed a disk around the newborn star and over time, this material began to collide and stick together forming larger clumps. 

These clumps then collide and merge with other clumps and gradually increase their size by accreting more matter. They will then become planetesimals and then protoplanets and finally a planet. This disk is violent when the gas and dust in it collide and merge to become planets. 

Once the process is finished, we will have a new planetary system which is stable enough to keep all the planets revolving around the star. But each planet in the system has its own unique characteristics. 

In our own Solar System, we can classify the planets into several different types based on their compositions. Let’s start from the planets near the Sun and go to the far side of the Solar System. 

The first type of planet in the Solar System is quite similar to the Earth. Their compositions are dominated by rocks and we call them rocky planets or terrestrial planets. In our Solar System, planets with this type are Mercury, Venus, Earth, and Mars.

Traveling farther from the Sun, leaving all those rocky planets behind us we will meet giant planets. The biggest one in the Solar System. Other similar planets to it are smaller. 

Yup. We met the king planet in the Solar System, Jupiter!

Jupiter has other similar planets in terms of size. The giants are Jupiter, Saturn, Uranus, and Neptune. All of them have other similarities too. They are all made of gas. These four are the gas giants. 

But still they have their differences too. Jupiter and Saturn are gas giants while Uranus and Neptune, which are farther than the first two giant planets, are colder and in fact their materials are frozen. So these two planets classify as ice giants. 

So what actually happened? Why are Uranus and Neptune different? Well this is related to their distance from the Sun. Or their location to be exact.  

When the Sun has formed and the material in the protoplanetary disk accreted to be planets, several planets formed near the Sun and several others are farther away. 

Close to the Sun the temperature was too high for volatiles (gases like water and methane) to condense, so only the materials with a higher melting point (and higher density) were able to form at this point. 

That’s why metals and silicates dominate this inner region and formed the rocky planets such as Mercury, Venus, Earth, and Mars.

The gas giants on the other hand, formed far enough away from the Sun that the temperature was cool enough for these volatile gases to condense, and form these huge, less dense planets. In this region, rock, metals, and ice, were accreted into dense cores. 

These cores then accrete gases in their surroundings to form giant gas mantles for the cores and by the end of the process, we have giant planets such as Jupiter and Saturn, with very thick atmospheres. This atmosphere is dominated by hydrogen and helium. 

But actually, back in the old days, the gas giant planet category included Uranus and Neptune. But after we learned more about both of these planets’ compositions, it looks like these planets are at least partially composed of heavy metals. These are materials heavier than hydrogen and helium.  

Located far away from the Sun, temperature in this area has decreased to the point that all material here is in icy form. Uranus and Neptune are made by heavy metals such as oxygen, carbon, nitrogen, and sulfur in their ice form. 

That means this material has reached their frozen point and that’s why we classify these two planets as ice giants.

At first astronomers thought that what we have in the Solar System will become a general model if we discover planets around other stars. 

For all this time, we only knew the Solar System as the only example of a planetary system. So that’s not odd if we would think that other planetary systems will be like our own Solar System. Planets formed close to the stars are rocky planets and those form far from the star are gas giants. 

In reality? That doesn’t apply!

An Exoplanet or Extrasolar Planet is a planet orbiting another star. And they have different stories, in which the planet types discovered in those systems are different from the one here in the Solar System.

The first exoplanet was actually found in 1992. But it was weird. This planet was orbiting a death star! A pulsar. So at that time, we thought it would be impossible. But then we discovered more and yes, there are planets orbiting pulsars.

So the first exoplanet found orbiting a sun-like star is 51 Pegasi b or Dimidium. It is indeed orbiting a sun-like star. But… the planet itself looks wrong. 

Wait.. what do you mean wrong?

51 Pegasi b or Dimidium was discovered in 1995. This planet is totally different from what we thought we would find. It is located very close to the star 51 Pegasi or Helvetios. So it should be a rocky planet, no?

Well… Nope! This planet is huge! It is in fact a gas giant planet. A hot gas giant planet

It is located close to the star and it only needs less than 5 days, or 4.2 days to be exact, to make a full trip around its star.  And… What’s more… in the coming years… we discovered more and more planets of this type. Looks like it is common to find hot gas giants circling stars from a close distance. 

So how does it form?

As we know, it would be hard to form gas planets close to a star. The light pressure pushes the gasses away. So the only possible way is, it forms far away from the star just like Jupiter in the Solar System. 

But then this planet migrated to the inner part of the system. There are two possibilities as to why these planets migrated. 

The first possibility is that the migration happened during the early stage of the system’s formation when the planets were young and the second one, when a planet interacts with another large object, it will create instability. 

As a result, the planets were either ejected from the system or sent to the inner part of the system, close to the star. 

The hot gas giant planets that are located very close to the stars are classified as hot Jupiters. 

The hot Jupiters were not the only weird thing in exoplanetary systems. There were more planet types found by astronomers that we don’t have in the Solar System. 

In 2005, astronomers discovered a planet revolving around a star named Gliese 876. The planet’s size is larger than the Earth, but smaller than Neptune. So they call it a Super Earth.  

Even though there is no super Earth in our Solar System, similar to hot Jupiters, this type of planet is commonly discovered in extrasolar systems. Especially circling the red dwarf stars. 

But here’s the deal. Not all super-Earths are Earth-like or rocky planets. Some of them are made mostly of hydrogen and helium gas just like the gas giants. So this type of planet gets another classification as a mini Neptune. 

So what are the differences between a Super-Earth and a Mini-Neptune? 

The super-Earths are exoplanets that have up to about four times the mass of Earth with 1.6 times the size of Earth and occasionally as large as 1.75 times the size of Earth, while the mini-Neptunes have masses 4 to 10 or more times that of Earth with size between two and four times the size of Earth. 

Few planets with sizes between these two planet types have been detected. One possible explanation for this gap is that mini-Neptunes are shedding their “fluffy” outermost shells and shrinking into super-Earths.

But the most significant difference is their density to differentiate the rocky planet from mini gas giants. Planets with low density means they were mostly made by hydrogen and helium, like a normal gas giant. On the contrary, if the planet’s density is high then they are mostly made of rocks and metals. 

As for the formation of these planets, there are two possibilities. The first one has a super-Earth planet forming close to the parent star. To support this theory, we need a larger mass of matter close to the star to create this type of planet. 

The second possibility is a super-Earth planet that formed far from the star and later migrated close to the star. And it can be said that mini-Neptunes were formed outside the frost line before migrating close to the star. 

End of podcast:

365 Days of Astronomy

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