What exoplanets are made of and how do we know it

(ORDO NEWS) — Astronomers recently detected carbon dioxide in the atmosphere of an exoplanet for the first time.

Astronomers know more than five thousand planets , and this is most likely a drop in the ocean. Whether an exoplanet is discovered or not depends on many things: the size of the planet, the distance to it, the period of revolution around the local sun, the orientation of the orbit in space.

And for each item, the restrictions are very strict. Since observers in such a situation manage to discover thousands of distant worlds, the total number of planets in the Galaxy should be comparable to the number of stars: hundreds of billions.

Some of these worlds are Earth-like and potentially habitable. Others are more like Jupiter or Neptune and are obviously lifeless. There are exoplanets so hot that even iron breaks into atoms there, and there are icy ones. Astronomers are interested in all of them, and now we will explain why.

There is a saying: who does not know a single foreign language, does not know anything about his native. It would be more accurate to say that in one language it is very difficult to judge what a language is in general.

After all, language as such is something that all languages ​​have in common, and they are very different. For example, there are 46 cases in the Tabasaran language, and 137 genders in the Barasana language.

Another example: in Chukchi or Aztec sentences often consist of two words – the subject and a very, very long predicate, where the entire content of the phrase (“the old man did not fish”) is squeezed in.

Any theory that claims to understand the universal laws of language development must explain how all these examples, exotic to our ears, arose.

Meanwhile, specialists in planets and planetary systems have been in the position of such a linguist for a very long time. They knew only one system – the Solar System, unaware of hot Jupiters or super-Earths. When the era of exoplanets began, the flow of new observational facts puzzled theorists.

But before facts can be explained, they must be collected. Some parameters of exoplanets (for example, the period of revolution around a star) are quite simple to determine. But others, including the composition, are much more complicated.

You can’t send a probe with a sampler a few light years away. Moreover, even with a telescope you cannot see a distant planet (at least with an optical one). How do astronomers figure out what exoplanets are made of?

Non-handy materials

We see the bodies of the solar system due to the light of the sun reflected from them. With exoplanets, this number does not work. At such a distance from the telescope, the planet and its star almost merge into one point, and the weak reflected light is drowned in the rays of the luminary.

Sometimes an exoplanet can be seen in the infrared (it emits much more of these rays than reflected light), and then if it is gigantic and hot. Of the thousands of discovered planets, there are infrared portraits for a few dozen.

Basically, exoplanets are discovered using the transit method and the radial velocity method, which Naked Science talked about in detail . The first allows you to measure the diameter of the planet, the second – the mass.

To determine the composition is already something. No matter how parochial our knowledge of the formation of planetary systems may be, it is difficult to imagine a silicate world the size of Jupiter or a gas world the mass of the Earth.

If only because there are hardly enough silicates for the first one in the protoplanetary disk, and the second one will not be stable: such a small mass of gas will not be held by its own gravity.

But in addition to extreme options, there are also intermediate ones, for example, super-Earths and mini-Neptunes.

At one time they turned out to be a surprise for observers: there are no such objects in the solar system. What are the planets made of, which are, say, 2-4 times larger than the Earth? Possibly from a small rocky core, water, methane and nitrogen.

Or maybe from a much larger core, enveloped in hydrogen and helium. At least the models allow for both options. It is quite possible that both occur in nature. “I blinded you from what was,” the Universe could say to such a planet.

When reading news about ocean worlds , it is important to understand that most often there is no direct evidence of the presence of water there.

Astronomers think something like this: “Yeah, judging by the size and mass, this planet is largely composed of something like water. And since H 2 O is one of the most common compounds in the universe, most likely this unknown component is water.”

It is better when the mass and radius of the exoplanet are known simultaneously. Then you can at least calculate its average density. However, this is so rare that the European Space Agency specifically launched the CHEOPS telescope into orbit to remedy the situation.

He observes by the method of transits of the planets, which were previously discovered by the method of radial velocities, in order to add diameters to their known masses.

The average density has been measured, for example, for seven planets of the famous TRAPPIST-1 system . All of them are similar in size to the Earth, and three of them lie in the habitable zone, that is, the temperature there allows the existence of liquid water.

But the average density of a planet doesn’t provide much information about its composition. It remains possible to play with the mass fractions of heavy rocks, ices, liquids and gases, and there is no need to talk about their exact chemical composition.

For example, the average density of the planet TRAPPIST-1e is higher than that of the Earth. What does this mean? Perhaps there is no atmosphere and hydrosphere, so the entire mass of the exoplanet falls on solid rocks.

This would be very sad, because in terms of the amount of heat received from the star, it is this world of the TRAPPIST system that is most similar to Earth. Or maybe there are streams murmuring and apple trees blooming, it’s just that TRAPPIST-1e has a huge iron core, it is this that increases the density.

What exoplanets are made of and how do we know it 2
When a planet passes between its star and an observer, the star’s rays shine through its atmosphere

Planet to the light

If an exoplanet is observed using the transit method, this means that it periodically passes between its sun and our telescopes.

Then the rays of the star shine through the atmosphere of the planet through and through, and traces of atmospheric gases appear in the spectrum of the star. Sounds tempting, doesn’t it? However, this method also has limitations.

First of all, the atmosphere is not the whole planet. Anyone who decides that the Earth is 76% nitrogen and 23% oxygen will be somewhat mistaken.

In addition, extracting the “fingerprints” of the planetary atmosphere from the spectrum of a star is a delicate job that is extremely demanding on data quality. Telescopes that are good enough for this can probably be counted on the fingers.

And no one will allow an instrument of this class to contemplate the planets day-to-day: astronomers line up for its observing time. It is not surprising that only dozens of planets out of thousands discovered by the transit method have been explored in this way.

The ARIEL orbital telescope , specially designed to “shine” the atmospheres, is called to correct the situation . It is planned that it examines at least a thousand exoplanets.

However, its launch is scheduled only for 2029. And considering how often the creators of advanced spacecraft do not meet the deadlines, we should rather talk about the 2030s.

Another limitation of the method: current telescopes allow us to study the atmosphere of only large planets. Super-Earths are accessible to them, albeit with difficulty, but worlds the size of Mars, Earth or Venus are not.

Finally, the planets have the unpleasant property of being covered with clouds, so that only the upper layers of the atmosphere are “shown through”, which can differ greatly in composition from the lower ones. True, there are cloudless worlds.

Such, for example, is KELT-9b, which at its temperature of 4300 °C is hotter than most of the stars in the Galaxy. In its hot atmosphere, there are simply no molecules that could form clouds.

Astronomers, of course, took full advantage of this luck. But perhaps the most important limitation of the spectral method played against them: not all substances have spectral lines in visible light.

As a result, out of 76 elements of the periodic table from lithium to platinum, nothing was found: magnesium, iron, titanium, sodium, chromium, scandium and yttrium. Calcium, cobalt and strontium are still under suspicion, but the authors are not sure about these results.

The full list of elements and compounds found on exoplanets by the spectral method is a little richer. It is worth adding hydrogen and helium (however, no one doubted the presence of the two most common elements in the Universe on exoplanets even without observations), as well as water and now carbon dioxide.

The last two substances are no longer found in light, but in infrared rays. Perhaps we forgot to mention something, but hardly much. The recently launched James Webb Telescope is able to detect several more compounds, including methane , the simplest organic matter, which, however, does not yet indicate the presence of life.

There is another very interesting way to study the composition of exoplanets, however, already destroyed. This is a search for the remnants of planets in the atmosphere of white dwarfs. Naked Science has already talked about him , so we will not repeat ourselves.

Summing up, we can say the following. Methods for determining the chemical composition of exoplanets exist and are working. But they don’t work as well as scientists would like. However, the same can be said about almost any method at the forefront of science – that’s why it is the cutting edge.


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