How astronomers reconstruct the surfaces of invisible alien worlds

The Universe is filled with planets. Astronomers have so far confirmed more than 4,500 worlds, of which more than 1,500 are rocky terrestrial planets. Within our solar system, the rocky planets – Mercury, Venus, Earth, and Mars – are very different from each other. But once you start looking at the systems around other stars, the diversity that we see in our solar system is left in the dust. These distant worlds can be incredibly bizarre, unlike anything we’ve imagined. Some are super-earths, others are rain rocks. Some have winds that rage thousands of miles an hour, and others are made of diamonds.

But how do astronomers know what are these worlds like? Bathed in the glow of their mother star, these planets are almost invisible. Scientists can only determine that these planets exist by looking at their parent star; maybe it wobbles a bit under the gravitational pull of the planet, or maybe the light dims as the planet passes in front of it. But see these planets directly? Unlikely. Still, astronomers have a few tricks up their sleeve that allow them to infer the properties of these alien worlds.

There is a model for that

Just because you can’t see something doesn’t mean you can’t predict its characteristics. Astronomers can make educated guesses about the properties of a planet to develop a detailed model.

That’s what McGill University graduate student Tue Giang Nguyen did with his colleagues. The planet they were looking at, K2-141b, was in orbit ridiculously close to its mother star, located about 200 light years from our solar system. To imagine what this world looked like, they made some key assumptions.

First, they assumed that the planet was locked by the tide on its star. This seemed like a reasonable assumption, given that the planet makes a complete revolution around its star in just 7 hours. “The star’s gravity is strong enough to transform the physical characteristics of the planet, and the side facing the star becomes denser than the other side,” Nguyen said. Great reflection. “This uneven mass distribution will, over time, force the planet to rotate such that one side will always face the star.” This means that one side of the planet is locked in an eternal sweltering day, while the other side is in a continuous night.

Nguyen and his team developed a one-dimensional model that takes into account how mass, momentum, and energy would shift from the scorching day side to the cold night side. What they found painted a picture of a hell planet. On the day side, temperatures reached 3,000 degrees Celsius – hot enough not only to melt the rock, but to spray this.

The winds would bring these vaporized rocks to the night side, where they would condense in the form of a shower of “pebbles”. These rocks would land in the ocean of magma, where they would flow back on the day side, only to evaporate again. Instead of a water cycle, as you see on Earth, you would see a rock cycle.

Comparison of NASA’s exoplanets. (Credit: NASA / Ames / JPL-Caltech)

Someday we may be able to observe this planet with JWST or maybe even Hubble. As this planet passes in front of its star, a small amount of stellar light will filter through the atmosphere, leaving signature lines on the star’s spectrum. Or conversely, when the planet passes behind the star, the star’s light will filter through the atmosphere, bounce off the planet’s surface, and then pass through the atmosphere again, heading towards us. We could then observe the changes it brings to the spectra of the star. Then we may be able to confirm some predictions about the atmosphere of K2-141b.

Planets “pollute” their stars

Keith Putirka, a geologist at California State University in Fresno, was at the annual Goldschmidt geochemistry conference. Putirka presented results, made with his student, predicting what types of planets were orbiting stars. They made simple assumptions that the planets were similar in composition to their host star, minus volatile elements like hydrogen, helium, and other noble gases. As he stood near his poster, Siyi Xu walked around. Xu, an astronomer at Gemini, asked him if he had ever heard of polluted white dwarfs.

When a star in the main sequence ends its life, it transforms into a red giant. This is in store for our sun, and when it does, the sun will engulf the orbits of Mercury and Venus, and possibly even Earth.

The planets orbiting these red giants will meet a very sad end. If they are close enough, they can be swallowed whole. Later, the red giant will expel its outer layers in the form of a planetary nebula, and the core will collapse into an Earth-sized stellar remnant, a white dwarf. Alternatively, the planets can be disturbed by the tides and gradually fall into the white dwarf.

Yet the planets will live – sort of. The rocks and minerals swallowed by the star will dissociate into their corresponding elements. Astronomers can observe these “polluted white dwarfs” and reconstruct what the planets orbiting the stars looked like.

By working together, that’s what Xu and Putirka decided to do. Taking detailed observations of the atmosphere of white dwarfs, they reconstructed these dead planets.

This approach – taking elemental compositions to deduce what types of minerals are present using standard mineralogy (or normative mineralogy, as it is known in the geological community) – has been in use since 20e century for rocks on Earth. “We just apply the same approach to the stars,” Putirka said. Great reflection.

And what a surprise it was. In their small sample of 23 white dwarfs, they found a wide variety of potential minerals. In fact, the variety was so huge that many of the minerals they found have no equivalent in our solar system. Some examples are the minerals Xu and Putirka called “quartz pyroxenites” or “periclase dunites”.

This diversity of minerals will affect the major characteristics of a planet. Will there be mountains? Tectonic plates ? A thick or thin crust? In fact, many planets potentially had a mantle made up of orthopyroxene (while olivine is dominant in the Earth’s mantle). This would change the thickness of the crust, affect the plate tectonics and possibly reject it altogether.

Not just this. “These are the mineral properties that will also determine things like whether a planet has a global water cycle or a global C. [carbon] cycle, which in turn affects things like how and when an atmosphere and oceans change and the resulting climate, ”Putirka said.

Geology – A case of life and death

A variety of things, like volcanoes or plate tectonics, can affect the habitability of a planet. Plate tectonics bring the surface of a planet to life. Having segments of the crust that can move helps a planet regulate its temperature. Volcanoes can also cycle a planet’s atmosphere, helping to replenish gases that would otherwise be lost in space.

Planetary geologist Paul Byrne of Washington University in St. Louis did not have a planet in mind when he developed his models. Instead, he wanted to understand the range of planetary properties and how the crusts of planets might affect their properties as a whole. He and his team “turned the dials,” Byrne told the University of Washington. Source. “We have literally run thousands of models. ”

By tinkering with the attributes of the planet – like its size, internal temperature, and composition, as well as the star’s properties and proximity to the planet – they were able to make predictions about the planet’s outer layer: the lithosphere. . They found that, generally, planets that are smaller, older, or far from their host star are more likely to have a thick outer layer. But there are exceptions, such as when the planets have a lithosphere only a few kilometers thick. They nicknamed these worlds “eggshell planets”.

So why are the planets so diverse? One possibility is how they were formed. “The protoplanetary disk could have different compositions and the planets formed under different conditions,” Xu said. These differences could have to do with previous generations of stars – a story passed down through millions of years, ultimately reflected in the characteristics of the newborn planet. Or they could be associated with the formation mechanisms and properties of the disc itself, such as temperature and pressure.

Even though we cannot see these planets directly, they should not remain unknown to us. When looking at stellar models or observations, one thing is for sure: our planetary zoo is more diverse than we ever imagined.

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