How astronomers discover exoplanets, which are outside our Solar System

In addition to the planets in our Solar System, astronomers also study so-called ‘exoplanets’ — those orbiting a stele that are not our Sun. According to NASA, 4,912 exoplanets have been discovered and there are at least 8,493 candidates.

The retired Kepler satellite discovered nearly half of these early ones in just one region of the sky, reports NASA.

The discovery of exoplanets not only helps humanity better understand the potential prevalence of life elsewhere in the universe, but also how the Earth and Solar System were formed.

And, although almost five thousand of them have already been discovered, there are some obstacles in the process of identifying some exoplanets, as I told the CNN Professor José Dias do Nascimento Júnior, PhD from the Paul Sabatier University (France) and research associate at Harvard-Smithsonian.

“Exoplanets are mostly discovered by indirect methods, as it is very difficult to observe them directly with telescopes. They are small bodies (without their own light) and that are dazzled by the intense brightness of the stars they orbit. ‘See’ would be like trying to find the glow of a firefly next to a naval beacon”, he comments.

According to NASA, the bad news about exoplanets is that we still don’t have a way to reach them and we won’t be leaving footprints on them anytime soon.

The good news is that we can observe them, measure their temperatures, taste their atmospheres and, perhaps one day soon, detect signs of life that may be hiding in captured pixels of light.

For comparison, even though exoplanets are far away – even the closest known exoplanet to Earth, Proxima Centauri b is still about four light years away (A light year is 9.46 trillion kilometers).

Aline Novais, a master in astronomy and a doctoral student at the Valongo Observatory, at the Federal University of Rio de Janeiro, told CNN that with the current technology we have, it would take much longer to travel there.

“For comparison purposes, traveling at the speed of the New Horizons spacecraft (16.2 km/s, which is 0.000054 times the speed of light), the trip would take more than 70,000 years”, he points out.

How exoplanets are discovered

Professor Helio Jaques Rocha-Pinto, astronomer and president of the Brazilian Astronomical Society (SAB), explained to CNN that the technique for identifying a planet consists of observing the light of the star that the planet orbits.

“This is how the planets of the Solar System become visible. Ideally, we would like to find other planets around distant stars using this same technique.”

Nascimento Júnior points out that astronomers use creative methods that reveal the presence of these distant planets. “These methods explore the effects the planet has on the stars it orbits. We have six main exoplanet detection techniques. Called the Radial Velocity technique; astrometry; Traffic; Gravitational microlenses; Imaging and Pulse.”

However, the stars are so far away that the separation (in the sky) between star and planet is very small — only large telescopes can do the optical separation between them.

“Furthermore, the star is many times brighter than its planets; this blurs the image of the planet, making it even more difficult to detect. Because of this, the most direct way we could use to find an exoplanet—that is, see it—was only possible in a few cases, when the star was a cold giant (in this case, the intensity contrast with the planet’s light). is smaller) and the planet was further away from the star”, adds Rocha-Pinto.

Most of the exoplanets detected to date were with the Transit technique, as explained by Nascimento Júnior. “It is today the most famous, due to the success of space missions (space satellites, CoRoT from ESA, Kepler from ESA and NASA)”, he points out.

The professor explains that the technique consists of observing because, “when the planet orbits its star, and this planet sometimes crosses the disk and in the crossing – which we call a planetary transit – it blocks some of the star’s light. As the blockage is small, this method is effective in space observations.”

Types of exoplanets

But not all these planets are the same. Novais explains that there are exoplanets with the most varied characteristics.

To get an idea of ​​comparison, Earth is considered a rocky planet, as well as Mars, Mercury and Venus.

Thus, when a rocky exoplanet about the size of Earth is discovered, it earns the nickname “Terrestrial Planet”. However, the similarity does not mean that it maintains the same conditions as Earth.

Novais explains that some may be similar to the planets we have in the Solar System, but others are quite different.

“For example, we call Jupiter-like planets those with main characteristics (radius, mass) similar to Jupiter. Same thing with Neptune-like planets. There are also rocky (or terrestrial) planets with a mass/radius comparable to Earth,” he comments.

Nascimento Júnior also points out that exoplanets that orbit other stars are very “strange” when compared to the examples found in our Solar System.

“Most are dramatically different. Generally speaking, small planets are rocky, large planets are gaseous, and those in between can be watery. Sometimes called Super-Earths, because they are much larger than Earth and likely to harbor oceans of liquid water,” said the Harvard-Smithsonian research associate.

Novais points out that, however, none of these definitions is official. According to the astronomer, many of the planets already detected are extremely close to their star, much closer than Mercury is to the Sun and they are often called “hot”.

“As is the case with hot Jupiters or hot Neptunes. We also have planets such as super Earths and mini Neptunes, whose name comes from the fact that they have masses greater than Earth’s, but smaller than Uranus/Neptune’s”, explains the astronomer.

Search by biosignature

Nascimento Júnior explains that we need to ‘go further’ with regard to the discovery of exoplanets. So far, Earth is the only planet we know of with life. However, scientists around the world, focused on biosignatures, are searching the galaxy for planets similar to ours and for signs of life.

The distance that the Earth orbits the Sun is considered ‘right’ for water to remain liquid and water, as we know it, is critical for survival.

According to NASA, this distance from the Sun is called the habitable zone, or the Goldilocks zone.

Rocky exoplanets found in the habitable zones of their stars are more likely targets for detecting liquid water on their surfaces.

The so-called Kepler-22b was the first planet discovered within the habitable zone of a Sun-like star, a region close to the star where liquid water may exist.

22-b was the first planet that the Kepler mission confirmed to be positioned in a habitable zone. The exoplanet is 2.4 times the size of Earth — the smallest ever found in an area that can support life.

Like the different beings on Earth, there is a constant adaptation to change such as temperature, radiation, salinity, acidity and aridity. In this way, it may be possible that life began in other worlds and adapted to conditions quite alien to what we are used to.

In the case of exoplanets, starting by classifying the type of atmosphere is an important step to know if life can remain there in a stable way, as explained by Professor Nascimento Júnior.

“The process of evolution of exoplanets follows laws of physics, chemistry and science in general and that can be symmetrical. Planets with volcanoes, for example, follow an evolution established by the relationships between them and their atmosphere with their gases”, he says.

Novais points out that studying the atmosphere is fundamental for understanding the formation and evolution processes undergone by a planet and its system, and may also give us clues about the nature and behavior of our own planet, our atmosphere, and the other planets in the System. Solar.

“The analysis of the atmosphere of an exoplanet is the best way we have to infer the habitability of that environment, through the characterization of its structure and composition, in search of biosignatures”, explains the astronomer.

For Nascimento Júnior, the Earth obeys a particular set of parameters. “Nothing prevents exoplanets, in this cosmic possibility of diversity, from being similar and very similar to each other”, he concludes.