What are exoplanets and how are they discovered?

What are exoplanets

The recent launch of the TESS mission has refocused the search for exoplanets. This term is not included in the Dictionary of the Real Academia Española, a common situation with most technicalities.

The Fundéu BBVA, on the other hand, does recognize this word, recommending that it be written together and without a hyphen.

What is an exoplanet? The question, though seemingly simple, is complex. In 2006, the International Astronomical Union (IAU) made a distinction between the terms “planet” and “dwarf planet”, which resulted in a striking piece of news: Pluto was no longer officially considered a planet and began to be described as a dwarf planet.

Both concepts, “planet” and “dwarf planet”, refer to celestial bodies orbiting around the Sun and possessing “sufficient mass for their own gravity to overcome the forces of a rigid body so that it acquires a hydrostatic equilibrium (practically spherical shape)”. It is not the same with the definition of exoplanet. According to the Paris Observatory, there is as yet no consensus on what should be considered as such.

It is now accepted that the term exoplanet refers to all planets outside the solar system, i.e. extrasolar planets. The IAU Working Group on this subject collected three characteristics that exoplanets should have: to be an object with a true mass below the limiting mass for thermonuclear fusion of deuterium, to rotate around a star or a stellar remnant [other than the Sun] and to present a mass and/or size greater than that used as a limit for a planet in the solar system.

However, the Paris Observatory continues, for the time being it has not been determined what limit exists in our case, so the provisional description of exoplanet is imprecise. A situation that, on the other hand, should not surprise us. The first exoplanet was discovered just over a quarter of a century ago. In 1992, two astronomers at the Arecibo Observatory (Puerto Rico) described a series of worlds that revolved around a peculiar star, called Lich, which emitted radiation at very short and regular intervals. In fact, the star functioned as a kind of lighthouse, that is to say, it was a pulsar.

Three years later, in 1995, two teams of scientists on both sides of the Atlantic found the first exoplanet revolving around a star similar to the Sun. The discovery was a real bombshell, as it demonstrated for the first time that there were planets outside the borders of the solar system and that they could also orbit stars similar to ours. Since then, the scientific community has managed to detect thousands of exoplanets thanks to different missions, the best known being the Kepler telescope. But how do we observe which worlds exist beyond the solar system?

Transit method

The most important technique when searching for exoplanets is called the transit method. Its objective is to measure the brightness coming from a star: the passage of an exoplanet between the star and the Earth will cause the luminosity that reaches us to decrease periodically, so we can indirectly infer that there is an extra-solar planet in that region.

his methodology will be used by the TESS mission, given the great success that the Kepler telescope and its successor K2 have had in recent years in finding worlds outside the solar system thanks to this approach.

Astrometry

One of the branches of astronomy is called astrometry, which consists of analyzing the position and movement of stars. Thanks to this type of studies, it is possible to detect exoplanets trying to measure the small perturbation that they exert on the orbiting stars. According to the European Southern Observatory (ESO), to date no extrasolar planet has been found using this technique, which also applies the successful Gaia mission.

Radial velocity tracking

As a star moves in the small orbit generated by the attraction of the exoplanet, it will move closer and further away from us until it completes its own orbit. The velocity of the star along the line of sight of an observer from Earth, explained since ESO, is called radial velocity. The small variations in this parameter cause changes in the spectrum of the star; that is to say, if we track its radial velocity we can detect new exoplanets, as does the HARPS instrument installed in La Silla (Chile).

Pulsar Chronometry

The first extrasolar planets revolved around a pulsar, that is, they orbited a kind of stellar beacon that emits radiation at short and regular intervals. If an exoplanet revolves around a star of this type, the beam that reaches the Earth will be affected, which will serve as a clue to know that an exoplanet actually revolves around that pulsar.

Gravitational microlenses

The latest technique for indirect detection of planets outside the solar system is based on the use of gravitational lenses. The gravity of a large object is able to bend the light coming from distant bodies, amplifying it. In other words, this phenomenon acts as a cosmic-level telescope, which allows us to study objects that emit little or no light, as in the case of exoplanets or black holes.

Why is it difficult to observe exoplanets directly?

When new planets are discovered outside the solar system, press releases and news are often accompanied by eye-catching illustrations that recreate what those worlds would be like revolving around stars other than our Sun. The problem? The direct detection of exoplanetases is extremely difficult: the enormous contrast between the light of the host star and that of the hypothetical world makes it very difficult to obtain real images of extrasolar planets.

Just because it’s complicated doesn’t mean it’s impossible: different teams of astronomers have managed to take direct photographs of exoplanets, as was the case for the first time in 2004 and then in 2008, 2017 or just a few days ago. These blurred images contrast with the brilliant illustrations we are used to when new worlds orbiting stars other than the Sun are unveiled. A reality that also demonstrates how difficult it remains today to discover planets outside the solar system.

 

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