June 14, 2021

Discovering exoplanets through stellar auroras



We live exciting times for the search for planets outside our Solar System. In just 25 years we have gone from not giving credit to the existence of planets around other stars to count by thousands the known planetary systems. Some of those systems are miniature versions of the Solar System; others have planets so close to the star that their temperature never drops thousands of degrees. In a few cases we have even been able to measure how temperatures are distributed on the planet’s surface and draw very crude maps; thermal maps, but maps after all.

And despite all these successes, the search for exoplanets remains a very difficult task. Stars are always much brighter than planets, and they are so close to them that it is often impossible to obtain an image of the planetary system. All these things we have said we find out by playing ingeniously with the light that reaches Earth, taking advantage of the fact that a small fraction of that light does not come from the star, but from the planet, and extracting from it all the information we can. But, with everything and with it, what we cannot do is miracles. Studying an exoplanet is much easier if it passes in front of the star and covers a bit of its light, and becomes more and more complicated if this does not happen.

That is why it is always good news that a new method for finding exoplanets appears. And this is what an international collaboration of scientists in the magazine just announced Nature Astronomy.

Listen to the stars on the radio

The idea is, in principle, simple: if the star is so bright that we are dazzled, try to find the planets in a wavelength where the star is dark and the planet is bright. Easy, apparently, but in delicate practice. The best option may be to try to observe the planet-star system in radio waves. The stars are not, in general, very bright on the radio, except for a few that suffer violent processes or that spin extremely fast. The planets, on the other hand, have a very natural way to shine on the radio: the auroras.

An aurora It is nothing but particles ejected by a star (especially protons and electrons) that when they reach a planet collide with their atmosphere and make it shine. If the planet has a magnetic field, like Earth, it catches the particles and accelerates them, crashing them harder against the atmosphere. This occurs with more intensity in the polar regions, which is where the magnetic field “points”, and that is why the auroras are more easily seen in Norway than in Spain.

All this movement of particles around the planet generates a good amount of radio waves, with characteristic properties that we have studied not only on Earth, but also on other planets in the Solar System. Thus, it would make sense to look for these radio emissions on planets around other stars: the star would be relatively silent in this channel and the planets could become very strident, depending on the amount of particles available and the intensity of their magnetic field. .

Unfortunately, when we go to the paper and do the calculations we find a click: the emissions of most planets cannot reach the earth’s surface. The ionosphere of our planet acts as a kind of “mirror” for low frequency radio waves, and most planets would emit at these frequencies. Only some giant planets, with very intense magnetic fields, could be visible from the Earth’s surface; To the rest, however bright they are, we are blind. To see them we would need a low frequency radio observatory in space, but for now we have none.

It seems, then, that this idea of ​​radio has led us to a dead end. Should we abandon it? Not yet.

A close example: Jupiter and its moons

A look at one of our planetary neighbors can give us a clue as to where we should continue: let’s take a look at the auroras of Jupiter, the giant of the Solar System. Jupiter is so large that it has a magnetic field lord and, of course, that produces beautiful auroras at its poles. But these auroras have something we don’t see on Earth: three points that they move, circling around the magnetic pole.

These “auroral points” are produced by the moons of Jupiter. Jupiter’s magnetic field is not only intense, but also extensive, and engulfs the orbits of the four largest moons, the Galilean satellites. When these moons move through Jupiter’s magnetic field they disturb him, as if they were ducks swimming on the surface of a pond. Io, the innermost moon, is surrounded by dust that expel its more than 400 volcanoes into space; Ganymede, the largest moon, has its own magnetic field. For one reason or another, when these moons cross Jupiter’s magnetic field they leave a mark on it, a trail. These steles are transported by the magnetic field to the planet’s poles, where they collide with the atmosphere and generate those luminous points, which revolve around the pole synchronized with the moons.

How can this help us in our search for exoplanets? Well, if we have exoplanets very close to its star, enough to be “furrowing” the magnetic field of the star in the same way that Jupiter’s moons do … that planet will also generate an auroral point, but this time in the star The planet will generate a stellar aurora.

Like every dawn, those points generated by the planets at the star’s pole are going to emit radio waves. And this time we are in luck: the magnetic fields of the stars are much more intense than those of the planets, so those radio waves are going to cross the Earth’s ionosphere. Those auroral points should be able to see them.

A preliminary detection

That is precisely what has just been announced in the article published a few days ago: the detection of radio waves compatible with a stellar aurora from a star that, otherwise, is not bright on the radio. That star is called GJ 1151 and is a red dwarf that is 27 light years from us in the constellation of the Big Dipper. What this group of astronomers has discovered is a radio broadcast that does not resemble the typical “radio noises” that stars usually make and that only appears intermittently.

GJ 1151 was observed in four intervals of about eight hours each. In three of them the star remained silent; in the room it shone with intensity in low frequency radio waves. The interpretation of this observation is that the star has a planet that orbits near it, immersed in its magnetic field, and that produces an auroral point near the star’s pole. When that point “points” towards us, we see it and the star lights up on the radio. When he looks away, the star is silent.

This interpretation is the most reasonable at the moment, especially after another article published in Astrophysical Journal Letters argue that GJ 1151 has no giant planet around it. A giant planet could produce this same effect if we were seeing their auroras, and not those of the star, but that possibility seems ruled out. So it seems plausible that GJ 1151 has a small planet around it, perhaps with its own magnetic field.

In any case, it is necessary to take this result still with caution. It is a first detection, and the authors have built a good case in favor of this detection method, but only new observations of the star, which prove that the radio emission is periodic, and others that find evidence of a planet with a period similar to radio broadcasts, they will confirm if this interpretation of what we have seen is correct. If it is, we will have seen the birth of a new way to detect exoplanets.

DON’T KEEP IT UP

  • This “auroras method” for detecting exoplanets will not be useful for all possible planets. Only those who orbit near the stars, in the area where they can create those “steles,” will produce a radio signal.
  • This method will be especially interesting to look for planets around red dwarfs. Being very cold stars, the planets that orbit near them can be at the appropriate temperature to house liquid water. And if they also have a magnetic field that protects them from stellar wind, better than better.
  • In no case do we intend to “photograph” the stellar auroras. The idea of ​​this method is to look for stars that are not violent and that, however, emit on the radio. If we observe that these radio broadcasts change periodically we may interpret them as an aurora generated by an exoplanet.

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