2023-04-17 17:36:28
Humans already know thousands of planets around other stars. We call them extrasolar planets, or exoplanets, and to find them we have applied techniques that could be within the reach of supposed extraterrestrial civilizations. Could an alien species with resources similar to ours detect or have detected Earth?
Suppose they could know the radial velocity method. Would they find us?
The ‘hot Jupiters’
The first extrasolar planet around a normal star was found in 1995 and left the astronomical community stumped.
The planet of the star 51 Pegasi inaugurated the category of “hot Jupiters” as a giant and massive world (double the diameter of Jupiter and half its mass), but located at a distance from its star of around one seventh of the that mediates between Mercury and the Sun. One would have expected that in the universe there would be large planets like Jupiter, but nobody expected them so close to their stars.
In these characteristics lies the key to its detection.
An exoplanet is easier to find the larger it is (in diameter or mass) and the closer it is to its star. The planet of the star 51 Pegasi is known today as Dimidio, while its star is called Helvetios.
In contrast to this first hot Jupiter, Earth is a tiny world orbiting much farther from the Sun. For a civilization with technology similar to ours, would detection be feasible?
Blinding light
Direct detection of the Earth, through reflected light, is ruled out. The orbits of any planets that may be around the stars appear tiny when viewed from afar. This destroys the possibility of distinguishing the Earth with means similar to ours if you want to do it thanks to the radiation it reflects, because it is lost in the glare of the Sun. A system with two planets captured by direct image: TYC 8998-760-1 . The star appears in the center affected by artifacts caused by the system used to dim its apparent brightness, while the two planets appear in the lower left, aligned. The other points are background stars. The feat is possible in this case because the star is close to the Sun, and its planets are huge and far away from it. ESO/Bohn et al.
There are very few cases of exoplanets captured directly by reflected light, in very favorable situations and using complex techniques. But there is not the slightest possibility of finding the Earth with those resources through interstellar space.
The aliens are blinded by sunlight, so they must resort to indirect methods.
Consider that of radial velocity, the one that led to the 1995 find of Dimidius.
«Bamboleo, bambolea…»
Forget everything you learned in school, because it is false that the Earth revolves around the Sun. We are not going to tell you that the Earth is flat (perhaps we will leave that surprise for another article), but we must clarify that it is not true that the Sun is in the center of the Earth’s orbit, not even in one of its foci if we consider it an ellipse.
Classical physics teaches that both the Sun and the Earth move throughout a year, and both do so around the common center of mass of the system they make up.
Of course, the contrast in masses between the Earth and the Sun is so enormous that this “center of gravity” falls very close to the star, so much so that… it lies inside the Solar sphere! But the important thing is that it does not coincide with the center of the luminous star. Therefore, the Sun, by having the Earth next to it, experiences a certain wobble. This happens to any star with planets, and the first extrasolar planets, including Dimidium, were discovered thanks to this effect. The challenge is that this reflex wobble of the stars occurs with minuscule velocities.
Both the planet and the star move around the common center of mass. Consequently, the star executes a to-and-fro motion in each planetary orbit.
The star Helvetios oscillates at more than 200 kilometers per hour every four days, in response to the attraction of Dimidius. It is not an astronomical speed, but it is more than considerable on a human scale. So much so that applied to a car would be worth a more than deserved fine for speeding. The reflex speed of a star is reduced for distant planets and for light planets.
Consider the examples of the Solar System:
Mercury, despite its closeness to the Sun, is so light that it wobbles the star at a paltry millimeter per second. At the opposite extreme, the record is held by Jupiter, which induces a reflex solar movement of more than 12 meters per second, almost 60 kilometers per hour, the speed of a moped on the road.
The reflex movement of the Sun due to the Earth amounts to only a third of a kilometer per hour: it does not reach ten centimeters per second.
Measure speed through color change
To detect the reflex motion of stars endowed with planets, the Doppler effect is used. Starlight is broken down into its constituents and compared to a light source at rest, which makes it possible to detect very slight color changes in the radiation caused by the speed of the star in its periodic swing. This procedure requires very precise measurements, repeated and distributed over a time interval that covers several times the orbital period of the planet.
Astronomy has reached the physical limit imposed by the agitation of the stellar photosphere itself, which places the best precision around one meter per second, just under 4 kilometers per hour.
As regards the time needed to detect the signal due to the planet, for Dimidius it could be enough to take data over a week, but to detect the Earth would require a minimum of two years of observation, while for Jupiter it would take almost a year. quarter of a century.
The Edge of the impossible
Extremely clever, ultra-sophisticated techniques are being developed that aim to improve the accuracy of the Doppler method to the threshold of several centimeters per second. There are hopes that this can be achieved with extremely large telescopes within the next few decades.
Therefore, finding Earth-like planets around Sun-like stars could be on the edge of the possible by the middle of the 21st century. We might expect the same from an extraterrestrial civilization similar to ours.
Detecting Earth by the radial velocity method would be a very difficult technical feat for aliens who were on par with humans in technological development, something on the verge of impossible.
The Doppler method has made it possible to detect planets similar in certain aspects to the Earth, but always around stars much lighter than the Sun, because the same planet is capable of causing a more intense reflex wobble if, other things being equal, it is placed next to a less massive star.
Related news
The Doppler method looks unpromising, too borderline for aliens to discover. What can we say about other approaches? If we consider the possibility of discovering the Earth from another planetary system with the method of transits, would they find us?
This article was originally published on The Conversation. Read the original.
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