The Kepler space telescope is amazing. It has found thousands of exoplanets. Why hasn’t it found life?

Searching for extraterrestrial life and civilization has been the dream of scientists for hundreds of years. Of course, scientists will not be like some common people who will have wild imaginations when they see something they cannot understand, such as a UFO (unidentified flying object), and insist on blaming aliens.

Scientists are talking about evidence. The current evidence is that not even a single cell has been found on other planets in the solar system except the earth.

The first thing is to find planets affiliated with other stars outside the solar system, especially Earth-like planets in the habitable zone. The so-called habitable zone is a region at a suitable distance from the star where the temperature of the planet is neither cold nor hot enough to maintain the existence of liquid water; and terrestrial planets are planets like the Earth with a layer of rock surrounding them and a solid surface.

Because scientists are convinced that habitable zones and Earth-like planets are necessary conditions to find life similar to Earth. Of course, there may be life or civilization in the universe that is completely different from the earth. Under what conditions these life and civilization can breed and evolve, there is currently no discovery or theory, and we will not discuss it today.

Since the advent of telescopes, scientists have been searching for planets outside the solar system for hundreds of years, but it is like finding a needle in a haystack, which is disappointing. It was not until 1992 that two American astronomers, Alexander Walzson and Dale Fryer, discovered for the first time two exoplanets orbiting pulsars, named PSR B1257+12 B and PSR B1257+ respectively. 12 C; in 1995, two Swiss astronomers, Mike Meyer and Didier Queloz, discovered for the first time an exoplanet orbiting a sun-like star, named 51 Pegasi b.

Since then, the search for exoplanets (planets outside the solar system) has become a hot topic among astronomers. Many ground telescopes and scientific project teams have joined the search for exoplanets, and new planets are constantly being discovered; Hubble, the first space telescope launched in 1990, was the first to use direct imaging to discover the planets around Fomalhaut. However, some of these discoveries are "amateur", and even if they are "professional", the efficiency is not high. The scientific community urgently needs a more professional and efficient exoplanet search plan.

The Kepler Space Telescope (sometimes referred to as "Kepler" later), also known as the Kepler Mission, is the world's first specialized spacecraft developed by NASA (National Space Administration) A space telescope to detect terrestrial planets outside the solar system, and was launched from Cape Canaveral Air Force Station in the United States on March 7, 2009.

Johannes Kepler was a German astronomer and mathematician in the 17th century. He discovered the three laws of planetary motion, namely the orbital law, the area law and the periodic law, which provided mankind with He made great contributions to the understanding of the universe and planetary motion, and the Kepler space telescope was named after him in his honor.

The Kepler telescope does not revolve around the earth like the Hubble telescope, but revolves around the sun like the earth. It follows the earth like a follower, but it will gradually move away from the earth. Four years later It is about 0.5 AU (75 million kilometers) from Earth. Its observation field points to the area where Cygnus and Lyra are located, away from the ecliptic plane, so that it will not be obscured by the earth and can continue to observe, but will not be affected by the diffusion of the earth and the moon, nor will it be affected by sunlight leakage. .

"Kepler" has a mass of about 1039 kilograms, a diameter of 0.96 meters, a main mirror of 1.4 meters, a field of view of about 105 square degrees (an apparent diameter of about 12 degrees), and the sky area it sees is about the same as a person's reach. The field of view covered by a fist seen with a straight arm is about the same, with an observation depth of about 3,000 light-years and a breadth of about 0.25% of the entire sky.

As soon as "Kepler" took off, it showed its unique and extraordinary strength. In more than 9 years of service, it has observed more than 5,000 candidate exoplanets, and those that have been confirmed after review There are 2,662 planets, including 49 candidates for habitable zone Earth-like planets, and more than 30 of them have been confirmed.

The global scientific community also uses a variety of methods to search for exoplanets. From the discovery of the first exoplanet in 1995, as of November 16, 2020, the total number of exoplanets discovered is 4,373. Judging from this data, the discoveries of the Kepler Space Telescope alone account for more than 60% of the total number of exoplanets discovered.

There are many methods to scientifically detect exoplanets. For example, the earliest planet was found using the radial velocity method. In addition, there are microlensing methods, astrometry methods, timing methods, occultation methods, etc. wait. These are all calculated through indirect observations and physical principles.

Exoplanets cannot be "seen" directly and can only be "measured" through stellar light changes and gravitational perturbations. Because the viewing angle of exoplanets is too small, only more than 100 planets have been found using direct imaging methods so far. The planets found by this method are all huge "hot Jupiters", that is, young Jupiter-like planets with temperatures between 600 and 2000 K. , most of which are dozens of times more massive than Jupiter.

Direct imaging does not mean that it can be "seen" directly, but can be "measured" through the invisible infrared light changes of stars.

These methods of searching for exoplanets have a very complicated theory, which I won’t go into today. I’ll just talk about the occultation method.

The Kepler Space Telescope uses the occultation method to search for exoplanets. The occultation method, also known as the transit method or the transit method, is a method of determining the existence of a planet by observing the subtle changes in the star's luminosity when an exoplanet crosses the star's surface in the visual direction.

Therefore, the most important instrument installed on "Kepler" is the high-precision space photometer developed by NASA. Its main function is not to obtain a clear image of the planet, but to obtain the luminosity of the star by measuring the star's photometry. changes, thereby calculating the conclusion whether there are planets orbiting it.

"Kepler" observed a sun-like star with m (V) = 12 for 6.5 hours, and could detect a comprehensive luminosity change of 20 ppm (20 parts per million), while a star similar to The luminosity change caused by the transit of the Earth's planet is about 84 ppm.

In this way, "Kepler" directionally observes the luminosity of 100,000 stars in orbit, detects whether there is a transiting planet through subtle changes in the luminosity of the stars, and calculates the revolution period and orbit of the transiting planet. The size, transit depth, planet size, etc. can also be measured by measuring the spectrum, luminosity and other parameters of the planet's parent star to obtain its mass, thereby deducing whether the planet is in the habitable zone.

Before the launch of "Kepler", the exoplanets discovered by the scientific community were basically "hot Jupiters" with large masses and volumes. After the launch of "Kepler", a large number of planets similar to the Earth were discovered. Similar smaller Earth-like planets, the first star system with 6 planets was discovered, the first planet orbiting two stars, the smallest planet orbiting a sun-like star was discovered, and so on.

For everyone’s convenience, I would like to explain the naming rules of planets: after the name of the star, according to the order of the planets close to the main star, they are named b, c, d, e, f, g, h, etc. . The planets discovered by "Kepler" are all preceded by Kepler star names. For example, Kepler 20 is the star name, and Kepler 20e is the fourth-order planet close to the Kepler 20 star.

Kepler 20e and Kepler 20f are the earliest asteroids discovered by humans. The radii of these two stars are 0.8 times and 1.03 times that of the Earth respectively. They are stars about 950 light-years away from us~ Two of the five planets in the Kepler-20 system. Kepler 20 is a sun-like star with a mass of about 91% that of the sun. Its surface temperature is slightly lower than that of the sun, about 5466K. These two Earth-like planets are too close to their host stars and are generally considered unsuitable for life, while several other planets are Jupiter-like planets.

But this discovery is an important turning point, ending the history of no discovery of exoplanets.

The smallest planet discovered by "Kepler" is a planet in the Kepler 42 star system. This is a red dwarf star only 126 light-years away from us. Its mass is only 0.13 times that of the sun, and its luminosity is only It is 0.24% of the Sun. It is currently found to be accompanied by three planets. Among them, Kepler 42b has a volume of about 0.78 times that of the Earth, Kepler 42c has a volume of about 0.73 times that of the Earth, and Kepler 42d has a volume that is only 1.97 times that of Mars. It is the smallest planet ever discovered.

Kepler 186 is a red dwarf star about 500 light years away from us. Its mass is about 0.54 times that of the sun. It is accompanied by five planets, among which Kepler 186f is the most eye-catching. It is about 1.1 times the size of the Earth and is in the habitable zone. Some scientists have analyzed that this is probably the most promising planet for life. However, some analyzes have found that although it is a cousin of the Earth, it is not a twin. In the outer edge of the habitable zone, it is likely to contain water ice rather than liquid water, which may not be suitable for habitation.

Kepler 1649c was an unexpected discovery. This planet that was missed during the review was obtained from data taken by Kepler in 2020, a few years after Kepler retired. Discovered by accident while re-reviewing. This planet, which is about 300 light-years away from us, is 1.06 times the size of the Earth and is a rocky planet. According to theoretical calculations, it is in the habitable zone of the star. It is considered to be the most Earth-like planet to date.

However, what is disappointing is that because its main star is a red dwarf star with a very small mass, the habitable zone is very close to the main star. The orbital period of Kepler 1649c is only 19 days, which is not suitable for life. exist.

"Kepler" also discovered the Kepler 90 galaxy, which is a solar-like system located 2,545 light-years away from us in the constellation Draco. It is the star system with the most exoplanets discovered so far, and it is the same as ours. The solar system is also accompanied by 8 planets.

"Kepler" has discovered a wide variety of exoplanets, including planets orbiting 2 or even 4 stars. There are many planets such as Earth's older cousins ??and Earth 2.0. Here It’s impossible to express them one by one.

In the past few years, the search for extraterrestrial planets has discovered many planets that look like Earth, such as Big Cousin and Earth 2.0. So are there aliens on these planets?

In fact, "Kepler" didn't know it either, and no one knew it. Because although humans continue to make progress in observing exoplanets, none of them can really "see clearly". Those so-called big cousins, Earth 2.0, etc. are just nonsense and are far from the ring shape of the Earth.

Just like it is difficult to find two identical leaves or two fingerprints on the earth, it is also difficult to find a planet that is exactly the same as the earth in the universe.

There may be extraterrestrial life and civilization in the universe, and there may also be life or civilization that is completely different from the earth, but so far there is no evidence, not even any clues in this regard. In this case, as a rigorous popular science article, we cannot speculate casually.

In 2018, the Kepler Space Telescope had to retire honorably due to running out of fuel. So who will fill its position? Will planetary search plans be shelved in the future? You don’t have to worry, the new generation of “Planet Hunter” developed by NASA has long been born. It is the new generation of “Transiting Exoplanet Survey Satellite”, or TESS for short. It was successfully launched on April 19, 2018, Beijing time. It was launched into space, replacing the older generation of "Kepler" and continuing its unfinished business.

TESS’s observation area is 400 times that of Kepler, which means it can conduct searches throughout the sky. It divides the sky into 26 different areas and surveys the sky every 27 days, focusing on 200,000 of the largest and brightest stars close to the earth. Scientists hope to use it to discover at least 20,000 exoplanets and find more habitable planets among them.

Searching for exoplanets is, on the one hand, to discover extraterrestrial life or civilization, and on the other hand, it is also to search for the most livable places in the future when the earth becomes uninhabitable and humans will have to immigrate to alien planets. This is a rainy day preparation for human reproduction, and it is a great move by a group of far-sighted scientists, industrialists, and politicians. As a member of the human race, I am gratified to have such outstanding people of my kind.

What do you think? Discussions welcome, thanks for reading.