In the Milky Way, there are at least 300 million habitable planets orbiting sun-like stars. The results of this research were published in the Astronomical Journal and were completed by cooperation from the National Aeronautics and Space Administration, the SETI Institute and other scientists around the world.
The idea of searching for habitable planets is to search for similar star systems in the Milky Way according to the parameters of the earth and the sun, and find those “habitable planets” in the habitable zone. These potentially habitable planets must meet at least two conditions: one is that the orbiting star has a similar temperature and age to the sun; the other is that the planet is in the habitable zone at the same distance from the star to the sun.
Being in the habitable zone does not mean that the planet is habitable, but there is a possibility of habitability. Specifically, the habitable zone is a belt-shaped area around the orbit of a star. The planets in this zone can support the existence of liquid water due to their atmospheric pressure and heat, so they are called the habitable zone.
Being in a livable zone is a necessary and insufficient condition for livability. But this is indeed a way to find a second home suitable for human survival.
The mission of the Kepler Project
The latest research results of the scientists this time are based on exoplanet data collected by the Kepler Space Telescope between 2009 and 2018.
The relative size of the Kepler 440b exoplanet and the Earth
The Kepler Space Telescope was launched by NASA in March 2009. Its mission is to search for planets orbiting other stars and similar in size to the Earth. By using a photometer, the brightness of 150,000 stars is continuously monitored in a fixed field of view, and these data are then transmitted back to the earth for analysis. The life of the plan was initially set at 3.5 years, but due to continued failure of the telescope, the entire plan was repeatedly extended until 2018 when the Kepler telescope officially announced its retirement.
Despite the long fate, the Kepler telescope still obtained a lot of valuable data and observation results. The Kepler project has observed more than 530,000 stars and discovered 2,800 exoplanets in its lifetime. In 2015, NASA released data on four planets located in the habitable zone discovered by Kepler. Among them, Kepler 438b, Kepler 442b and Kepler 452b are close to the size of the earth and may be rocks. The fourth is Kepler 440b, which is a super earth.
The radius is 1.86 times that of the earth, and its orbit is in the habitable zone.
Although Kepler’s mission is over, the subsequent data analysis work is still ongoing. This time, scientists have dealt with some thorny issues inferred from Kepler’s data, especially the question of how many sun-like stars in the Milky Way have habitable planets in their orbits. Previously, relevant studies gave different answers, ranging from less than 1% to more than 100% (there are multiple habitable planets around a star).
There are two main reasons for the disparity in analysis results: incomplete data and incorrect detection results in the Kepler data set. This time, the scientists involved in the data analysis used the complete Kepler exoplanet data set for the first time, or used the star data provided by Gaia (the European Space Agency’s mission to map stars in the Milky Way). All this helps to improve the accuracy of the final result and reduce uncertainty. Before that, scientists spent several years analyzing Kepler data to remove uncertainties and ensure that only true exoplanets remain.
Steve Bryson of NASA Ames Research Center in California is responsible for this new work. Its team determined the formation rate of sun-like stars in the Milky Way, the number of stars that may have rocky planets (0.5 to 1.5 times the radius of the Earth) orbiting them, and the habitability of rocky planets.
Bryson and his team predict that, on average, 37% to 60% of the sun-like stars in the Milky Way galaxy have at least one habitable planet orbiting it. If you are optimistic, this number may be as high as 88%; conservatively, this number will drop to 7% (ie 300 million) of sun-like stars in the Milky Way. On this basis, the research team predicts that within 30 light-years of the Earth, there will be 4 sun-like stars orbiting habitable planets.
Exoplanet concept map
Bryson commented on this: “One of the initial goals of the Kepler mission is to accurately calculate this number, and we have always planned to do so.” By implication, Bryson and others finally completed the Kepler set. The original goal, although 2 years have passed since Kepler’s retirement.
The habitability of a planet is related to the presence of liquid water on its surface. Most studies judge the distance between the exoplanet and the Zhu star, and it is not appropriate to be too close or too far. According to Bryson, when observing a particular star, its brightness can be used to determine the amount of heat it transfers to surrounding planets to determine whether it is habitable. Bryson also emphasized that it is imprecise to rely solely on distance to judge. In addition, his team will directly measure the true temperature of the planet.
Although the Bryson team stated that the uncertainty of this study is smaller than before, it is still large. This is mainly because the samples of exoplanets are too small. Kepler has identified more than 2,800 exoplanets, only some of which revolve around sun-like stars. Using this number to infer that the accuracy of hundreds of millions of such planets in the Milky Way is not ideal.
But this is only the first step. The whole point of the Kepler project is to allow scientists to figure out what stellar bodies should be invested more resources in to find extraterrestrial life. Because funds, equipment and observation time are all limited. At the same time, these tools (such as NASA’s James Webb Space Telescope and European Space Agency’s Plato Telescope) can be used to determine whether a potentially habitable exoplanet has an atmosphere or any potential biological features, and this type of research can help Engineers design telescopes more suitable for these tasks.
The premise of finding a second home is to protect the earth
After Kepler satellites retired in 2018, NASA immediately launched the TESS program. TESS refers to the extrasolar planet survey satellite, which was launched into a predetermined orbit by SpaceX’s Falcon 9 rocket in April 2018. Compared with Kepler satellites, the space area scanned by TESS is 400 times larger.
TESS is equipped with 4 wide-angle telescopes and a charge-coupled device (CCD) to transmit data back to Earth every two weeks. It can also transmit full-frame images with an effective exposure time of two hours, allowing scientists to search for unexpected transient phenomena, such as gamma-ray bursts.
“Dark Blue Dot” Photo
As of May 2020, TESS has identified 1835 candidate exoplanets, 46 of which have been confirmed.
Human beings have never stopped exploring extraterrestrial civilization and a second homeland apart from the earth, although this process is full of frustrations and often nothing. As recently as September of this year, a research report in the “Astronomical Society of Australia Publications” magazine showed that researchers at Curtin University in Australia used 4096 antenna arrays to scan a wide area of space, hoping to capture the radio transmission signals of alien civilizations. After scanning 10 million stars, nothing was found. The scope of this search is the largest ever.
Therefore, humans cannot help asking whether there are exoplanets that are as suitable for human habitation as the Earth, or whether there is an extraterrestrial civilization?
Scientists have had many discussions about this. One explanation is called the “earth-specific hypothesis.” Those who support this theory believe that the formation of life requires a combination of various events and factors, which means that the earth may be a lonely existence in the universe.
There are also scientists who believe that although alien civilizations may be common, they cannot cross the interstellar distance or will be destroyed before mastering the technology.
According to German astrophysicist and radio astronomer Sebastian von Horner, the average duration of human civilization is 6,500 years. After that, it will disappear due to external reasons or internal reasons (mental or physical degradation).
Thirty years ago, Voyager 1 flew to the edge of the solar system. Before permanently turning off the camera, NASA manipulated it back to take a “family portrait” of the solar system. In the photo, the earth is a “dark blue dot” which occupies only 0.12 pixels.
Mankind will continue to explore the universe, and the grand plan of finding extraterrestrial civilization and a second home will be inherited from generation to generation. But at the same time, it is even more important to protect this only “dark blue spot” because this is the only home known to date.