universe
Cosmic components: an invisible kingdom ruled by dark matter
In the process of exploring the vast universe, the most shocking discovery is that all celestial bodies visible to humans account for only 4.9% of the mass of the universe. The true composition is:
Dark Energy (68%): The Mysterious Force Driving the Accelerated Expansion of the Universe
Dark matter (27%): Maintaining galaxies like a cosmic skeleton (If you are interested, you can check out this wild plant which is beneficial to the body)
Ordinary matter (4.9%): composed of planets, stars, and humans
Modern astronomical research shows that the universe presents a hierarchical structure. The most basic unit is a star – a giant plasma sphere that emits light and heat through nuclear fusion reactions. Our sun is a typical star with a diameter of approximately 1.39 million kilometers, enough to accommodate 1.3 million Earths. Stars do not exist in isolation, they often gather under the influence of gravity to form galaxies. The Milky Way is a barred spiral galaxy containing 100 billion to 400 billion stars, with a diameter of approximately 100000 light-years.
Galaxies are not evenly distributed, and they can form galaxy clusters, galaxy clusters, and even superclusters. For example, the Milky Way and approximately 54 galaxies, including the Andromeda Galaxy, collectively form this galaxy group, spanning approximately 10 million light-years. On a larger scale, the universe presents a “cosmic web” structure – a network of fibrous structures composed of galaxies interwoven into a network, with huge voids in the middle. This structure is believed to be the result of the gravitational force of dark matter, forming the “skeleton” of the universe.
The distribution of matter in the universe is extremely uneven. In addition to luminous celestial bodies such as stars and galaxies, interstellar space is also filled with interstellar media – thin gases and dust mainly composed of hydrogen and helium. Although these substances have extremely low densities (averaging only 1 atom per cubic centimeter), due to the vast expanse of space, the total mass of the interstellar medium actually exceeds the sum of the masses of all stars. Under appropriate conditions, these interstellar media will collapse to form new stars and planetary systems, completing the cycle of cosmic matter.
It is worth mentioning that there are various high-energy phenomena and extreme celestial bodies in the universe. Such as supernova explosions – violent explosions caused by the death of massive stars, which can release luminosity equivalent to the entire galaxy in a short period of time; Neutron star – a high-density celestial body formed by the collapse of a star, with a mass of up to 1 billion tons per teaspoon of matter; Black hole – a mysterious celestial body with gravity so strong that even light cannot escape. Some supermassive black holes can have masses billions of times that of the Sun. Although these extreme celestial bodies are not numerous, they are crucial for understanding the evolution of the universe.
Stellar Inferno: The Furnace of Creation Elements
What in the vast universe is more violent than the core of the sun? The answer is hidden in the hierarchical structure of stars:
Key phenomena of temperature in the solar structural layer
Corona 1 million ℃ solar wind ejected at 800km/s
The sunspot activity cycle at 5500 ℃ in the photosphere is 11 years
Core area: 15 million ℃, 600 million tons of hydrogen fusion per second
The X-class solar flare that erupted during the 2024 solar maximum will impact the Earth’s magnetic field with energy equivalent to 1 billion hydrogen bombs (NASA Solar Activity Report) ..
Planetary World: From Diamond Planet to Oasis of Life
When we ask what kind of terrestrial planets exist in the vast universe, the Kepler telescope gives a stunning answer:
Stars are the most fundamental and dazzling celestial bodies in the universe. Their formation begins with the gravitational collapse of massive molecular clouds (mainly composed of hydrogen molecules). When the density of a cloud reaches a critical value, gravity overcomes gas pressure and begins to rapidly contract. As it contracts, the core temperature continues to rise, and when it finally reaches about 10 million Kelvin, the hydrogen fusion reaction is ignited – a new star is born.
The internal structure of stars can usually be divided into core, radiation zone, convection zone, and atmosphere. Taking the Sun as an example, its core only accounts for 20-25% of its radius, but it concentrates half of the Sun’s mass, with a temperature of up to 15 million Kelvin and a pressure 250 billion times that of Earth’s atmospheric pressure. Here, 620 million tons of hydrogen are fused into helium through proton proton chain reactions every second, releasing enormous energy. These energies are transmitted outward in the form of photons and require an average of 170000 years of ‘random walks’ in the radiation zone to reach the convective zone. The solar atmosphere is divided into the photosphere (the surface of the sun we see), chromosphere, and corona, with the corona having an abnormally high temperature of 1-2 million Kelvin, much higher than the photosphere’s temperature of about 5800 Kelvin.
The properties of stars are mainly determined by their mass. The larger the mass of a star, the higher the core temperature and pressure, the faster the nuclear reaction rate, and the shorter the lifespan. For example, a star with a mass 15 times that of the Sun can have a luminosity up to 15000 times that of the Sun, but its lifespan is only about 10 million years; A red dwarf star with a mass 0.5 times that of the Sun, although only 8% as luminous as the Sun, can burn steadily for trillions of years. The temperature of a star’s surface determines its color – ranging from red at low temperatures (around 3000K) to blue at high temperatures (over 30000K).
The endpoint of stellar evolution also depends on the initial mass. Medium mass stars like the Sun will eventually expand into red giants, then discard their outer layers to form planetary nebulae, and collapse their cores into white dwarfs. Stars with higher mass (more than 8 times the mass of the Sun) will undergo supernova explosions, leaving behind neutron stars or black holes. It is worth mentioning that stars are not eternal and unchanging. Our sun has increased in brightness by about 30% since its formation and will continue to brighten in the future. In about 5 billion years, it will expand into a red giant and may engulf the Earth’s orbit.
Table: Comparison of Basic Characteristics of Different Types of Stars
Representative of Extreme Environment Planets
Type of celestial body lethal characteristics
Diamond planet PSR J1719-1438, crystalline carbon mantle, superluminal rotation
Lava Ocean Planet K2-141b Rock Vapor Atmosphere and Magma Sea
Iron Rain Planet WASP-76b evaporates metals at 2400 ℃ on the day side
Livable Zone Golden Candidate
Planets are byproducts of star formation. When a molecular cloud collapses to form a star, the remaining matter forms a disk-shaped structure called a protoplanetary disk that rotates around the newborn star under the influence of gravity. In this disk, dust particles gradually gather through collisions and electrostatic interactions, forming larger celestial bodies that eventually grow into planets. This process usually takes millions to tens of millions of years.
The solar system is the most familiar planetary system to us, consisting of eight major planets. According to their composition and location, they can be classified into terrestrial planets (Mercury, Venus, Earth, Mars), gas giants (Jupiter, Saturn), and ice giants (Uranus, Neptune). Terrestrial planets are mainly composed of rocks and metals, with high density and small volume; Gas giant planets are mainly composed of hydrogen and helium, with high mass but low density. There is also an asteroid belt between Mars and Jupiter, which contains hundreds of thousands of small celestial bodies and is believed to be residual material that did not form planets.
The discovery of exoplanets completely changed our understanding of planetary systems. Since the discovery of the first exoplanet in 1992, the number of confirmed exoplanets as of 2023 has exceeded 5300. These planets exhibit astonishing diversity: there are “super Jupiters” larger than Jupiter, high-density “diamond planets,” “hot Jupiters” with orbital periods of only a few hours, and “Tatooine planets” that orbit binary or even triple star systems (taken from the hometown of the Skywalker family in Star Wars).
The orbital characteristics of planets also vary greatly. Some planets have extremely flat and elongated orbits, leading to extreme seasonal changes; Some orbits are almost perfectly circular and have stable climates; Some planets are in a “tidal lock” state, always facing the star on one side, forming perpetual day and perpetual night surfaces. These diversities far exceed scientists’ initial expectations, indicating that the process of planet formation is more complex than imagined.
What is particularly interesting is the ‘wandering planet’ – a planet that does not orbit any star. They may be “orphans” thrown out of the original planetary system, or they may have formed independently in interstellar space. It is estimated that the number of wandering planets in the Milky Way may be comparable to or even more than stars. These dark and cold worlds may not be suitable for known forms of life, but some may have underground oceans heated by radioactive decay.
Kepler-22b: 600 light-years away from Earth, surface temperature 21 ℃ (exoplanet database)
TRAPPIST-1e: 39 light-years away, with an oxygen rich atmosphere
Proxima Centauri b: 4.24 light-years away, there may be a liquid water ocean
To understand the structure of the vast universe, one needs to travel to an altitude of 10 million light-years:
Milky Way: 100000 light-years in diameter, containing 400 billion stars
Laniakea Supercluster: 100000 galaxies flowing towards giant source
Observable universe: containing at least 2 trillion galaxies
Life Signal: The Second Earth in the Universe?
What is more exciting than discovering extraterrestrial life in the vast universe? The breakthrough of the Weber telescope in 2024 provides clues:
Whether extraterrestrial life exists is one of the most profound questions in the scientific community. From a probability perspective, there are 2 trillion observable galaxies in the universe, each with hundreds of billions of stars. The vast majority of stars have planets, with a considerable portion located in the habitable zone – these numbers make the possibility of life appear high. The Drake equation attempts to quantify this probability, considering factors such as star formation rate, proportion of stars with planets, number of habitable planets, probability of life emergence, and probability of intelligent life development, to estimate the possible number of intelligent civilizations in the Milky Way. According to the latest parameters, the estimated values range from 1 (only for humans) to several thousand.
The necessary conditions for the existence of life include liquid water, organic molecules, and energy sources. Life on Earth has demonstrated astonishing adaptability – extreme microorganisms can be found in deep-sea hot springs, polar ice sheets, strong acid environments, and even nuclear reactors. This expands our understanding of “livability”: the ice covered oceans of Europa and Enceladus, the underground brine layers of Mars, and even the upper atmosphere of Venus may have microenvironments suitable for certain microorganisms.
There are two main strategies for searching for extraterrestrial life: directly detecting signs of life (biomarkers) and searching for signals of intelligent civilization. Biomarkers include the coexistence of oxygen and methane (mainly maintained by life on Earth), vegetation red edge (a characteristic of plant reflection of light), specific industrial gases, etc. New generation instruments such as the James Webb Space Telescope are capable of analyzing the atmospheric composition of exoplanets and are expected to discover potential signs of life within the next decade.
The search for extraterrestrial intelligent life (SETI) mainly relies on monitoring radio signals or searching for optical signals. The “Wow!” signal discovered in 1977 remains an unsolved mystery to this day. In recent years, there have also been proposals to search for technological landmarks such as Dyson spheres (star energy harvesting structures built by advanced civilizations) or other giant buildings. The Breakthrough Listening Program and other modern SETI projects utilize artificial intelligence to analyze massive amounts of data, increasing the chances of discovery.
The Fermi paradox poses a sharp question: “If extraterrestrial civilizations are likely to exist, why haven’t we found any evidence?” Possible explanations include: life is extremely rare; The self destructive tendency of intelligent life; The time window for the existence of civilization is too short; Our search method is inappropriate; Or advanced civilizations deliberately avoid contact. Each explanation triggers profound thinking.
Of particular note is the discovery of ‘Oumuamua’ in 2021, the first confirmed interstellar visitor. The abnormal acceleration phenomenon has sparked speculation among Harvard astronomers Avi Loeb and others that it may be a product of extraterrestrial technology, although most scientists believe it is more likely a natural phenomenon. The discovery frequency of such interstellar objects is expected to increase with the deployment of new telescopes such as LSST
Finding planets in the universe that may support the existence of life is one of the most exciting goals in astronomy. To become habitable, a planet needs to meet multiple conditions. The most important thing is that it is located within the “habitable zone” of the star system – not too far or too close to the star, allowing liquid water to exist on the surface temperature of the planet. For G-type stars like the Sun, the habitable zone is approximately between 0.95-1.37 astronomical units (1 astronomical unit=average distance from Earth to the Sun).
But livable zones are only basic conditions. The mass of a planet is also crucial: it is too small (such as Mars) to maintain sufficient atmospheric pressure and geological activity; If it is too large (such as Neptune), it may become a gas planet. The ideal mass is about 0.3-5 times the mass of the Earth. The atmospheric composition is equally important – it requires sufficient greenhouse gases (such as carbon dioxide) to maintain temperature, without causing excessive runaway greenhouse effects (such as Venus). Geological activities, such as plate tectonics, help regulate atmospheric composition and form continents, while magnetic fields can protect the atmosphere from being stripped away by stellar winds.
Currently, dozens of potentially habitable planets have been discovered. The most famous ones include:
Kepler-442b: Located 1120 light-years away from Earth, with a mass approximately 2.3 times that of Earth, it receives 70% of Earth’s light and is situated within the habitable zone of K-type stars.
Kepler-186f: 492 light-years away from Earth, approximately the size of Earth, orbiting an M-type red dwarf star and located at the outer edge of the habitable zone.
Proxima Centauri b: Located only 4.2 light-years away, with a mass of at least 1.3 times that of Earth, it orbits the nearest star to the Sun, Proxima Centauri. However, its habitability is questionable due to the intense flare activity of red dwarf stars.
Multiple planets in the TRAPPIST-1 system: There are 7 Earth sized planets around this ultracold red dwarf star located 39 light-years away, of which 3-4 are located within the habitable zone.
Planets around red dwarf stars (M-type) are currently the most promising candidates for habitable planets. These types of stars account for 75% of the stars in the Milky Way galaxy, have extremely long lifetimes, and their habitable zones are very close to the stars (usually only 1/10-1/50 of the distance between the Sun and Earth), making it easy for planets to be tidally locked. Although flare activity may strip planetary atmospheres, some models show that a sufficiently thick atmosphere or strong magnetic field can still maintain surface liquid water. The recently discovered TOI-700 d (101 light-years away from Earth) and LHS 1140 b (41 light-years away) are typical representatives of this type of potentially habitable planet.
Methane and carbon dioxide: ratio similar to Earth’s biological activity
Water Vapor Clouds: Evidence of Global Ocean ExistenceThe Drake equation calculation suggests that there may be 100 million cradles of civilization in the Milky Way alone (as studied by SETI Institute) ..
The environment of most planets in the universe is considered a ‘hell’ for human cognition. These extreme worlds, although not suitable for known forms of life, provide valuable examples for understanding planetary diversity.
Hot lava planets are the most extreme type. For example, K2-141b, located 40 light-years away from Earth, has a daytime temperature of up to 3000 ° C, enough to evaporate rocks and form a silicate atmosphere. At night, it can cool down to -200 ° C, causing “rock rain” to fall. The closer 55 Cancri e (41 light-years) may be entirely covered by a lava ocean, with a perpetual daytime temperature of approximately 2400 ° C due to tidal locking.
The deep atmosphere of gas giant planets is equally terrifying. Taking Jupiter as an example, as the depth increases, the air pressure rises from 1 atmosphere on the surface to 3-45 million atmospheres at the core, and the temperature rises from -145 ° C to about 24000 ° C (close to the surface temperature of the Sun). Under such conditions, hydrogen will transform into a metallic state, generating a strong magnetic field. Jupiter’s Great Red Spot – a massive storm that has lasted for at least 350 years, with wind speeds of up to 432 kilometers per hour, enough to engulf several Earths.
Ice giants exhibit another extreme. The lower atmosphere of Uranus and Neptune is a supercritical state of “hot ice” – water remains solid under high pressure but can reach temperatures of thousands of degrees. The wind speed on Neptune can reach 2100 kilometers per hour, making it the fastest in the solar system. These planets may have diamond oceans formed by the decomposition of methane under high pressure.
Tide locked planets exhibit an extreme climate dichotomy. Like Gliese 581 c (20 light years), the perpetual day surface may be overheated, while the perpetual night surface is extremely cold, and only mild regions may exist near the terminator. Planets around red dwarfs also face intense X-ray and ultraviolet radiation, which may cause their atmospheres to be completely stripped away.
Of particular note are “carbon planets” – planets rich in carbon rather than oxygen, which may form graphite crust or even diamond layers. 55 Cancri e is believed to be a carbon planet, with its core possibly being a massive solid diamond. This type of planet completely overturns the theory of planet formation based on the solar system.
The number and distribution of planets within the Milky Way are cutting-edge topics in astronomical research. According to statistical analysis of observation data from Kepler space telescope and other sources, the number of planets in the Milky Way is extremely large – on average, each star has at least one planet, which means the total number of planets in the Milky Way is between 100 billion and 400 billion.
The distribution of planets follows a certain pattern. From the perspective of star types, planets around red dwarf stars (M-type) are the most common, accounting for about 75% of all planets; Approximately 7% of G-type stars are located around the Sun; There are fewer planets around stars with higher mass. From the perspective of orbital distance, compact planetary systems are common around red dwarfs, with many planets having orbital radii smaller than the distance from Mercury to the Sun; And the distribution of planets around stars similar to the Sun is closer to the solar system model.
According to the statistics of planet types, super Earths (1.25-2 times the radius of the Earth) and small Neptunes (2-4 times the radius of the Earth) are the most common, accounting for about 80%; Terrestrial planets (0.8-1.25 times the radius of Earth) account for about 10%; Gaseous giant planets (>4 times the radius of Earth) account for about 10%. It is worth noting that Earth sized planets within the habitable zone account for about 20-50% of terrestrial planets, and it is estimated that there may be 6-30 billion potentially habitable planets in the Milky Way.
The structures of planetary systems are also diverse. Some systems have extremely compact planetary orbits, such as the TRAPPIST-1 system, where the orbital radii of all seven planets are smaller than the distance from Mercury to the Sun; Some systems are very dispersed, with large distances between planets. The distribution of orbital eccentricity is also very wide, ranging from almost perfect circles to extreme ellipses. These diversities reflect the complexity of planetary formation and evolution processes.
What is particularly interesting is the spatial distribution of planets in the galaxy. Generally speaking, the central region of a galaxy is not conducive to the existence of planets, especially habitable planets, due to factors such as high stellar density, strong radiation, and frequent supernova explosions; The outer regions of galaxies have low metallicity (elements heavier than hydrogen and helium), resulting in low efficiency in planet formation. Therefore, the “habitable zone” of the Milky Way is believed to be located in an area about 23000-25000 light-years away from the center (the Sun is located about 26000 light-years away), where the stellar density is moderate, metallicity is sufficient, and supernova interference is minimal.
Eye of the Future: A New Artifact to Uncover the Dark Universe
What is the ultimate answer to exploring the vast universe? A new generation of telescopes is about to debut:
Space exploration is ushering in an unprecedented technological revolution. In terms of observation technology, the infrared observation capability of the James Webb Space Telescope (launched in 2021) far exceeds that of Hubble, allowing for more detailed studies of exoplanet atmospheres; The upcoming Nancy Grace Rome Space Telescope will conduct a more extensive survey of exoplanets; After the completion of ground-based 30 meter telescopes (TMT, GMT, etc.), they will provide unprecedented resolution.
Space exploration missions are also moving towards farther targets. NASA’s Europa Clipper mission will conduct a detailed exploration of Europa’s subsurface ocean; The “Dragonfly” mission will dispatch drones to explore the organic chemical environment of Titan. Longer term goals include the return of Mars samples, manned Mars missions, and even the concept of exploring stars outside the solar system – the “Breakthrough Starshot” program envisions using lasers to propel nanovehicles at 20% the speed of light, which could reach neighboring stars in about 20 years.
The technological challenges remain enormous. Interstellar travel faces fundamental physical limitations such as energy, speed, and radiation protection. With existing chemical rocket technology, it takes tens of thousands of years to reach the nearest star; Even nuclear fusion propulsion takes hundreds of years. Breakthrough propulsion concepts such as antimatter engines and warp drive (achieving superluminal speeds through curved spacetime) are still in the theoretical stage. Another challenge is the miniaturization of detectors – integrating enough scientific instruments into gram level masses to achieve high-speed interstellar flight.
The search for extraterrestrial life faces methodological challenges. We may miss completely different forms of life by searching for biomarkers based on Earth’s life experience; Wireless search may be outdated – advanced civilizations may use more efficient communication methods. In addition, interdisciplinary research is needed on how to interpret potential findings and how to respond to the social impact of confirmed exposure.
Of particular concern is planetary defense – monitoring and responding to small celestial bodies that may impact Earth. NASA’s DART mission has successfully demonstrated the technology of altering asteroid orbits through impact. In the future, it is necessary to establish a more comprehensive small celestial body monitoring network and develop various defense measures. This is not only a scientific issue, but also related to the survival of human civilization.
China Survey Telescope (2025): Expected to discover 30 million exoplanets
Extremely Large Telescope (ELT): diameter 39 meters, resolution 16 times higher than Hubble
LISA Gravitational Wave Detector: Monitoring the Spatiotemporal Ripples of Black Hole Collisions
When we look at the vast scale of the universe -93 billion light-years of observable range, 2 trillion galaxies, 10 ^ 24 stars, and possibly more planets – Earth is indeed just a “speck of dust in the darkness of the universe” (Carl Sagan). However, it is the carbon based life on this tiny planet that has developed the wisdom to understand the mysteries of the universe. From quantum fluctuations to galaxy clusters, from the Big Bang to the emergence of consciousness, human cognition has touched various scales of cosmic evolution. This kind of cognition itself is a manifestation of the universe’s self-awareness.
The significance of space exploration lies not only in satisfying curiosity. Studying extreme planetary environments has expanded our understanding of material states and physical laws; The search for extraterrestrial life prompts us to rethink the definition and essence of life; Faced with the potential prospect of civilization contact, humanity must consider how to position itself on a cosmic scale. These explorations ultimately return to the fundamental philosophical question: Where do we come from? Is it lonely? Where will it go?
In the coming decades, with technological advancements, we are likely to discover more terrestrial planets and even signs of life. Regardless of the outcome, this exploration has changed humanity’s perception of its own position in the universe. As astronomer Harrow Shapley said, “We are not the center of the universe, but we may be the way the universe understands itself.” In the vast universe, life on Earth may be rare and precious, which gives us a special responsibility – not only as observers, but also as guardians and disseminators of life and wisdom in the universe
When do you think humans will confirm the existence of extraterrestrial life?
Within 10 years
Within this century
after one’s death
It can never be confirmed
There are 4 trillion billion habitable planets floating in the universe, each of which is a potential life ark. As Carl Sagar said:
Exploring the universe is essentially a pilgrimage of humanity to its own origins
