Since the beginning of the space age, people have used chemical rockets to get into space. While this method is certainly effective, it is quite expensive and resource intensive. Scientists became interested in the question: would hypothetical aliens be able to leave their planets using similar technologies?
Two studies
Harvard professor Abraham Loeb and astronomer Michael Gippke, an independent researcher associated with the Sonneberg Observatory, attempted to analyze this question in two recently published papers. Professor Loeb looked at the problems that extraterrestrial beings might encounter when launching rockets from Proxima b. Hippke's research addresses a similar question - whether aliens living on super-Earths can get to .
In his study, Loeb argues that we humans are lucky to live on a planet that is well suited for space launches. In order for the rocket to leave the surface of the Earth and begin to orbit the Sun as its satellite, it needs to reach a speed of 11.186 km/s. The speed required to escape Earth's orbit and leave the solar system is about 42 km/s relative to the Sun.
Professor Loeb says:
“Accelerating a rocket to cosmic speeds requires a huge mass of fuel, which grows exponentially. By a happy coincidence, the rate of escape from Earth's orbit around the Sun is at the limit of the speed achievable by chemical rockets. However, the habitable zone around fainter stars is closer to them, making it very difficult for chemical rockets to escape their gravitational clutches. your star."
As Loeb points out in his essay, escape velocity is calculated as the square root of the stellar mass divided by the distance from the star. This means that the rate of escape from the habitable zone is directly proportional to the stellar mass and inversely proportional to the distance from the star.
This infographic compares the planet's orbit around Proxima Centauri (Proxima b) with the same region of the Solar System.
Proximity to a star is not good for planets that orbit M-type stars (red dwarfs). These stars are the most common type of star in the Universe, making up about 75% of such objects in the Milky Way Galaxy. In addition, recent research has discovered many rocky planets orbiting red dwarf stars, and some scientists believe such planets are the most promising place to look for potentially habitable worlds.
Is it possible to fly away from Proxima b?
Using the closest star to our own world (Proxima Centauri) as an example, Loeb explains that a rocket using chemical fuel would have a much harder time achieving escape velocity from a planet located in its habitable zone.
“The closest star to the Sun, Proxima Centauri, is an example of a faint star with only 12% of the mass of the Sun,” he said. “A couple of years ago, this star was discovered to have a planet the size of , called Proxima b. It is located in the habitable zone, which is 20 times closer to the star than the Earth is from the Sun. At this location, the ejection speed is 50% greater than in the Earth's orbit around the Sun. It will be difficult for the civilization on Proxima b to leave their world using chemical rockets."
What did Hippke research?
Hippke's research begins with the assertion that Earth may not, in fact, be the most common type of planet in our universe. For example, planets that are more massive than Earth will have higher surface gravity, meaning that they will be able to retain a denser atmosphere, which will provide it with protection from harmful cosmic rays and solar radiation.
An artist's impression of a Super-Earth, a class of planets that has many Earth masses but is smaller than the planet Uranus or Neptune. Source: NASA/Ames/JPL-Caltech. Additionally, a planet with higher gravity would have a flatter topography, resulting in archipelagos instead of continents and smaller oceans—an ideal situation when it comes to promoting biodiversity. However, when it comes to launching rockets, increased surface gravity will mean the need to reach higher flight speeds. As Hippke pointed out in his study:
"Rocket propulsion obeys the Tsiolkovsky equation (1903): if the rocket is carrying fuel, the ratio of the total rocket mass to the final velocity is an exponential function, making high speeds (or heavy loads) more expensive."
For his calculations, Hippke uses Kepler-20 b, a “Super-Earth” located 950 light-years away. This planet is 1.6 times the size of Earth and has 9.7 times the mass of our planet. While the speed at which a body leaves orbit around Earth is approximately 11 km/s, a rocket attempting to leave a super-Earth like Kepler-20 b would have to achieve an exit speed of ~27.1 km/s. As a result, a single-stage rocket on Kepler-20 b would have to burn 104 times more fuel than a rocket on Earth in order to reach orbit.
To put this all into perspective, Hippke looks at specific payloads launched from Earth. “To launch a payload of 6.2 tons, as is required in the case of the space telescope. James Webb from the planet Kepler-20 b, the fuel mass will increase to 55,000 tons, which is equal to the mass of the largest ocean battleships,” he writes. "For a classic Apollo to the Moon (45 t) the rocket would have to be significantly larger, ~400,000 t."
Project Starshot, designed to be humanity's first interstellar journey. Hippke's analysis allows us to conclude that chemical rockets will still provide the speeds necessary for departure from the planet on super-Earths with up to 10 Earth masses. However, the amount of fuel required makes this method impractical. As Hippke noted, this could seriously affect the development of an alien civilization.
“I'm amazed at how lucky we humans are to find ourselves on a planet that's good enough for spaceflight,” he said. “Other civilizations, if they exist, may not be so lucky. On more massive planets, spaceflight will be more expensive, and its opportunity will decrease exponentially depending on the mass of the planet. Such civilizations will not have satellite television, a mission to the moon or the Hubble Space Telescope.”
Both of these articles provide some clear conclusions that relate to the search for extraterrestrial intelligence (). First, it means that civilizations on planets that orbit red dwarfs or super-Earths are less likely to explore space, making them harder to detect. The results of the study also indicate that humanity may be one of the few civilizations that have been given the opportunity to explore space by.
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An artist's impression of the surface of Proxima Centauri b
Computer modeling has shown that conditions on Proxima Centauri b may be suitable for life if the planet has an atmosphere similar in density to Earth's. In this case, the gas shell of the exoplanet will be able to protect its inhabitants from the harmful effects of radiation from the central star. Article published in the magazine Monthly Notices of the Royal Astronomical Society: Letters.
The planet Proxima Centauri was discovered by astronomers this year and immediately aroused great interest from the scientific community. This is not surprising: firstly, it revolves around the closest star to Earth - Proxima Centauri, and secondly, it is located in the potentially habitable zone, that is, where liquid water can exist. However, there is not yet enough observational data to reliably judge whether life can exist on Proxima b. In fact, all that scientists know is the planet's rotation period (11.2 Earth days), approximate mass (1.2 Earth masses) and distance from the star (0.05 astronomical units). Nevertheless, based on this information, as well as from information about Proxima Centauri, it is possible to build models, albeit based on different assumptions.
Thus, calculations show that a large amount of extreme ultraviolet and X-ray radiation will fall on the surface of the planet. Researchers estimate that Proxima b should receive at least 250 times more X-rays than Earth. This imposes certain restrictions on the possibility of the existence of life on the planet, since X-rays are ionizing, that is, harmful to living organisms. Thus, the lethal dose of radiation for humans is 5 to 10 sieverts, for a goldfish - 100 sieverts, 100-1000 sieverts for insects, 10,000 sieverts for viruses and 10,000 - 100,000 sieverts for bacteria Deinococcus radiodurans.
However, the author of the new paper, Dimitra Atri, believes that life on Proxima b could exist even if flares and coronal mass ejections occur on Proxima Centauri, accompanied by bursts of high-energy protons. He came to this conclusion by estimating the likely size of stellar flares (based on recorded solar flares), the possible density of the planet's atmosphere and the strength of the magnetic field.
The dose of radiation that would reach Proxima b as a result of the Proxima Centauri flare if the atmosphere had a "bar density" of 100 (solid line), 300 (dotted line), 700 (short dashes) and 1000 (long dashes) grams per centimeter in square. The horizontal line shows the average daily dose of radiation from the natural background radiation on Earth.
The astrophysicist’s calculations showed that if the planet has a gaseous shell with a density not inferior to that of the Earth (“column density” is about 1000 grams per centimeter squared) and a magnetic field the same as that of the Earth, then organisms on its surface will be able to survive most outbreaks on Proxima Centauri. According to the researcher's estimates, even powerful flares with an energy of more than 1 × 10 35 erg are unlikely to harm the planet's biosphere (large flares on the Sun have an energy of about 1 × 10 32 erg). If the atmosphere is less dense ("columnar density" 700 grams per centimeter squared), emissions of high-energy particles will become a threat to some species, such as humans. In the worst case, if Proxima b has almost no magnetic field, the flares will “blow away” its atmosphere and then all life similar to Earth will die.
However, the author of the work emphasizes that when assessing the habitability of a planet, it is necessary to take into account not only the one-time, but also the cumulative effect created by bursts of high-energy protons. On red dwarfs of spectral class M (which includes Proxima Centauri), flares with a power of 10 34 -10 35 erg occur approximately once a decade, and with a power of 10 32 - every five days. The surface of a planet with an atmosphere and magnetic field strength like Earth will still receive a relatively small dose of radiation (about 0.01 sievert), but with deviations - for example, with a thinner atmosphere - they will already harm living organisms (for example, A radiation dose of 50 millisieverts is considered harmful for humans.
Recently, astrophysicists tried to calculate the possible composition and structure of Proxima b. According to their estimates, there is an ocean on the surface of the planet, but these calculations are very approximate. Models by other researchers show that dim stars may well form Earth-like planets, and scientists also suggest the existence of oceans on them.
Kristina Ulasovich
Researchers, using computer simulations similar to those used to study climate change on Earth, have found that Proxima Centauri b may have huge volumes of liquid water on its surface. This discovery significantly increases the chances of living organisms on this exoplanet.
“The main conclusion from the simulations that have been conducted is that there is a decent chance that the planet will be habitable,” said Anthony Del Genio, a planetary scientist at NASA's Space Science Institute. Del Genio is also the lead author of a paper describing the new research, which was published September 5 in the journal Astrobiology.
Proxima Centauri is a small red dwarf star located just 4.2 light years away. Despite its proximity to Earth, scientists still know little about Proxima Centauri's planetary companion, Proxima Centauri b. What is known is that its mass is at least 1.3 times that of Earth, and that it orbits its parent star once every 11 days. Therefore, Del Genio and his colleagues had to make some reasonable assumptions about the exoplanet, namely that it has an atmosphere and an ocean on its surface, in order to continue their work.
Proxima Centauri b is in orbit around its star in the habitable zone. That is, at a distance that allows it to receive enough energy to maintain its surface temperature above the freezing point of water. But at the same time, the exoplanet is very close to the star. Therefore, it is likely that the exoplanet's rotation is blocked by tidal gravitational forces. This means that one side of Proxima Centauri b always faces its parent star, just as the same side faces Earth.
Previous research, published in the journal Astronomy and Astrophysics in 2016, described a simulated hypothetical atmosphere of Proxima Centauri b. It has been suggested that the exoplanet's hemisphere, always facing the star, would be a hot desert. At the same time, the ocean facing space will be frozen. Therefore, on Proxima Centauri b there can only be a piece of warm sea at the junction of the hemispheres.
But the new simulations were more global than the previous ones. They implied the presence of a dynamic, circulating ocean that could very efficiently transfer heat from one side of the exoplanet to the other. According to the researchers, the movement of the atmosphere combines in such a way that "although the night side never sees the light of the host star, there is a narrow strip of liquid water that is maintained around the equatorial region," Del Genio said.
In total, the team ran 18 separate simulation scenarios. Scientists have looked at the effects of giant continents, thin atmospheres, different atmospheric compositions, and even changes in the amount of salt in the global ocean. In virtually all models, Proxima Centauri b had open ocean that persisted on at least some portion of its surface.
“The more of a planet's surface is covered with liquid water, the higher the likelihood that there will be life there. We will be able to find evidence of this life with future uses,” Del Genio said.
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If you weren't on Earth on Wednesday, you missed this: Astronomers have found a planet that's as close to us as it gets - in the nearby star system Alpha Centauri. The planet, called Proxima Centauri b, orbits its star every 11.2 days. And yes, it is in the “potentially habitable zone,” the Goldilocks zone where liquid water could (just might) be on the surface. Its mass - 1.3 Earth's - implies that the planet should (for now only should) be solid. It is not surprising that we have already included it in the list of the greatest discoveries of the century.
But wait a minute. Astronomers have found other Earth-like planets in habitable zones in recent years. According to the Planetary Habitability Laboratory at the University of Puerto Rico, there are currently 15 potentially habitable "Earth-sized" exoplanets (in terms of mass or radius) confirmed to exist. And although yes, Proxima Centauri b has a mass very close to that of Earth, in other respects it may not be so earthy.
What really sets this planet apart from its competitors and what caught our attention in the first place is its location. Just as the closest supermarket to your home will be your most visited store, Proxima Centauri b warms the soul of scientists with its proximity and attractiveness. However, this attractiveness remains in question.
First, scientists so far only know the minimum mass of Proxima Centauri b - the smallest mass it can have - and do not know its radius. That is, they don’t know for sure whether it is solid or not.
"Don't forget we only have a minimal mass for this planet," astronomer Elizabeth Tasker tweeted. “By these measures, I would be the twin of most life forms on Earth.”
In addition, the star covers the planet with high-energy radiation, which means it dried up all the water a long time ago. If there is any hydration left, it will only be in the most sunlit places. While "warm enough to have water" is an important fact, it does not equate to actual habitability.
Rory Barnes, an astronomer at Washington State University, has developed a habitability index that ranks potentially pleasant planets based on a wide variety of factors. And his conclusions are disappointing. "I'm pessimistic that any planet can be habitable because there are too many demands to be met," he says. "But Proxima gives us a great chance to find out if I'm right or wrong."
In light of all this uncertainty, the excitement, or as it is now fashionably called - hype, about this planet seems premature or simply downright wrong. But there is definitely hype.
“I was surprised when I read a paper from the European Southern Observatory that said very clearly: This is the most Earth-like planet we've found yet,” says Lisa Messery, an anthropologist at the University of Virginia, “even though the same article wrote that the planet has no seasons, the year lasts 11 days, the sky should be red, and Proxima Centauri is an active star with solar flares. In other words: this is a completely uninhabited world.”
Messeri does not study exoplanets. She studies people who study planets - following and interviewing them for many years. The reaction to this discovery, she says, has a lot to do with Proxima's proximity to us. “The reason we care about this planet is because it’s a place we can go to and be.” In the case of most planets, we can only imagine. But Proxima Centauri b is the first exoplanet that could become physically reachable.
Earth's geography similarly influences our perceptions. “We feel connected to places that are close to us because we can go there on the weekend,” says Messeri. “Even if I’m not going to New York on Saturday, the fact that I can makes it part of my world.”
Forward and with a song
While scientists aren't heading to Alpha Centauri on Saturday, they're definitely planning to fly there. The Breakthrough Starshot Initiative, about which was announced in April, plans to send postage stamp-sized probes to the star. At the time of Proxima's announcement, the people behind the project had not yet decided which star in the triple system they wanted to visit (but now the choice is almost obvious).
True, it is too early to discuss interstellar space travel - I mean, seriously discuss it. Scientists try to stay away from such discussions. But now there is a reason and a place to point to, says Messeri. With such a place, scientists can speak quite honestly and with passion publicly about interstellar intentions.
The proximity of the new planet also makes the search for alien life more feasible. There is a real star, a real Earth-like planet, really nearby. If a Contact-style signal came from this system, you could bet a million dollars that the world's governments would band together and send people there. Because the human-powered interstellar journey to Proxima is the cosmic version of Messeri's weekend trip to New York.
Sending an interstellar message to aliens rather than waiting to receive it has historically been considered a philosophical exercise. A message can take many generations to reach its destination, as can a hypothetical answer. But with Proxima, we get something like a real conversation with aliens, like meeting a stranger, says Douglas Vakoch, head of METI International. "In less than ten years, we could send a message and receive a response from interested Centauri."
More traditional sciences will also share in the benefits: for example, the European Extremely Large Telescope will be able to take pictures of this planet that will provide at least a little new information (maybe a lot). It will allow scientists to find possible biosignatures indicating the existence of life. Scientists go crazy at the thought of this possibility, because the closer a planet is, the more information we can learn about it.
Looking in the mirror
Yet it is not an Earth twin, no matter what the headlines say, and scientists have not yet found an Earth twin. Hot Jupiters are cool; planets with glass rain are also fun; super-Earths are generally something supernatural. Compiling a complete census of exoplanets will be a very valuable asset. But most scientists, according to Messeri, really just want to find another Earth. This is reflected in scientific priorities. The Kepler Space Telescope, which has found more planets than anyone else on this planet, was "specially designed to explore part of our region of the Milky Way galaxy, searching for dozens of Earth-sized planets in or near the habitable zone," according to NASA.
The search for an "Earth twin" is a pursuit of a Platonic ideal, says Messeri. “It allows us to see the Earth in its prime, as we would like to see it, not disfigured by climate change, war or disease.”
But we haven’t found such a place yet. And we may never find it. In your quest to find your perfect match, you usually find someone who is super cool but yells at you when he's hungry or hates your mom. In your search for the perfect job, you find yourself in the role of a dishwasher. In this sense, the discovery of Proxima Centauri b is a representation of humanity's desire for perfection, for a pure and virgin Earth.
Most likely, this will happen all the time. We set ourselves a great goal and failed. Not because we are so unlucky, but because we were doomed to it from the very beginning, because this is the nature of humanity: to always strive for the ideal and never achieve it.
