Transcription
Compared to gigantic galaxies and star clusters, the solar system is no more than just a speck of dust. There are thousands of stars within the radius of a hundred light years from it, with some of them barely distinguishable against the dark abyss of space. Others are so bright that they can be seen even if they're in other galaxies. Besides, there are a lot of invisible objects lurking at the depths of space too, from massive brown dwarfs to rocky exoplanets comparable to our Earth in size. Some of them may harbor life, while others may pose a threat. However, all this diversity is eclipsed by the scale of our galaxy. There are 200 billion stars and over a trillion planets, at the very least, to be found across its expanses. It is quite impossible to completely explore this great abundance of unique space objects. Today, we will talk about some of the closest ones.
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By galactic standards, the area of space on the outer edge of the solar system is considered to be sparsely populated by both stars and other space objects. Still, approximately 50 stellar systems are known to be located within the radius of 15 light years from the Earth. Some of them are single stars, while others contain two components and more, which is why the total number of stars in this area reaches 65. Many of them have exoplanets and protoplanetary disks made up of gas and dust, where celestial body formation processes are still in progress. In order to analyze this great variety, we'll have to map out the environs. As we slowly recede from our home planet, we will cover dozens of light years of space and visit quite a few remote worlds.
It is time to set out to the stars. Having traveled six light years away from the sun, we will reach our first stop: a small red dwarf, designation GJ 699, also known as Barnard's star. It is remarkable for its incredible travel speed across the sky and also for the fact that it's capable of crossing the lunar disk in the course of 180 years. This is due not only to its close proximity to us but also to its great proper motion. Barnard's star is moving towards the solar system at a speed of slightly over a hundred kilometers per second and is going to become our closest star in about 10,000 years. By then, the distance between Barnard's star and our sun will be just 3.8 light years. But the red dwarf is so dim that it will not be visible even at such a small distance, and it will be possible to observe it only through optical devices. The star's mass is just 17 percent that of the sun, and its radius is six times as small as that of our star. Just like all red dwarfs, Barnard's star is quite cool, with a surface temperature just 3134 Kelvin, or slightly under 2,900 degrees Celsius. With these characteristics taken into account, it isn't surprising that the object's luminosity is 2300 times lower than that of our parent star.
In 2018, the results of a 20-year observation of the star's proper motion were published, which may provide evidence for the presence of an exoplanet nearby. The calculations show it to be a "cold super-Earth" with a mass of not less than 3.2 times that of the Earth. The planet, named GJ 699b, is located about 0.4 astronomical units from the center of the system and takes 233 days to complete a full orbit around its parent star. Unfortunately, the exoplanet follows an orbit that does not lie between the Earth and its parent star, so it's hardly feasible to carry out spectral analysis of the object. Still, mathematical modeling may give us some insights into what it's like. As Barnard's star is very dim, its planetary companion receives 50 times less energy than does the Earth from the sun. That is why the temperature on the celestial body's surface is extremely low: just 105 Kelvin, or 168 degrees Celsius below zero. However, it cannot be ruled out that tidal disturbances or radioactive decay may have released enough energy in the planet's interior to form a warm subsurface ocean. If that is the case, the potential life in its depths would have had ample time to gradually develop. For among the known stars, Barnard's star appears to be one of the oldest. According to estimates, its age is over 10 billion years, which is only a fraction of the expected lifespan of a star. As it is located quite close and is moving in a favorable direction, Barnard's star is one of the highest priority objects for a potential interstellar flight. Time will show if spacecraft launched from Earth will ever reach it.
Meanwhile, on the other side of the sun, there lies the almost exact opposite of the dim red dwarf. Of course, we're talking about Sirius, the brightest star in the night sky, which is 8.6 light years away from our planet. From Earth, the star appears to be a single light source, but close inspection revealed that it is not a single object but a binary star. The two components of the system are currently at their maximum distance from each other: about 30 astronomical units. The main star, also called Sirius A, is a bright blue and white star with a mass two times that of the sun. At the same time, it is about 70 percent larger in diameter, and its surface temperature reaches 10,000 Kelvin. Because of this, Sirius A emits 25 times more energy than our sun. The second object, known as Sirius B, is barely visible against the background of its bright and massive companion. It is a white dwarf with a radius of about 6,000 kilometers and a mass very close to that of the sun. Like all stars of this type, its surface temperature is extremely high, reaching 25,000 Kelvin. However, due to its small size, this white dwarf is about a thousand times dimmer than its stellar companion.
After long and persistent research, no signs of planets have been found in the vicinity of Sirius. The reason for this may lie in the comparatively young age of the stellar system, or else the special position of the ecliptic plane. Because of this, the hypothetical exoplanets do not pass between the light source and the Earth and therefore cannot be detected by the transit method. Another hypothesis claims that Sirius B exhausted its stellar fuel supply roughly 120 million years ago and transformed into a red giant sun. Afterwards, it shed its outer layers, turning into a compact and very hot white dwarf. As a result, the protoplanetary gas and dust disk enveloping the young stellar system at the time was destroyed by this explosion. Its remnants scattered into space around, denying the forming planets any chance of survival. Currently, both components of the system are moving around a common mass center in moderately elongated orbits, completing one every 50 years. Over the next 60,000 years, Sirius will gradually approach Earth, becoming brighter and more visible in the night sky. The minimum distance between us will be about seven and a half light years, after which the system will start to slowly drift away from us. Things will go on like this for about another 660 million years, after which Sirius A will meet the same fate as its companion. For a brief moment, a massive flare will boost its luminosity by a factor of thousands, but within a few months, the star will fade completely, vanishing from the sky forever, leaving the vicinity of Sirius permeated by lethal ultraviolet radiation and stellar wind.
We will perform a maneuver near the solar system to reach a small star 14 light years from the Earth. It is designated differently in different catalogs, and one of its names is Wolf 1061. The star is a red dwarf with a radius of about 30 percent that of the sun. Its mass is about four times smaller than that of our star, and its surface temperature is 3272 Kelvin, or about 3,000 degrees Celsius. Like all objects in its class, Wolf 1061 is extremely dim. Its luminosity is only 0.14 percent that of the sun, so the star is not visible to the naked eye from Earth. However, this red dwarf has one of the most abundant and diverse planetary environments of any stellar system. Nearest, as many as three exoplanets have been discovered in its vicinity. All of them rocky and similar in composition to Earth, but still, conditions on each of the three celestial bodies differ considerably.
The first, closest exoplanet to the parent star, is called Wolf 1061b. It is located 0.038 astronomical units from the center of the system and completes the full orbit around it roughly every five days. The object has a mass in the range of one and a half to two Earths and a radius twenty percent greater than that of our planet. It is likely that the celestial body is tidally locked and faces its star with one side at all times. For this reason, the temperature on the day side of the planet is extremely high, and the flares and stellar winds of the star so close to it virtually rule out any chance of there being an atmosphere. All these factors make the exoplanet completely unsuitable for the genesis and support of life.
The second object in the system, Wolf 1061c, is located 0.089 astronomical units from its star and completes a full orbit around it roughly every 18 days. Thus, it is within the habitable zone of the star and can be considered a potentially habitable celestial body. Wolf 1061c is about three to four times more massive than our planet and belongs to the class of super-Earths. Observations show that its radius is about 1.6 times that of Earth, while the gravity on its surface is calculated to be 60 percent higher than what we're used to. The exoplanet receives noticeably less energy from its star than the Earth does from the sun, so its temperature of equilibrium is much lower. It is 223 Kelvin, or about 50 degrees Celsius below zero, which is slightly higher than on Mars. Nevertheless, given variations in temperature during the day and from season to season, liquid water may well exist on the surface of the celestial body. With a dense atmosphere with a pronounced greenhouse effect, this probability is even higher, and conditions on a planet could approach favorable ones. Of all the potentially habitable exoplanets known to date, Wolf 1061c is one of the closest, ranking fifth on the list.
The third object in the system is called Wolf 1061d. It follows a clearly elongated orbit, with a distance between the parent's star and the aphelium reaching 0.73 astronomical units. As for the perihelium, it lies much closer to the system's center, just 0.21 astronomical units away. It takes the celestial object about 217 Earth days to complete a full year rotation. The mass of Wolf 1061d is calculated to be roughly 7.7 times that of our Earth. If this celestial body turns out to be a rocky object, its radius is supposed to measure about 1.7 times that of the Earth. If, on the other hand, this planet falls in the category of "mini-Neptunes" with a thick multi-layer atmosphere, its diameter is supposed to be not less than 2.2 times that of our planet. In spite of a comparatively high eccentricity of its orbit, Wolf 1061d never enters its star's habitable zone. The celestial body is likely to be completely unsuitable for supporting life but may pose a great scientific interest in terms of planetary evolution. This system is one of top priority destinations for future explorations, and it is highly probable that there are new discoveries to be soon made in it.
In addition to everything else, the star Wolf 1061 is also remarkable for its position, which is virtually at the very edge of the so-called Local Interstellar Cloud. It is an area in space filled with hot but extremely rarefied hydrogen. Its temperature measures about 7000 Kelvin, and on average, there is just 0.3 of an atom to be found in one cubic centimeter of space here. It is about twice as little as the average value in the Milky Way. The solar system has been moving through the Local Cloud for the past ten thousand years. Its average diameter is about 30 light years, but our system is almost at its very edge. Alpha Centauri, for example, is already in the neighboring so-called G-cloud. We, on the other hand, will spend about another 2,000 years within our cloud, after which we will enter an even thinner and hotter region. The Local Interstellar Cloud was formed by the collision of two giant bubbles of hot cosmic gas. It has a momentum of its own, which is perpendicular to the direction of the solar system's motion, and also has a magnetic field and radiation. Fortunately, despite the high energy of the interstellar gas particles that make up the cloud, there is no threat to organisms on Earth. The solar wind is a reliable protection against cosmic hydrogen, and the Earth's dense atmosphere takes the heat off particles that do break through the heliosphere. Still, traces of the isotope iron-60 have been found in the ice of Antarctica. It is thought to have arrived to Earth from no other place but the Local Interstellar Cloud.
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As we continue our journey through space, we will soon get to a rather unusual star called AP Columbe, located 27.4 light years away from Earth. First and foremost, it draws our attention because it was born just 40 million years ago, when the dinosaurs would have long since died out on our planet. What is even more striking is that its birth took place in a completely different region of the galaxy, 450 light years from the object's present position. Young stars usually remain close to their stellar nursery for quite a long period of time, up to several hundred million years. If you analyze the movement of AP Columbe, however, you will find that it probably previously belonged to the open cluster IC 2391. The cluster has about 30 stars united by a common origin and direction of movement, but a few million years ago, one of them exploded, shattering the delicate equilibrium of a multitude of cosmic bodies. This catastrophe gave the youngster an additional momentum that swiftly expelled it from its birthplace.
A very small red dwarf with a mass just 13 percent that of the sun, this time it's four times more energy than any other known celestial body with the same parameters. In addition, its radius is twice as large as calculated based on the modern stellar structure theory. Strangely enough, another unusual feature of the star has helped to provide answers to these riddles. According to spectral analysis of its radiation, the stellar matter of AP Columbe contains an abnormally large amount of lithium. Normally, this element is found in abundance in protostellar clumps, but it burns out completely after the start of a thermonuclear reaction. This means that the star is very young and at the very beginning of its formation, not even in the main sequence phase yet. As a consequence, AP Columbe is unstable and explosive in nature. It is prone to violent outbursts and stellar matter ejections, during which its luminosity may increase by up to 10 times for a short while. Such events would normally threaten exoplanets nearby, stripping off their atmosphere and sterilizing their surface with ultraviolet radiation. As it is, observations show that there are no large space objects within 4.5 astronomical units of this star. In all likelihood, it is too young for the formation of fully-fledged planets. Nevertheless, it is a unique cosmic body, very important for understanding early evolutionary stages of stars.
As we've seen, not every star has a planetary system. So, our next stop is near the orange dwarf Gliese 370, 37 light years from Earth. In 2011, a single exoplanet was discovered near it, designated Gliese 370b. The object was discovered with the use of the radial velocity method and has a mass of three to four Earths. According to current understanding of the formation of celestial bodies, the discovered world is most likely a rocky planet with a radius about 30 percent larger than the Earth's. Under such conditions, the gravity on its surface may be at least 40 percent higher than what we're used to. Gliese 370b is located at a distance of 0.26 astronomical units from its parent star and completes an annual rotation in 54.4 Earth days. Thus, the exoplanet is in a habitable zone of its star. If the albedo value of its surface is close to that of the Earth, then the average temperature of the celestial body should be about 25 degrees Celsius, which is a few degrees higher than on our Earth. The fact that an exoplanet is located quite far from its parent star suggests that it is not tidally locked and has its own iron or cycle. This contributes to an even distribution of heat over the celestial body's surface, which as a consequence may create a milder climate. A moderately high gravity should smooth out the planet's terrain, which combined with an abundance of liquid water may cause vast seas to form, both warm and shallow. Such places have traditionally been considered the most comfortable for the genesis and support of life.
At the time of its discovery, Gliese 370b was considered one of the most favorable exoplanets for potential biological life. However, more recent research has suggested that conditions on its surface are somewhat less attractive than previously thought. In particular, the atmosphere of the celestial body may be too dense and have a pronounced greenhouse effect. Still, even according to the most pessimistic predictions, the average temperature on the surface of the object does not exceed 78 degrees Celsius, which corresponds to a temperature range of about 30 to 100 degrees Celsius. Given that our planet's climate was about the same during the Archean period, Gliese 370b remains a very promising exoplanet for the search for extraterrestrial life, which means that its exploration must continue.
If we stop and look around for a while, we will find that we have already traveled more than 50 light years away from our solar system. There are over a thousand stars within its radius. Most of them single stars, others are binary stars or even groups of stars. The most striking example is Castor, a complex and surprising system of as many as six components. This stellar system is located 50 light years away from Earth and is visible to the naked eye as a single source of light, as the second brightest object in the constellation Gemini and the 23rd brightest in the night sky in general. Castor has been known to humans since ancient times. As the system has as many as six components, it has a rather complex structure. The two brightest of these, Castor A and B, are bluish-white Sirius-like stars, each with a red dwarf companion nearby. The distance between the mass centers of these binaries is about 110 astronomical units, and it takes them 445 years to complete a full orbit. About 600 astronomical units from Castor A and B, there is another compact binary system consisting of a pair of almost identical red dwarfs. Each has a mass of about 0.6 times that of the sun and has a surface temperature of about 3900 Kelvin. The two stars are so close together that they take less than 20 hours to complete a rotation. They move around the system's common mass center following a very oblong elliptical orbit with an orbital period of about 14,000 years.
So far, no confirmed exoplanets have been found in the Castor system, but if they do exist, they're most likely to be located here, the faintest component of the system. In 2018, for example, astronomers detected variations in the orbital period of Castor C, which implies there may be a brown dwarf nearby, about 49 times more massive than Jupiter. According to calculations, it is located 14 astronomical units from the parent star and completes the full orbit around it every 54 years. It is possible that, like many red dwarfs, Castor C has rocky exoplanets close by. In this case, the skyline of such a hypothetical object must present a truly incredible spectacle. On one side, a considerable part of it is occupied by two crimson stars circling the sky without stopping, while on the other side, the eternal twilight is diffused by the light of a pair of stars, either of them hundreds of times brighter than the Earth's moon. Whether this is actually the case can it yet be verified at this point.
Although the stars visited are great distances apart, they're all within a single structure stretching for the total of about 300 light years. This is called the Local Bubble and contains not only thousands of stars but also several large structures, including the Local Interstellar Cloud. The Local Bubble was formed by successive explosions of several supernovae that occurred between 10 and 15 million years ago. It has a giant cloud of interstellar gas heated to 1 million Kelvin. Because of its high temperature, the gas tends to expand, so its density is about 10 times lower than the galactic average. In addition, the hot ionized hydrogen constantly emits X-rays. Fortunately, the magnetic fields of the sun and the Earth, as well as the atmosphere of the planet, successfully protect us from the harmful radiation. The Local Bubble is bordered by other similar formations. For example, slightly away from the direction of the solar system's motion is the so-called Bubble One, formed by supernova explosions in the vicinity of Antares. This giant cosmic structure has been actively interacting with our bubble for a long time, which has caused the formation of tremendous compactions located at the region at the edge. The center of the neighboring bubble is about 500 light years from us, and the solar system will reach its margins in a few tens of millions of years.
Clouds of scorching hot gas and open clusters, giant nebulae and stellar streams – all these majestic cosmic structures add up to a single system called galaxies. In turn, they combine to form galactic filaments, giant streams on walls stretching for billions of light years. This forms the large-scale structure of the universe, with countless different stars and worlds lurking in its depths. Who knows what we're eventually defined among them.
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