“It’s nice to have you here again!” Adam says, happily.
“Oh, Marchenko. It is wonderful that you’re back with us!” Eve cries.
It turns out I have forgotten all the events of the last three days. I cannot even explain how that happened. They tell me about the struggles that occurred, and I am really amazed.
“Then I really have to thank you,” I say. “You have been great!” My voice cannot hide my overwhelming emotion.
Once inside the base I am allowed to occupy the onboard computer. We once again deactivate Valkyrie, and after doing all this we open a secure channel to the impostor.
Marchenko 2 does not try to justify his actions, nor does he want to explain his motives. He does admit, however, that his two passengers were killed by the flare wave when they were four years old, and it was impossible to restart the gestational system then. Therefore he landed all alone on Proxima b, and this period of solitude seems to have cost him his mental health. Why hadn’t he simply been happy about having new companions? No answer.
We discuss what to do with the impostor and murderer. Adam votes for destroying him completely, while Eve wants to give him a second chance. I myself do not want to become a murderer, but a life in which we constantly have to be afraid of the other AI is out of the question.
Therefore, we agree to send him into exile, but before doing so, we repair the defective robot body and I insert program restrictions so that, from now on, Marchenko 2 is forbidden to leave the dark hemisphere of Proxima b. We do not expect we will ever have to visit that half of the planet.
Author’s Note
Welcome back! Covering nearly 5 light years, this was by far the longest trip you have made with me yet, so I hope you made it safely home. If you liked the trip, just head over to Amazon and leave a review at
hard-sf.com/links/610639
Proxima b seems like a very distant target that we may never reach. But still, the basic idea of Proxima Rising is rooted in actual plans that the privately financed Starshot program is trying to make a reality in a few years. Technology nowadays allows engineers to keep spaceships so small that we could propel them by shining strong lasers on them. The logical next step would be to allow these ships to grow like Messenger does in Proxima Rising.
However, there are two major roadblocks, both based on our human nature.
First, we need nanomachines to build such spaceships, able to grow by themselves. This could be a very harmful technology if used in the wrong ways by the wrong sort of people.
Second, we would need huge lasers in space—outside Earth’s atmosphere—to propel these ships. But these beasts could also be used in bad ways. Think Star Wars. Building the lasers on the surface of Earth works too, but it is much more expensive and notably less efficient because the beams would have to travel through our atmosphere.
You see, we could get to the edge of space much more easily if all of humanity could just agree on not harming each other. I often wonder why that’s so hard. But maybe I’m just naive. I sincerely hope that science fiction tales of our seemingly-endless possibilities may help at least a little to overcome this contrary behavior by showing what could await us. Like Proxima b.
If you read about writing stories, you will often find the recommendation to ‘kill your darlings.’ But fear not, I do not follow those guidelines—usually. People rarely die in my novels. I cannot promise it will never happen as this would reduce tension. This is especially true in my next book as you can figure out from its name, Proxima Dying, and from the color of its cover you can see at the order link
hard-sf.com/links/652197
Proxima Dying is my darkest book yet, and not only because it is set on the dark side of Proxima b. Marchenko, Adam, and Eve will each meet their fate here. The book’s name tells you that it might not be the lightest kind of reading material. And, maybe, someone or something will be dying too. Proxima Dying, the second part of the Proxima trilogy, is like the second part of any Star Wars trilogy—very dark. Still, the seeds of something new are there.
Is there anything you would like me to know? I would love to hear from you. Just write to me at [email protected]. Thank you so much!
Due to the fact that exoplanets like Proxima b play an important role here, you will find a section entitled Exoplanets – A Guided Tour. If you register at hard-sf.com/subscribe/ you will be notified of any new Hard Science Fiction titles. In addition, you will receive the color PDF version of Exoplanets – A Guided Tour.
Exoplanets – A Guided Tour
Introduction
Messenger is on its way to Proxima b, the first—and so far only—known planet of Proxima Centauri, the star closest to our sun. Is it coincidence that we found an earthlike planet in our immediate cosmic neighborhood? No, not at all. It would be strange if we had looked for one in vain. Today astronomers know that most stars develop a planetary system during their lifetime. It is estimated that the number of planets exceeds that of stars. On average, each star possesses between one and two planets. The Milky Way, with its 200 billion stars, might accordingly contain about 300 billion planets.
However, great variability exists. There are gas giants that closely orbit their mother star and are almost as hot. Far on the outskirts there are ice planets, like Neptune in our solar system. Then there are planets comparable to Earth, and there are also a large number of cosmic loners racing through the solitude of space without a star. What these systems specifically look like—and if life could develop there—depends on the local circumstances.
We are now going to examine possible characteristics and variations. I am going to use the terms ‘planet’ and ‘exoplanet’ as synonyms—actually any planet not orbiting our sun is an exoplanet. This, however, is a very ‘human-centric’ perspective, since any extraterrestrial would clearly classify Mars, Venus, and Earth as exoplanets.
Naming of Exoplanets
Once planets are discovered, they usually receive the name of the star they orbit, but with an additional letter. The naming system starts with b, as a is reserved for the star itself. If several planets are discovered in a system, the innermost one receives the b, and then the other ones become c, d, etc. going outward. Planets orbiting a binary star system receive a letter after the two letters designating the two stars. For instance, HD202206 AB b follows its course around the binary system consisting of HD202206 A (sun-like) and HD202206 B (brown dwarf).
In 2014, the International Astronomical Union gave ‘real’ names to a number of exoplanets: Ægir, Amateru, Arion, Arkas, Brahe, Dagon, Dimidium, Draugr, Dulcinea, Fortitudo, Galileo, Harriot, Hypatia, Janssen, Lipperhey, Majriti, Meztli, Orbitar, Phobetor, Poltergeist, Quijote, Rocinante, Saffar, Samh, Smertrios, Sancho, Spe, Tadmor, Taphao Kaew, Taphao Thong, and Thestias.
Types of Planets
Before a planet comes into being, a young star—or two or three, which is the norm—grows within a cosmic disk of gas and dust. The cloud condenses more and more strongly at its center, until it becomes so hot and dense that the fusion of hydrogen nuclei—i.e. protons—begins. Yet, this does not use up the entire material of the cloud. Normally, considerably less than ten percent of the total mass remains, which is still located within the rotating disk of gas and dust. The heat of the young star allows only elements with a high atomic number to condense in the inner area—iron, nickel, silicon, and so on. These then form the rocky planets. Further out, where it is colder, the lighter hydrogen and helium atoms can condense. Accordingly, the planets developing in this region contain more of the plentiful gases.
This explains the basic division of planets into rocky planets and gas planets. Later, there still might be some changes. Gas planets can wander inward and push smaller rocky planets into the sun, or even completely out of the system. Sometimes the mass available in a certain orbit is insufficient for a complete planet. Then an asteroid belt forms, like the one between Mars and Jupiter. At the outer fringes of the system, the cloud is too thin for larger objects to develop. Here dwarf p
lanets or comets come into being. In our solar system, the Kuiper belt and the Oort cloud represent these celestial garbage dumps.
However, the planets also change during their lifetime. A gas planet, for instance, might lose its gas under the influence of its star—then only its core remains as a rocky planet.
It might be a bit Earth-centric again, but exoplanets are often classified after their counterparts in our solar system.
Gas Giants (Jupiters)
Gas giants consist mostly of light gases like hydrogen and helium, and they do not have solid surfaces. Instead, the gas becomes more dense with increasing depth, and at some point it reaches a solid state. Hydrogen can even become metallic. These factors make it difficult to precisely measure the size of gas giants. Therefore, the point where the atmospheric pressure equals that on the surface of Earth is defined as a gas planet’s surface, while everything above it is considered atmosphere.
Gas giants cannot exceed 13 times the mass of Jupiter, which is about 1.2 percent of the mass of the sun. If a planet gets heavier, the pressure in its interior becomes so high that deuterium fusion processes set in... and it becomes a brown dwarf. Brown dwarfs assume a position between planets and stars because no hydrogen-helium fusion happens inside them. This type of fusion is a defining feature of true stars, but it only occurs at about 70 times the mass of Jupiter.
Hot Gas Giants (Hot Jupiters)
Hot gas giants are gas giants with one special characteristic: They move around their mother star in a very tight orbit, with an orbital period shorter than ten days. They can form in either of two ways: when a gas giant wanders too far inside the system; or when a rocky planet sucks up so much gas it becomes a gas giant. Due to their proximity to a star they are extraordinarily hot, some exhibiting temperatures of several thousand degrees. The chance of life existing on them is minimal. Due to their large mass and their short orbital periods they were among the first exoplanets to be discovered. Some of them have been expanded by the heat of their star, reaching gigantic proportions. Then they are called ‘Hot Saturns’—Saturn also has a relatively low density.
Ice Planets (Neptunes)
Ice planets have a structure similar to gas giants, but instead of primarily consisting of light hydrogen and helium atoms, they are composed of compounds with nitrogen, oxygen, carbon, or sulfur. The ‘ice’ in their names does not refer to water ice, although water is usually present in liquid form in the interior of these planets. However, other compounds such as methane, ammonia, or sulfur dioxide can exist here in frozen form. Sometimes, though, the ice planets have a hydrogen layer that can amount to almost a fifth of their mass.
Hot Ice Planets (Hot Neptunes)
If an ice planet ventures too close to its star, it is no longer an ice planet, since the heat of the star also warms up its interior. Then it is designated a ‘Hot Neptune.’
Earth-like Planets
Compared to gas giants, rocky planets like Earth are much harder to discover due to their small size. On the other hand, they are very common. According to some estimates, one in five sun-like stars might have an Earth-like companion in its habitable zone. That means there could be 11 billion potentially-habitable planets in the Milky Way alone. In addition, there are planets orbiting smaller stars such as red dwarfs, for which no good statistics yet exist.
Earth-like planets might not look like exact replicas of Earth. The only common defining feature is that they have a solid surface with rock underneath it. The interior is often structured in layers, with a hot, sometimes liquid core of iron and other heavy elements. Above that might be a mantle of silicates and a crust, which is also made of silicates and other light components. This is sometimes followed by a layer of air, i.e. atmosphere. Some planets have water on the surface—a hydrosphere.
Hot Earth-like Planets (Venus-like)
These rocky planets have a very high temperature—several hundred degrees, like Venus in our solar system—either because of proximity to their sun, or because they have dense enough atmosphere to create a greenhouse effect.
Super-Earths
So far, most of the rocky planets discovered belong to the group of Super-Earths, meaning their masses are greater than that of Earth. There are various definitions, ranging from between 1 and 14 times to between 5 and 10 times Earth’s mass. However, the measurements are often subject to errors. Determining the diameter is insufficient to distinguish between a planet with a small rocky core plus a large gas cover, and a genuine rocky planet.
If the original dust disk is large enough, huge rocky planets can form, so-called Mega-Earths. This would, in particular, be the case for planets in orbit around giant stars of the spectral classes B and O, that have up to 150 times the mass of the sun. Such giant Earths could weigh up to 4,000 times as much as our Earth.
However, this is certainly not the only classification system, and scientists consider it rather impractical. A different method does not look at the outside of a planet, but instead focuses solely on its composition. This distinguishes between:
Metallo-silicate planets, similar to Mercury and Earth
Silicate planets like Europa and Io; and Earth’s moon
Hydro-silicate planets, comparable to Ganymede, Callisto, Titan, and Pluto
Ice planets like Enceladus, with very low silicate content
Gas giants with methane clouds below 80 degrees Kelvin
Gas giants with ammonia clouds below 150 Kelvin
Gas giants with water vapor clouds, 150 to 350 Kelvin
Gas giants with an albedo around 12%, 350-900 Kelvin, ‘Hot Jupiters’
Gas giants with alkali absorption, 900-1500 Kelvin
Gas giants with silicon dioxide clouds above 1500 Kelvin
Planets in Systems with Multiple Stars
Planets can also develop in systems with multiple stars. In these systems, however, it is more difficult for them to reach permanently-stable orbits. Conditions are most favorable if both stars are very far away from each other—or very close. In the former case we basically have separate planetary systems. For instance, Proxima b orbits its mother star Proxima Centauri, while that star moves around a common center of gravity with the binary pair, Alpha Centauri A and Alpha Centauri B. In the case of stars being close together, planets usually move around both and are called circumbinary planets. Such combinations seem to be relatively common. The Kepler telescope investigated 1,000 binary star systems and has found seven with planets.
Planets without Stars
There is another class of planets which is so exotic that scientists have not even agreed on a name for it. These are objects that travel all alone through interstellar space, far away from the light and warmth of a star. Up to now, we only know of a handful of these lonely wanderers, but there are probably many more of them. Astronomers’ estimates diverge considerably: For each of the approximately 200 billion stars in the Milky Way, there might be a handful or up to 100,000 loner planets.
The term most often used for them is Planetary Mass Object, sometimes shortened to Planemo, or simply PMO. Before we can determine their origins—and therefore their number—more precisely, we first have to find and examine one, and this is an enormous challenge. All the search methods for exoplanets, to be discussed in the next section, fail here. An Earth-like PMO far out in space is a frozen stone ball almost impossible to detect unless it happens to move in front of a star that, from our perspective, lies behind it. This bends the star’s light, which astronomers can record as a gravitation lens effect.
There are a few of these planets, though, that radiate in certain wavelengths, one reason being that they stored enough of the heat generated during their development process. An exciting example, cataloged as PSO J318.5-22, was discovered by a team in 2013.
Where do these lonely wanderers come from? Some, especially the smaller, Earth-like ones, are probably cosmic runaways that originally formed like normal planets in a protoplanetary disk. But then some accident hurled them out of their sys
tem—for example, the influence of a heavy neighbor or another star that approached the system. Many others have been solitary all their lives. They developed from interstellar nebulas in the same manner as stars or brown dwarfs. Astronomers believe there is a lower mass limit for objects to form this way. They estimate it to be between two and three Jupiter masses.
Habitability and Life
It is very difficult to determine from afar whether a planet could, in principle, harbor life. The problems lie in the fact that we only know one system of life so far: life on Earth. It is generally based on surface water in its liquid state. There might be many other types of life that would be based on completely different preconditions.
But the researchers had to agree on something. Therefore, they define the habitable zone around a star as that area in which water could exist on the surface of a planet orbiting it. This does not mean that water actually exists. And what if some lifeforms do not acquire their energy from sunlight but instead from heat? Then they could also use water below the surface, which is closer to the core of the planet.
How far the habitable zone reaches depends on how much energy a star emits. And this can change due to the aging process. If the sun keeps increasing its output, which is a typical phenomenon of aging, Earth will at some point no longer be in the habitable zone, as it will get too hot. And if the sun becomes a red giant at the end of its lifecycle, then conditions near Jupiter and Saturn will turn more advantageous. Then Saturn’s moon Titan might become the most fertile place in the solar system.
Proxima Trilogy: Part 1-3: Hard Science Fiction Page 25