Artist impression (generated with Midjourney AI) of a sunset on Proxima b.

EP. 13: FIVE REALISTIC WAYS TO REACH THE STARS.

On September 29, 2023February 12, 2024 By AlessandraIn Extrasolar Planets, S.E.T.I.

Are there any realistic ways to reach the stars?

In the future, humans will explore the stars. This may happen in a few decades or centuries, but it is inevitable. The long period is due to the stars being incredibly distant, beyond what we can imagine. Our current technology is not advanced enough to travel through interstellar space. However, as we advance in our understanding of physics and technology, we will likely develop new propulsion methods and ways to overcome the barriers that separate us from distant planetary systems.

Our present task is to describe and compare five realistic ways to reach what is currently considered the closest (only about 4.2 ly) habitable planet to Earth, that is, Proxima Centauri b. We begin with something realistic and then move on to more fantastic possibilities.

Why should we travel to Proxima Centauri b?

Traveling to Proxima Centauri b is extremely important for science, the economy, and human understanding.

Venturing to Proxima Centauri b could help us learn more about exoplanets and potentially discover extraterrestrial life. It is located in a region where liquid water could exist, making it a possible habitat for life. Studying this planet would provide valuable information about its atmosphere, geology, and signs of life. These findings would significantly advance our knowledge of the universe and our own existence, helping us answer longstanding questions about life beyond Earth.

Journeying to Proxima Centauri b can lead to groundbreaking technologies, industries, and advancements. Developing efficient propulsion systems, life support technologies, and navigation methods for interstellar travel can have wide-ranging impacts, including benefits for transportation, energy generation, and resource management on Earth. Investing in these endeavors can bring economic growth, job opportunities, and technological progress.

Human nature is driven by a strong urge to explore and push boundaries. The idea of traveling to another habitable planet represents the ultimate achievement, reflecting our curiosity and thirst for knowledge. Interstellar travel represents a future where humanity goes beyond our planet, uniting us and inspiring future generations to pursue science and exploration. This endeavor would have a profound psychological and societal impact, fostering a sense of unity on a global scale.

In summary, traveling to Proxima Centauri b would allow us to gain new scientific knowledge, possibly find alien life, and create innovative technologies. It could also boost our economy and inspire us to explore beyond our limits. This journey would advance our understanding of the universe, unite humanity, and pave the way for interstellar travel.

What Kind of Planet is Proxima Centauri b?

**Fig. 1**: An imaginary landscape of Proxima Centauri b, made by the author with Midjourney AI.

With a minimum mass of at least 1.07 M_E(M_E= 5.9722 x 10²⁴ kg) and a radius only slightly larger than that of Earth, Proxima b is deemed a potentially Earth-like planet. This planet is situated within the habitable zone of Proxima Centauri, although it remains uncertain whether or not it possesses an atmosphere. Proxima Centauri, being a flare star emitting intense electromagnetic radiation, has the potential to strip away any atmospheric layer surrounding the planet. Furthermore, Proxima b is expected to be tidally locked with its host star, meaning that one side of the world would always face Proxima Centauri due to a 1:1 orbit where the rotation period matches the time taken to complete one orbit. The consequences of such tidal locking are still ambiguous regarding whether habitable conditions can arise. In such a scenario, the planet would experience an extreme climate, with only a portion of it being habitable.

Proxima b may not be tidally locked if:

Its eccentricity is higher than 0.1 – 0.06 (that is, the orbit is much flatter than a perfect circle); in this case, the planet would probably enter a Mercury-like 3:2 resonance (three rotations around the axis for every two revolutions around the primary star);
The planet isn’t symmetrical (e.g., triaxial). In this case, capture into a non-tidally locked orbit would be possible even with low eccentricity.

In a non-tidally locked orbit, there are disadvantages. For instance, the planet’s mantle would experience tidal heating, leading to more volcanic activity and a possible loss of a magnetic field. Protecting the atmosphere from the stellar wind is challenging without a strong magnetic field, like Mars.

Proxima Centauri b’s atmosphere has two possible scenarios: either it lost hydrogen and retained oxygen and carbon dioxide, or it still has hydrogen and formed farther away from its star, which would have helped preserve its water.

However, red dwarfs may not be suitable for supporting life due to various challenges and uncertainties.

Among others:

The stellar wind from Proxima Centauri is more substantial than the Sun’s and may remove parts of the planet’s atmosphere;
If a planet is tidally locked to its star, the atmosphere can collapse on its night side;
Proxima b may not always be in the habitable zone due to its eccentric orbit;
Proxima Centauri, a star different from the Sun, had its habitable zone further away in the past. This means that if a planet like Proxima Centauri b formed in its current orbit, it might have been too close to the star for water to exist for up to 180 million years. This could have caused a runaway greenhouse effect, where the planet’s water evaporates into steam and is lost into space, similar to what happened on Venus.

Still, red dwarfs like Proxima Centauri live for a very long time, much longer than the Sun. This gives life a lot of time to develop.

How to travel to Proxima b

There are several ways to travel to Proxima b, and here are five of them that scientists have proposed. One method is the “generation ship,” which was one of the first ways to reach the stars discussed in scientific literature. It is a potential option with our current technology.

(a) Generation Ship:

This idea involves creating a spacecraft that can support many generations of people during a long journey. The ship would travel at sub-luminal speeds, possibly using nuclear power. It’s hard to know precisely how long it would take for the starship to reach its destination, but it could be thousands of years or even more.

This idea is technically feasible with our current technologies. However, it is essential to consider the drawbacks associated with such a venture.

Living your entire life on a spaceship without ever experiencing life on a planet could be really tough for your mental health. Being confined in a limited space, having a boring routine, and not being able to interact with others much can make you feel down. Also, being unable to see different places or try new things may make you feel like you’re missing out and disconnected from the natural world.

Health concerns are also significant when planning a generation ship. Extended space travel can lead to problems like weakened bones and muscles, vision impairments, and increased radiation exposure. The lack of proper medical facilities and resources onboard makes it extremely difficult to maintain the overall health and well-being of the crew.

On top of that, the people living on the ship would have to create their own society. They would need to make rules, govern themselves, and develop their own way of life. It would be a big challenge to keep everyone happy and treat everyone fairly. There might be problems with people wanting too much power or causing trouble. It’s essential to think about all of these things before embarking on a journey like this.

Finally, there are ethical concerns to consider. Is it fair to force future generations into space travel without their consent? Their descendants would have no choice in the matter and would live and die on the spaceship, missing out on the joys of life on a planet. This raises questions about our responsibility to future generations.

(b) Ion Propulsion:

**Fig. 3**: A starship using ion propulsion to reach the stars. Image made by the author with Midjourney AI.

Ion propulsion utilizes electrically charged particles (ions) to generate thrust. This technology is already used in some spacecraft missions, like NASA’s Dawn mission. Ion thrusters provide low acceleration but can maintain continuous and efficient propulsion over a long period. With current capabilities, ion propulsion could potentially reduce travel time to Proxima Centauri to several thousand years, but significant advancements in this technology would be required for it to become a practical option for interstellar travel.

(c) Anti-matter Propulsion:

**Fig.4**: An anti-matter-propelled starship approaching an exoplanet. Image made by the author with Midjourney AI.

Anti-matter propulsion involves using anti-matter to generate thrust by converting mass into energy. This technology has great potential for faster space travel. However, producing, storing, and containing anti-matter is very challenging. Currently, only small amounts of anti-matter can be produced. If these challenges can be overcome, we could potentially reach speeds close to the speed of light, enabling us to travel to Proxima Centauri in a matter of decades or less.

(d) Travel Through a Wormhole:

**Fig.5:** A futuristic starship entering a wormhole. Wormholes, or Einstein-Rosen bridges, are hypothetical shortcuts through space-time. Image made by the author with Midjourney AI.

Wormholes involve creating tunnels or shortcuts in spacetime that could potentially connect distant locations. There is ongoing research in theoretical physics regarding wormholes, but it is important to note that there is no definitive consensus on the existence or feasibility of traversable wormholes.

According to conventional theories of general relativity, wormholes would require exotic matter with negative energy density to stabilize them and keep them open. Exotic matter, which has properties contrary to ordinary matter, has not been observed in nature, and its existence is purely speculative at this point. However, some theoretical physicists have proposed alternative models that aim to avoid using exotic matter or colossal energies. One such approach is the concept of “traversable wormholes without exotic matter,” first put forth by Eric Davis in 1997. This model utilizes a form of matter known as “phantom energy,” which has negative energy but does not violate any physical energy conditions. Phantom energy is a hypothetical concept that arises from quantum field theory and has negative pressure. It remains an area of ongoing theoretical exploration and debate.

If wormholes could be discovered and harnessed, they would allow almost instantaneous travel between Proxima Centauri and Earth.

Furthermore, researchers have also speculated about the possibility of harnessing the effects of quantum entanglement or exploiting so-called warp bubbles to achieve some form of shortcut through space. A warp bubble is a concept derived from the theory of general relativity, which describes the gravitational interactions between objects. In simple terms, a warp bubble refers to a hypothetical method of achieving faster-than-light travel by distorting the fabric of spacetime.

According to general relativity, massive objects like stars and planets create a curvature in spacetime, which we perceive as gravity. The idea behind a warp bubble is to manipulate this curvature in a way that allows for faster-than-light travel. By creating a region of spacetime that is compressed in front of a spacecraft and expanded behind it, the spaceship would effectively be “warped” or propelled through space at speeds greater than the speed of light.

The concept of a warp bubble was popularized by the science fiction series Star Trek, where it is referred to as a “warp drive.” Scientists have proposed various theoretical frameworks, such as the Alcubierre drive, which mathematically describes how a warp bubble could potentially be created. The Alcubierre drive suggests that by contracting spacetime in front of a spacecraft and expanding it behind, the spaceship could ride on a wave of distorted spacetime, effectively bypassing the cosmic speed limit imposed by the speed of light.

However, significant challenges and limitations are associated with the warp bubble concept. One major obstacle is again the requirement of exotic matter with negative energy density. The energy requirements for creating and sustaining a warp bubble are also immense, potentially requiring amounts of energy far beyond our current technological capabilities.

(e) Solar Sail:

**Fig.6**: A spaceship driven by a solar sail is an intriguing possibility to reach the stars. Image made by the author with Midjourney AI:

Solar sails are a fascinating spacecraft propulsion technology that harnesses the power of sunlight to propel a spacecraft through space. They work by utilizing the gentle pressure exerted by photons, or particles of light, emitted by the Sun. These photons can transfer momentum to the surface of large reflective sails, creating a slight but continuous acceleration.

One notable project exploring the potential of solar sails is the Breakthrough Starshot Project. This ambitious undertaking aims to send tiny, gram-scale spacecraft to the nearest star system, Alpha Centauri. The envisioned spacecraft would be equipped with ultra-lightweight sails and propelled by an array of powerful lasers from Earth. By leveraging the momentum provided by the laser beams, these tiny probes could potentially reach speeds of up to 20% the speed of light, significantly reducing the travel time to another star system.

As a final remark, we report an intriguing speculation by Harvard astrophysicist Avi Loeb. In 2018, he proposed that the peculiar interstellar object named Oumuamua, which means “scout” or “messenger” in Hawaiian could be an alien spacecraft propelled by a solar sail.

However, it is essential to note that this speculation remains highly speculative and controversial within the scientific community. The available data on Oumuamua is limited, and alternative natural explanations, such as cometary outgassing or a peculiar shape resulting from its formation, have also been proposed. Further studies and observations are necessary to determine its true nature definitively.

EP. 8: WHAT WOULD BE A GREAT PLACE TO SEARCH FOR ET?

On October 9, 2021October 1, 2023 By AlessandraIn Extrasolar Planets, S.E.T.I.

The Dyson sphere is a hypothetical megastructure physicist Freeman Dyson proposed in 1960.

According to his paper published in Science magazine, a technologically advanced alien civilization would use increasing energy as it grew. As the most significant source of energy in any solar system is the parent star, sooner or later, the civilization would build orbiting solar panels to try to capture it. Such structures would take up more and more space until they eventually covered the entire star like a sphere.

In a 2008 interview with Slate, Dyson also credited the concept to writer Olaf Stapledon, who introduced it in his novel Star Maker in 1937.

Dyson’s hypothesis turned out to be hard to verify because a complete Dyson sphere, absorbing all of the light from the star, would be invisible to an exo-planet hunting telescope (such as NASA’s Kepler). Only half-completed spheres would have a chance to be discovered.

Unfortunately, a Dyson sphere is unlikely to remain under construction for long. The time it takes to make a Dyson sphere is relatively short. A 2013 paper by Stuart Armstrong and Anders Sandberg (“Eternity in six hours: Intergalactic spreading of intelligent life and sharpening the Fermi paradox”) estimates that disassembling Mercury to make a partial Dyson shell could be done in 31 years.

An alternative would be to look for waste heat in the infrared. After being absorbed and used, the energy from a star needs to be reradiated, or else it would build up and eventually melt the Dyson sphere. This energy would be shifted to longer wavelengths so that a Dyson sphere might give off a peculiar energy signature in the infrared. In other words, Freeman Dyson saw a search for his namesake spheres as a complement in the infrared to what Frank Drake’s Search for extraterrestrial intelligence (SETI, see previous blog post) had begun to do with radiotelescopes.

Carl Sagan and Russell Walker first voiced an issue with Dyson’s SETI notion in their 1966 paper “The Infrared Detectability of Dyson’s Civilizations” for the Astrophysical Journal. The authors noted that:

discrimination of Dyson civilizations from naturally occurring low temperature objects is very difficult, unless Dyson civilizations have some further distinguishing feature, such as monocromatic radio-frequency emission.

In the following decades, the search for Dyson spheres expanded dramatically. Starting from the 1980’s researchers went to work using sources identified by the Infrared Astronomical Satellite (IRAS). These early searches produced little o no results, as most Dyson sphere candidates had either non-technological explanations or needed further study. Subsequent investigations using NASA’s space-based WISE (Wide Field Infrared Survey), with higher resolution than IRAS, have all concluded that the identification of a promising source would not in itself be proof of an extraterrestrial civilization unless the object could be followed up with more conventional methods, such as laser or radio search.

Among the latest developments concerning Dyson spheres are the following:

Dyson spheres could be built around black holes instead of stars.

Black holes can radiate incredible amounts of energy (10⁵ more energy than the Sun) produced by the so-called “accretion disk” of gas and dust falling into the black hole’s maw. As a consequence of their spiraling and spinning motions, these materials heat up through friction to millions of degrees, emitting extremely energetic X-ray photons.

But why would an alien civilization decide to build a Dyson sphere around a distant black hole (if it weren’t “distant,” the civilization would have been “eaten” long before it managed to construct the sphere) rather than using their much closer parent star? Black holes concentrate an enormous mass into a space area that is orders of magnitude smaller than a star’s, and are therefore easier to encircle. On the downside, black holes often have bursts of activity followed by quiet periods as they consume varying lumps of matter in their disks. An alien species woulod have to protect their orbiting structures from the huge explosions that might destroy them.

Dyson spheres could be circling the husks of sunlike stars known as white dwarfs.

Every star has a finite lifetime. If a civilization arose around a typical sun-like star, then someday that star would turn into a red giant and leave behind a white dwarf. That process would roast its solar system’s inner planets and freeze the outer ones as the white dwarf cooled off. Consequently, the civilization would have to choose between moving to another system or building a series of habitats that harvest the radiation from the remaining white dwarf. It seems unlikely that a civilization, no matter how advanced, would go through the enormous effort of traveling to another star only to build a Dyson sphere.

This allows a direct connection between stellar lifetimes and the prevalence of Dyson spheres.

If enough aliens decided to build Dyson spheres around their white dwarf homes, then astronomers should find at least one Dyson sphere in white dwarf surveys. The presence of a megastructure like a Dyson sphere around a white dwarf would absorb part of its radiation and convert it into reusable energy. Since no conversion is 100% efficient, this process would leave behind waste heat that would escape as infrared light.

Astronomers have already found many white dwarfs with excess infrared emission, usually explained as dust in those systems, not megastructures. According to a paper by Ben Zuckerman and recently accepted for publication in the journal Monthly Notices of the Royal Astronomical Society, no more than 3% of habitable planets around sunlike stars give rise to a white dwarf sphere-building civilization. Still, there are so many planets orbiting sunlike stars that this calculation only provides an upper limit of 9 million potential alien civilizations in the Milky Way.

EP. 5: ARE ROGUE WORLDS THE ULTIMATE ABODE FOR LIFE?

On February 15, 2021December 1, 2023 By AlessandraIn Extrasolar Planets, S.E.T.I.1 Comment

The search for extraterrestrial life has captivated humanity for centuries. Countless questions arise in our quest to discover if we are alone in the vast universe. The Drake Equation, a mathematical formula introduced by astronomer Frank Drake in 1961, attempts to estimate the number of civilizations within our Milky Way Galaxy. However, recent scientific discoveries have unveiled a new intriguing possibility – rogue worlds. These wandering bodies, expelled from their original solar systems, may hold the potential for harboring life. In this blog post, we will explore the fascinating intersection of the Drake Equation and the enigmatic realm of rogue worlds, exploring the tantalizing notion of life beyond our home planet.

The original form of the equation is the following:

N = R* f(p) n(e) f(i) f(l) f (c) L

• N is the number of civilizations trying to communicate with us right now;

• R* is the rate of star formation in stars per year;

• f(p) is the fraction of those stars which have planetary systems;

• n(e) is the number of Goldilocks (i.e., Earth-type) planets in a planetary system);

• f(l) is the fraction of habitable planets that are inhabited;

• f(i) is the fraction of inhabited planets that possess intelligent technological civilizations;

• f (c) is the fraction of intelligent technological civilizations that choose to emit detectable signals;

• L is the length of time signals will be sent.

The first three factors are astronomical, the fourth and fifth are biological, and the last two factors are social. There are several issues with the equation. Among these:

(1) The uncertainties are large enough for the astronomical factors and increase as one progresses from the astronomical to the biological to the social.

(2) Most factors depend on theoretical insights of star and planet formation, new discoveries about exoplanets, and varying subjective opinions on the evolution of life and intelligence. The presumed longevity of civilization must also be taken into account.

(3) The equation has many hidden assumptions: a uniform star formation rate (SFR) over the Galaxy’s lifetime and a steady state of civilization birth and death.

(4) No matter what value one chooses for R*, the assumption is always that a habitable planet must have a star. However, rogue worlds (bodies that have been thrown out of their own nascent solar system) wander around the Galaxy unattached to a star.

This last item has recently awakened great interest in the scientific community.

Theoretical calculations (Imagined Life, by James S. Trefil and Michael Summers, 2019) suggest that:

“[…] the number of rogues might be between twice and thousands of times the number of conventional planets. Interstellar space must be littered with them!”

Also, rogue planets need not be uninteresting ice balls with no life and energy. Lacking direct radiation from a star, a world can be heated by the residual power from its formation and the radioactive decay of elements in its interior. If provided with one or more moons, the planet can draw energy from a process known as tidal heating (which is responsible for the subsurface oceans on some of Jupiter and Saturn’s moons).

All in all, rogue planets can be compared to (Imagined Life by James S. Trefil and Michael Summers, 2019):

“[…] houses whose lights have been turned off but whose furnaces are still operating.”

Interestingly, rogue planets had been predicted as early as the 1930s by American horror and S.F. author Howard Phillips Lovecraft.

In his short story: The Haunter of the Dark, he wrote:

“[…] remember Yuggoth, and more distant Shaggai, and the ultimate void of the black planets… […].”

When the planet Pluto had just been discovered by Clyde Tombaugh (1906-97) at Lowell Observatory (Flagstaff, Arizona), he wrote another short story: The Whisperer in Darkness.

Here are a few quotes:

“[…] Their main immediate abode is a still undiscovered and almost lightless planet at the very edge of our solar system – beyond Neptune and the ninth in distance from the [S]un. It is, as we have inferred, the object mystically hinted at as ‘Yuggoth’ in certain ancient and forbidden writings; […] I would not be surprised if astronomers become sufficiently sensitive to these thought-currents to discover Yuggoth when the Outer Ones wish them to do so. But Yuggoth, of course, is only the stepping-stone. The main body of the beings inhabits strangely organised abysses wholly beyond the utmost reach of any human imagination.”

And also:

“[…] Those wild hills are surely the outpost of a frightful cosmic race – as I doubt all the less since reading that a new ninth planet has been glimpsed beyond Neptune, just as those influences had said it would be glimpsed. Astronomers, with a hideous appropriateness they little suspect, have named this thing ‘Pluto.’ I feel, beyond question, that it is nothing less than nighted Yuggoth […].”

What would life be like on a rogue planet?

According to Imagined Life, by J.S. Trefil and M. Summers:

“It’s dark. Not midnight-on-a-side-street dark, but trapped-in-a-cave dark. And no wonder—there’s no sun in the sky, for this is a rogue world, one that circles no star. There is a moon up there somewhere, but without a source of light for it to reflect, it’s just a darker patch in the sky. Whatever life forms live on this planet had better be able to see in infrared because there’s simply no other light to be had. You’re wearing infrared sensors, fortunately, and you spot a few of these creatures scurrying back to the planet’s subterranean tunnels, where they can bask in the heat emanating from the planet’s interior. […]”

Life on a dark planet has been described by British author Arthur C. Clarke in his 1950 short story: A Walk in the Dark:

“[…] Here at the edge of the Galaxy, the stars were so few and scattered that their light was negligible. […]”

“[…] Here at this outpost of the Universe, the sky held perhaps a hundred faintly gleaming points of light, as useless as the five ridiculous moons on which no one had ever bothered to land. […]”

“[…] No one could deny that the tunnels out in the wasteland were rather puzzling, but everyone believed them to be volcanic vents. Though, of course, life often crept into such places. With a shudder, he remembered the giant polyps that had snared the first explorers of Vargon III […]

The Drake Equation is not meant to give a precise answer but to stimulate scientific discussion and exploration. It is based on several factors that affect the likelihood of finding intelligent life, such as the rate of star formation, the fraction of stars with planets, the fraction of planets suitable for life, and the fraction of civilizations that develop radio technology. Each factor is multiplied by the previous one, resulting in the number of detectable civilizations in our galaxy. However, many of these factors are uncertain, and different assumptions can lead to different outcomes. For example, some estimates suggest that there could be millions of civilizations in our Galaxy, while others suggest that we might be the only one.

According to a recent study, under the strictest set of assumptions, where life forms between 4.5 billion and 5.5 billion years after star formation, there are likely between four and 211 civilizations in the Milky Way today capable of communicating with others, with 36 the most likely figure. Another study yielded two main results: an optimistic one and a pessimistic one. In the optimistic situation, the researchers suggested the aforementioned 42,777 communicating extraterrestrial intelligent civilizations (CETIs) with an error margin of plus 267 and minus 369, and they would need to survive 2,000 years on average to communicate with us.

The Drake Equation is a fascinating way to explore the possibilities of extraterrestrial life and communication. It helps us understand what we know and don’t know about our place in the universe. It also inspires us to keep searching for signs of other civilizations and to wonder what they might be like.

Read more about this topic in this post and this other post.

EP. 2: IS THE UNIVERSE AN AWFUL WASTE OF SPACE?

On September 5, 2020December 1, 2023 By AlessandraIn Extrasolar Planets, S.E.T.I.

“The universe is a pretty big place. If it’s just us, seems like an awful waste of space.”

This quote is attributed to Carl Sagan from his novel Contact (1985). It is often interpreted as reflecting Sagan’s optimism and belief in the possibility of extraterrestrial life. He strongly advocated for the search for extraterrestrial intelligence (SETI) and believed that the discovery of intelligent life beyond Earth would have profound implications for humanity.

In other words, Sagan suggested that if the Universe is so vast and we are the only intelligent life in it, it would be a shame to waste all that space on just one civilization.

A recent estimate (Conselice C.J. et al. 2016) says the observable Universe contains two trillion – or two million million – galaxies. Of course, this is a huge number, which math buffs can probably better appreciate if I translate it into scientific notation:

two trillion = two million million = one thousand billion = 2 x 10¹²

Even if we neglect 99.9999% of the Universe and consider only the Milky Way, we are left with a staggering number of about 100 to 400 billion stars.

Of course, these hundreds of billion stars vastly differ in age, mass, and chemical composition.

According to the stellar luminosity function:

A small percentage of stars are massive, young, and very bright (the so-called O, B, and A spectral types, with colors ranging from ultraviolet/white to blue);

A relatively large number of stars are medium-sized (the F and G spectral types, yellow to orange in color). Our “dull” Sun is one of them;

The majority of stars are small, old, low-mass stars (the K, M spectral types, a.k.a. red dwarfs);

Many stars are brown dwarfs (dark, spherical lumps of stellar material that never reached the star stage).

In the last few decades, roughly from the early nineties, it has become known that most, if not all, stars possess planets. Our Sun has eight major ones (excluding the KBOs or Kuiper Belt Objects). The former planet Pluto, now demoted to “dwarf planet,” is one).

Just like stars, planets also show a vast range of types.

I found a helpful classification in Imagined Life: A Speculative Scientific Journey among the Exoplanets in Search of Intelligent Aliens, Ice Creatures, and Supergravity Animals by James S. Trefil and Michael Summers. We can envisage the following kinds of exoplanetary environments as the most promising for alien hunters:

(1) Goldilocks Planets: planets like Earth, located at a distance from their star that allows them to have oceans of liquid water on their surface for extended periods;

(2) Subsurface Ocean Worlds: planets on which oceans of liquid water are bounded below by solid rock and above by ice. Examples in our solar system: the planet Pluto and several moons of Jupiter, Saturn, Uranus, and Neptune);

(3) Rogue Worlds: planets without a parent star. Such planets have been ejected from their solar system of origin and now wander through space. An example is OTS 44, a free-floating planetary-mass object located at 550 light-years, with approximately the mass of Jupiter;

(4) Water Worlds: planets with no dry land at all. That’s what a post-apocalyptic Earth would look like. (See, e.g., Kevin Reynolds’ 1995 movie Waterworld);

(5) Tidally Locked Worlds: planets that always present the same face to their star, much as the Moon does with Earth. Their peculiarity is that one side is perennially hot, while the other is an eternal Antarctica;

(6) Super-Earths: planets whose size falls between Earth and Neptune. Given their mass, the main characteristic of these planets is their intense gravity. Creatures must live in oceans or evolve a strategy to deal with this crushing force. A nice fictionalization of this is Edmond Hamilton‘s Starwolf series (1967-68), where Morgan Chane, the son of a human missionary family, grows up in a heavier-than-Earth world.

If these worlds exist, and there’s a tiny chance some might be inhabited, well… I want to see them. I’ll probably never do it in person (sadly, I’m not an astronaut). However, I can still dream about them, hoping someone will get there someday.

I wish someone to be able to say, just like the replicant Roy Batty in Ridley Scott’s 1982 movie Blade Runner:

“I’ve seen things you people wouldn’t believe.
Attack ships on fire off the shoulder of Orion.
I watched C-beams glitter in the dark near the Tannhauser Gate.
All those moments will be lost in time, like tears in the rain.
Time to die.”