We went hiking in the Solar System [link], but what lies beyond? It’s hard to imagine now, but before the 1990s astronomers were not even certain that any other star hosted planets: discovering the first exoplanets in 1992 and 1994 made headlines, and they all got individual names (Poltergeist, Phobetor and Draugr, a variety of undead creatures, ghosts and beasts – perhaps reflecting the lack of sleep of the astronomers involved?).
Our methods and telescopes have evolved by leaps and bounds since the 1990s, and finding exoplanets have become everyday news. Now we know that, statistically, almost every star in the galaxy is likely to have at least one planet, and many even have multiple. In NASA’s excellent online exoplanet catalogue [link] there are 5535 confirmed discoveries today, and the counter moves up very frequently. But how relevant are these discoveries? Finding a Neptune-sized gas planet with unique properties 500 light-years away could stir scientific curiosity, but it’s totally out of reach to visit, and why would we go there anyway?
Naturally, we have special interest in habitable planets, and the closer, the better. In the crudest manner, habitable = liquid water on rocky surface. Liquid water means warm, but not too warm surface temperatures: the planets needs to be located in the “Goldilocks” or habitable zone, just the right distance from the star for optimal temperature range.
Rocky surface currently is assumed in function of the exoplanet’s size: if it’s roughly Earth-sized or smaller, it is thought to be composed of rocks. We also have “super-Earths”, a class of planets unlike any in our solar system, which are between twice the size of Earth and up to 10 times its mass. These could be made of gas, rock or a combination of both: if the surface is solid enough, they could be habitable.
So far, this is all the information we have about most exoplanets: their distance to their star, their size, and often, the star’s stable or unstable nature. Many of the red dwarf stars hosting exoplanets are volatile types, and frequently emit deadly flares that would strip the planets of their atmosphere. As red dwarfs are cooler than our sun, their habitable zone is located very close to the star, and these flares really hit the planets at a short distance.
A good-natured star, suitable distance in the habitable zone, the right size – our solar system harbours three such planets (Venus, Earth, Mars), but to our best knowledge, only one of them is habitable. With more research, some already undergoing, astronomers hope to confirm life-friendliness to a greater certainty. For “Habitable+”, the exoplanet would need to hold an atmosphere, though preferably not too dense (to avoid runaway greenhouse effect); a magnetic field would be helpful to protect from radiation; and finding oxygen in the atmosphere, a telling sign of life, would be bingo.
There are a great number of promising candidates in the exoplanet catalogue, but we soon reach the other limiting factor: distance.
What would be still a realistic distance to look at when we talk about visiting exoplanets? Naturally, we’re not talking hiking and day trips, but rather a one-way ticket with a view of colonising. Nonetheless, a lot of pessimists/realists would point out that even at the velocity of the fastest man-made spacecraft, NASA’s Parker Solar Probe (535,000 kilometers per hour – 0.05% of the speed of light), it would take 8400 years to reach the nearest exoplanet. And then, the solar probe achieved this speed by a Venus flyby, dipping into the planet’s gravity to accelerate, and the whole probe is about the size of an average car, with a payload 50 kilograms – a far cry from a manned spaceship.
Then come the optimists/dreamers, who would still entertain the idea of humankind reaching and settling exoplanets within a few centuries. Obviously, we will need an entirely new type of propulsion to reach greater speeds, and a combination of suspended animation/intergenerational spaceship. At a velocity of 1% of the speed of light, it would take 420 years to reach the nearest exoplanet. At 2% of the speed of light, it would be 600 years to a bunch of exoplanets located at 12 light years away from us. Not (entirely) out of the realm of possible!
The exoplanet catalogue figures are promising: 54 of the confirmed exoplanets are maximum 20 light years away, and a good part of them are Earth-sized or super-Earth type (2-10 bigger than Earth, with a chance of a solid enough surface). At such early days as we are with exoplanet exploration, we’ll probably find a lot more in the coming years, and also find out more of these candidates. While waiting for these results, I’ve compiled a list of the 10 most promising candidates: here are the 10 best and nearest exoplanets for setting up home!
- THE CLOSEST: Proxima Centauri b
- THE MOST SOUGHT-AFTER: Alpha Centauri
- THE CALMEST: Ross 128 b
- THE BALMIEST: Luyten b
- THE LEAST NOTICED: Gliese 1061 c, d
- THE SUNNIEST: Tau Ceti f
- THE FANCIEST NAME: Teegarden b, c
- THE WILDEST: Wolf 1061c
- THE COOLEST TWINS: Gliese 1002 b, c
- THE BEST ASSORTMENT: TRAPPIST-1 system
1. THE CLOSEST: Proxima Centauri b
These centaurs can be quite confusing, so a quick introduction: we have the star system Alpha Centauri, with 3 stars, of which dimmest – called Proxima Centauri – is the nearest star to the Sun, 4.2 light years away. The other two brighter stars Alpha Centauri A and B form a close binary system, and we’ll have a look at them later.
The “closeness” of Proxima Centauri is relative: it is 40,208,000,000,000 [read: 40 trillion] km away. Our fastest interstellar probe, Voyager 1, would take 73,000 years to arrive. In this mighty large galaxy, this is our nearest neighbour.
The good news is that Proxima Centauri has at least 3 exoplanets, one of them in the habitable zone: Proxima Centauri b is the second planet from the star, and it is a terrestrial-type rocky planet, only slightly heavier than Earth.
Proxima Centauri is a small red dwarf star, its mass hardly more than 10% of our Sun, so the habitable zone is located close to the star. Indeed, planet b is on very close orbit to its sun, at a distance of 7.5 million km – for comparison, this is only 20 times further away than Moon from Earth, and any asteroid coming this close to Earth is automatically considered to be a “potentially hazardous asteroid” by NASA.
Which is bad news for Proxima Centauri b, because this red dwarf is one of the volatile types, with regular stellar flares producing extreme ultraviolet radiation. Such an active star could not only irradiate any life, but could also sweep away any atmosphere – so the habitability of Proxima b is highly uncertain.
2. THE MOST SOUGHT-AFTER: Alpha Centauri
As explained above, the other centaurs, Alpha Centauri A and B, form a binary system, which is still in our closest neighbourhood, 4.3 light years away. Due to their closeness, and as they are both quite similar to our Sun, the system has long been in the eye of avid planet-hunters. Estimates place the probability of finding a habitable planet around Alpha Centauri A or B at roughly 75%. There have been some false positives around 2012-13, when scientists believed to have found evidence for exoplanets around Alpha Centari B, but these have not been confirmed, and the hypothetical orbits seemed outside the habitable zone, too hot to be comfortable.
Search is going on – currently, Alpha Centauri is the target of several exoplanet-finding missions. Breakthrough Starshot, the brainchild of a group of social media savvy billionaires and scientists, conceptualised a flyby to Alpha Centauri, with the probe reaching 10% of the speed of light with a combination of nuclear propulsion and solar sail. At such velocity, the mission could reach Alpha Centauri and report back to Earth in a span of 24 years – such a feat would certainly change our way of looking at interstellar distances. As to the launch of the mission, the timeline is a broad “within a generation”, with the speed of funding playing a big role in reaching the speed of interstellar travel.
Since then, NASA also released a mission concept to launch a probe to Alpha Centauri, suggesting the use of solar sails propelled by high energy lasers, and a flight time of 44 years to Alpha Centauri. While they have a proposed launch date (2069), so far they have no budget whatsoever. For the time being, in our telescopes we trust: a confirmed discovery of exoplanet(s) in the habitable zone of Alpha Centauri would boost interest (and funding) for the projects.
3. THE CALMEST: Ross 128 b
With Ross 128, we enter the realm of strange-sounding suns. If anyone wonders, yes, it was a guy called Frank Elmer Ross who first spotted this sun, in the same manner as it was a guy called Luyten, Teegarden, etc. in the following. We also move considerably further away from our Solar System, into a region of space where we have a good number of systems with confirmed exoplanets at a distance of 12 light years.
Ross 128 b is the nearest exoplanet found to orbit a quiet red dwarf, and it is considered one of the best candidates for habitability. It is Earth-sized, likely rocky, and located in the habitable zone of its sun, at about 11 light years from Earth. Star Ross 128 is only one fifth of our Sun in terms of mass and radius, and it emits weaker starlight, so any habitable planets needs to stay on close orbit. Due to this close distance, Ross 128b is probably tidally locked, always showing the same face to its star.
Not long ago, tidally locked planets were considered too inhospitable to settle.
Take Mercury, for instance: on the side facing the sun, temperatures reach 800°F (425°C), while on the side facing away, they drop to -300°F (-185°C). Recent thinking, however, has become more lenient. If the planet has a thick enough atmosphere, it could spread the warmth and cold across the dark/light dividing line, creating liveable areas. Oceans can play a role, too, by creating evaporation and clouds, and by cold and warm current (just think of the Gulf current on Earth). A good summary can be found here [link]. All in all, this is good news for exoplanet fans, as most what has been found so far is orbiting red dwarf stars – not just because red dwarfs far outnumber other type of stars (see below), but also because it is more difficult to detect planets of brighter host stars.
4. THE BALMIEST: Luyten b
Another nearby red dwarf 12 light years away, and another planet in the habitable zone. Luyten b is in the super-Earth category, meaning it’s almost 3 times bigger than Earth, but it’s still a good chance that it’s rocky. It has approx. the same volume of starlight as Earth, which makes it one of the best candidates for real habitability. We don’t know yet if it has an atmosphere, but with an Earth-like atmosphere, average surface temperature would be around 292 K (19 °C), very similar to our home planet. Cherry on top, it is one of the rare exoplanets orbiting a red dwarf that is not tidally locked. Another cherry – Luyten seems to be one of those good-natured, inactive stars without dramatic flares.
The presence of so much fruit prompted METI (very similar to SETI, but instead of “search”, read “messaging”) to send a musical/scientific message to the planet in 2017. The only musical piece I recognised was from Jean-Michel Jarre, but you can listen to them here: https://www.sonarcalling.com/. Let’s watch out for the Luyteniens’ response around 2041.
5. THE LEAST NOTICED: Gliese 1061 c, d
Three Earth-sized planets orbiting a peaceful red dwarf barely 12 light years away, two of them in the habitable zone, and they are entirely missing from the headlines? Definitely “up and coming” neighbourhood. Only recently discovered in 2020, the system has one planet on the innermost cusp of the habitable zone (c) and one in the zone (d): both planets are less than twice the mass of Earth, with a good probability of being rocky.
1061c could make an excellent jungle planet, with considerably more starlight than Earth, and an estimated average surface temperature of 34 C (provided it has an Earth-like atmosphere, which we obviously don’t know at this point). 1061d, on the other hand, looks rather chilly: with a lot less starlight than Earth, even with the warming effect of an Earth-like atmosphere the average surface temperature would be around 250 K (−23 °C). People from the frostiest corners of Siberia would find this familiar, as some of their coldest settlements have comparable annual mean temperatures. Other factors such as oceans, a denser atmosphere etc. could make a difference to the climate of this planet – with a bit of luck, it could be more Sweden, less Siberia.
6. THE SUNNIEST: Tau Ceti f
There are ten red dwarfs to every G-type star in our galaxy, and with our current methods it’s easier to detect planets around dimmer stars (like red dwarfs). Star Tau Ceti is therefore the closest single G-type star to us (Alpha Centauri, mentioned above, is closer, but it’s a multiple star system). G-type stars are the familiar kind of suns, similar in luminosity, strength and spectrum of light to Earth’s Sun. They are also the shortest lived with an average 10 billion years of lifetime, but they have at least two massive advantages when it comes to hosting liveable planets: their habitable zone is much bigger, and they emit on average 400 times less harmful radiation compared to red dwarves.
So finding yellow-light bathed planets around Tau Ceti, 12 lights years away, is definitely good news. So far, five planets have been identified, two of them orbiting in what could be considered, generously, the habitable zone. Good news are ending here, however. All five planets fall in the Super-Earth category, meaning that they may be gaseous rather than rocky. A further hiccup is location: Tau Ceti e is just a tad too close, while Tau Ceti f a tad too far. There are many factors affecting a planet’s surface temperature, a big number of which (inner core activity, oceans, atmosphere) we know nothing about yet – so no need to drop our optimism entirely. Estimates for the two planets range from Venus-like hellhole to hot-ocean planet in case of Tau Ceti e, and from frozen ball of ice to thawing frozen ball of ice in case of Tau Ceti f. The latter is explained by a rise of the star’s luminosity in the last 1 billion years – it may just have been enough time to allow for liquid water to appear on the planet’s surface.
Some perceive an added headache in the form of frequent bombardment by asteroids and comets from the Tau Ceti’s massive debris ring, a phenomenon that could affect the hospitableness of the two planets – but it needs more studying.
7. THE FANCIEST NAME: Teegarden b, c
Call your planet “Tea Garden”, and you’ll catch the eye of investors. Even though it was a coincidence that astrophysicist Bonnard Teegarden had a pleasant-sounding name, it still gives the star system a slight competitive edge.
Teegarden’s star is a red dwarf approximately 12.5 light years away, and it has two planets orbiting in the habitable zone. Both seem to be near in size to Earth. Teegarden Star is so weak that habitable planets need to stick close: planet b is the innermost planet, completing an orbit around the star in 5 days. As the planet is vastly closer to its star than Mercury to Sun, the red dwarf star must be enormous on the (tidally locked) horizon.
Teegarden’s research team made all kinds of modelling, calculating for example the range of atmospheric density that would allow for liquid water to exist on the planets’ surface – a broad range, it seems, from one-third of Earth’s atmosphere to something 17 times as dense. Working with similar models, they also calculated that Teegarden b has a good chance of having surface temperatures between zero and 50 °C. Encouraging news for settlers and tea-growers.
8. THE WILDEST: Wolf 1061c
On the planetary lottery, play the number 1061! Even in our small sample of 10, the number comes up for the second time.
Wolf 1061 is another red dwarf of the quiet type, not in the habit of deadly irradiation, approximately 14 light year away. We have spotted 3 planets in the system so far, one of which, a planet of Super-Earth category, is in the habitable zone. Wolf 1061c is about 4 times more massive than Earth, but also with a bigger radius, so surface gravity would be only about 1.6 times higher than on our planet. There is a good chance that the planet is rocky.
Altogether, it is an excellent candidate for hospitable exoplanet, with one outstanding concern, however, regarding its orbit. According to some modelling, a massive outer planet in the system, Wolf 1061d, could have a highly eccentric orbit that perturbs the orbits of the inner planets. Eccentric orbits could cause the planet to flip from much further away to much closer to the star on an irregular basis, possibly within the span of a few thousand years. This on the one hand could lead to unpleasant ice ages varying with tropical heat on a short timescale. However, there is an even darker side to high orbital eccentricity: tidal heating. The planet’s interior can warm up: in small dosage, this can actually help to preserve habitability during ice ages, but excessive tidal heating can cause savage volcanic activity, causing the planet to turn into sterile ball of molten rock. Then again, we’ll need to know a lot more about the dynamics of the Wolf system to be certain.
9. THE COOLEST TWINS: Gliese 1002 b, c
Yet another red dwarf star without dramatic flair, located 16 light years away, with two planets in the habitable zone. GJ 1002 b and c are only slightly more massive than Earth, making them very likely rocky planets, and they are on close orbit (10 and 20 days to complete, respectively) to the star.
Of the twins, planet b looks more attractive: mass and radius practically identical to Earth, with an estimated (given certain factors, including an atmosphere) surface temperature of 261 Kelvin (- 12 C). The distance between the two planets is minimal, only 4 million kilometres on average – for comparison, this is less ten times the average Earth-Moon distance. Imagine having not a moon, but a whole planet bigger than ours at such close quarters! When a biggish asteroid passed by Earth at about 4 million kilometres in 2022, headlines called it a “close miss”. Given the minimal distance, planet c (all else being equal) would only be slightly chillier than planet b.
10. THE BEST ASSORTMENT: TRAPPIST-1 system
This system is a lot more far-flung than the stars above, located 40 light years away from Earth. Still, it has been the focus of quite a lot of interest due to a unique assortment of seven planets in a small system, all of them about the same size or smaller than Earth, so probably rocky. Then again, the name also may play a role, evoking the famous monastic (“Trappist”) beers from Belgium. Though officially the denomination stands for “Transiting Planets and Planetesimals Small Telescope”, one of the universities involved in the project is located, where else, in Belgium.
The whole the Trappist system is tiny: all seven planets would fit the within the orbit of Mercury and our sun.
Their star is a very cold red dwarf, roughly the size of our Jupiter. At least three, possible more of the planets are in the habitable zone. Trappist 1 b and 1 c are rather too hot for comfort, with estimated surface temperatures of 450 F (225 C) and 225 F (107 C), but they are tidally locked, so these temperatures describe their day side – in their twilight zone, they could be more hospitable. Trappist 1 d has a better chance of being habitable, receiving slightly more starlight than Earth, but it’s a smallish planet, so its gravity may not be sufficient to retain an atmosphere.
Trappist 1e is the current favourite candidate for habitable environment, with a slightly smaller size than Earth, but comparable gravity. It is smack right in the middle of the Goldilocks zone, and receives about the same volume of starlight as Earth. Should we be able to confirm the presence of an atmosphere, liquid water on the surface would be a very high probability.
Then we still have Trappist 1f and 1 g, two maybes on the outer edge of the habitable zone – they would be cold, but with a dense atmosphere and/or some volcanic activity, they could retain just enough heat. The list ends with the seventh planet, h, which is definitely too cold for comfort.
For the moment, flybys to the Trappist system are not a real option: even with the nano-spacecrafts of the Breakthrough Starshot project, it would take 200 years to reach. But many of our telescopes will focus attention on this intriguing exoplanet reserve in the hope of confirming the presence of atmosphere and detect specific molecules such as carbon dioxide, methane, nitrogen, oxygen and water. In the meantime, there is a popular initiative to name the seven planets properly – ideas ranging from the Greek seven sisters (Pleiades) to the seven sons of Feanor to, obviously, Doc, Grumpy, Happy, Sleepy, Bashful, Sneezy, and Dopey.
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