If you were looking for Earth 2.0, realistically…
You would not find any within 10 ly (light-years)
When I say Earth 2.0
I mean an exoplanet that has a chance of having Earth-like life. That means similar orbits, similar atmospheres, similar planetary composition, similar amounts of time to develop, similar parent stars, and even similar neighboring planets, though I won’t get into that too much in this article.
In this article and the upcoming series let’s investigate the existing stars and exoplanets and see if any of them fit the criteria laid out to make sure that it is a viable planet for Earth-type life.
Let’s see what we have
Here are all of the stars within 10 ly (light-years).
None of them are viable enough to allow for the creation of an Earth-like planet.
You can see the reasons why shown as checkboxes:
The values and Spectral types are rounded off or simplified to be easier to take on an overview.
Apparently, binary stars and dwarf stars are very common. Here are my criteria for the Star of Earth 2.0:
- Stable orbits
- No extreme stellar events
- The right size
1 — Stable orbits
Just by this criteria, we can rule out all binary stars. This is because a star, even a small star is much heavier than a planet. A brown dwarf would likely be 50 times heavier than Jupiter and red dwarfs would be hundreds of times heavier. Such a heavy object orbiting another star even at large distances would create orbital imbalances. Just take a look at what Jupiter and Saturn can do to our solar system. This shows how big planet movements can cause other objects to have very unstable orbits. Zero eccentricity means stable orbits. The different colors show the different materials in our early solar system and how they were scattered over the first 500 kyr (500 000 years).
Even if there is a safe zone for binary star systems, for now, let’s only consider them without accepting them, as there are too many unknowns. What I’d like to do is replicate the solar system - without assuming too much or playing God.
Here’s an example of what Alpha Centauri A and B binary star system would do to the solar system planets:
Unsurprisingly, Alpha Centauri A and B do not have any discovered planets, which makes sense given the gravitational chaos of two 1000x Jupiter mass objects orbiting each other at the same distance as Pluto is from the Sun.
All of the outer planets would be ejected out of the system while the inner planets would undergo strong gravitational instability that ultimately causes the planets to change orbits enough to be too cold or too hot at times.
2 — No extreme stellar events
Small stars like red drawfs might seem as suitable as bigger stars for life if the exoplanet is close enough, but unfortunately, their stellar winds have much more powerful, unstable bursts. Unlike from a larger star, these winds would likely sterilize developing life on an exoplanet. It might be fitting for an advanced civilization, but not for a primitive one.
And again, even if life is possible around M-type or L-type stars (red or brown dwarfs) there are too many suppositions and unknowns. The planet would orbit the star within a week or weeks, there might be tidal locking, and the red light might also influence life development.
3 — The right size
A larger star such as Sirius has a lifespan of only 1 billion years. Also, its wavelength has much more UV and other destructive wavelengths.
Of course, we could imagine life evolving somehow much more quickly than on Earth. But I prefer not to guess, but to copy. Life on Earth took a billion years to appear and another 3 billion to develop into multicellular organisms. I would prefer an exoplanet with life already on it and not just bacteria, so 1 billion years isn’t going to be enough. I would consider stars older than 2 billion years.
The UV and higher wavelength light, on the other hand, is definitely a deal breaker as it literally sterilizes Earth-like bacteria and life.
Small stars have the previously described issue of extreme stellar events, tidal locking, too-fast periods of rotation, etc.
These stars do not have Earth-like exoplanets nor would they have life on them even if they did
Here is a list of all of the known exoplanets within 10 ly.
They have been found around the red dwarf Proxima Centauri, which obviously garners a lot of attention being the closest star to Earth, and another red dwarf Lalande 21185, which as the numerical name suggests - is not that famous.
Let’s ignore the fact that neither of the systems would work even if we had the perfect planet at the perfect distance, due to the chaotic nature of red dwarfs.
Here are my criteria for an Earth 2.0 exoplanet:
- In the habitable zone
- Made of rocks
- 0.3 — 2 times Earth's mass
1 — In the habitable zone
I think this one is self-explanatory, although you might debate the boundaries and strictness of the habitable zone — the planet has to be in that habitable zone for a very long time. For example, both Venus and Mars are very close to the Sun’s habitable zone, while also being completely inhospitable, I would say just being in the habitable zone is not enough, although there might be atmospheric compensations for an atmosphere that’s too cold or too hot, so let’s keep the habitable zone as is.
2 —Made of rocks
Planet composition is actually very dependent on the distance to its star at the time of formation. I would like a surface in my new world and so a water world is not acceptable. In the case of Earth, we have a rocky-metal planet and yet even from the small number of comets disrupted by Jupiter, that crashed into Earth, we still have over 70% of Earth’s surface covered in water. So really Earth 2.0 should ideally be as dense as Earth is or at most along the 100% rocky line:
3 —0.3–2 times Earth’s mass
A planet that’s too small might have issues holding its atmosphere, while a planet that's too big might have an atmosphere that's too dense and could cause greenhouse effects or other issues. Too many unknowns.
Our six candidates:
Proxima Centauri d is at the right distance, but too small. Not clear if rocky.
Proxima Centauri b is at the right distance, the right size and radius indicating it’s rocky. So, pretty good.
Proxima Centauri C is a Neptune-type planet.
Lalande 21185 B has the density of Earth, but is bigger and in the habitable zone. It might, however, be even stripped of its atmosphere due to the flaring character of Proxima Centauri. It's also likely tidally locked to the star giving it a permanent inferno on one side and a frosty desert on the other.
Lalande 21185 D is a mini-Neptune type. Gas, water — not good.
Lalande 21185 C is a Neptune type. Gas, water — not good.
Perhaps there could be a candidate at the 20 ly mark? 30 ly mark?
I further explore the viability of exoplanets in my other posts, however, you will soon see that such a structure like our beloved Solar system is uncommonly rare in the galaxy, or at least it is very hard to detect.
Out of the 4000-ish exoplanets discovered, most of them are mini-Neptunes and almost none are like Earth orbiting a star like the Sun.
Credits: written by me, edited by friend:)
