Making an Earth (Part 2): A Beautiful Day in the Neighborhood

Article May 9, 2026

If you read last week’s blog post then you already know what this week’s is about, but in case you haven’t let’s catch you up. Last week we looked at all the inherent characteristics of the Earth itself that helped it turn into a wonderful place for life to form, evolve complex forms, and flourish magnificently over long periods of time.

This image of the Earth was taken by the Artemis 2 astronauts on their way to the Moon in April 2026. Credit: NASA/Reid Wiseman
This image of the Earth was taken by the Artemis 2 astronauts on their way to the Moon in April 2026. Credit: NASA/Reid Wiseman

But it turns out that it’s not just Earth itself that is responsible for achieving that state. If every single thing about Earth was the same but its environs were different, it would have been hard to get to a point where I, a fairly intelligent animal creature, am typing this article into a machine of a level of complexity that I cannot explain (plenty of people can, to be fair. I’m not one of them).

So we’re moving on from taking a look at some the ways we lucked out with our planet to some of the ways our planet lucked out with its surroundings, so we’ll all be able to count our blessings properly tonight.

 

Star of the Show

This is a big one: the Sun. We got very lucky with the Sun, or at least that our planet chose to form around a G2V yellow dwarf star (Sun-like, let’s just call it Sun-like). There’s many types of stars out there and a lot of them pose problems if you want to find a life-supporting planet around them.

Let’s imagine the Sun being replaced by a more massive star. This star will be bigger and hotter than the Sun. That’s going to shift the location of the habitable zone, the region where a planet could possibly have the right temperature range to have liquid water on its surface, around the star. With a hotter, brighter star, the habitable zone would be farther out compared to the one around our Sun, and have a greater extent. That’s great, more habitable real estate, right?

Well. Hotter stars are more energetic than the Sun. Their peak energy output is at higher wavelengths of light beyond the visible part we’re mostly used to. An A type star (see this article for more on how stars are classified, but tl;dr an A-type is bigger and hotter than the Sun, maybe twice its mass with a surface several thousand degrees hotter, but nowhere near as big and hot as these things can get) will have its peak energy output in the UV part of the spectrum.

That means any planet in its system is going to be getting absolutely soaked in UV rays. Our Sun obviously puts out UV light as well (wear sunscreen kids!), but it puts out most of its energy in the visible part of the spectrum. That’s why it’s the visible part for us—it’s no coincidence that our eyes evolved to best detect the wavelengths of light our Sun puts out most. 

This diagram of what are known as blackbody curves shows what wavelengths of light are the peak energy outputs of three types of stars, an A-type (larger than the Sun), a G-type (like the Sun), and an M-type (smaller than the Sun). Credit: NASA
This diagram of what are known as blackbody curves shows what wavelengths of light are the peak energy outputs of three types of stars, an A-type (larger than the Sun), a G-type (like the Sun), and an M-type (smaller than the Sun). Credit: NASA

Even if there is a theoretical planet in the theoretical habitable zone around this theoretical A-type star, it’s not habitable. Water molecules split apart when exposed to enough UV light, and loose hydrogen atoms can be easily lost to space from an atmosphere (if the planet can hang onto an atmosphere at all—water isn’t the only thing that gets blasted when exposed to UV).

But let’s say that, hey, we’re making stuff up here anyway, maybe there exists some set of circumstances where a perfectly habitable world can be set up around this star. There’s another problem. Bigger stars live fast and die young. If the Sun was an A-type star it would be dead already. On our entirely made-up habitable world there would be time for life to form, if it did so on the same timeline it did on Earth, but it’s not going to have time to become anything more than single cells before the star gives up the ghost.

 

Think Small?

The seven planets of the TRAPPIST-1 system orbit very closely to their small red star, with several in the star’s habitable zone. Credit: NASA/JPL-Caltech
The seven planets of the TRAPPIST-1 system orbit very closely to their small red star, with several in the star’s habitable zone. Credit: NASA/JPL-Caltech

Okay, what about replacing the Sun with a smaller star? The most common type of star in the universe is M-type red dwarfs. These can get very small indeed, but the bigger ones are roughly half the Sun’s mass. They’re colder than the Sun as well, so the habitable zone around such a star would be close in.

But we know for a fact that these stars form rocky worlds around them and we know for a fact that even with the reduced habitable real estate they can form complex solar systems of many planets that can settle several of them into that zone (looking at you, TRAPPIST-1 system). 

What’s more, these stars live forever. Okay, that’s an exaggeration, but not much. They live very, very long lives. In fact no red dwarf has ever reached the end of its lifespan because the universe has only been around for 13.8 billion years and these guys live way longer than that. Long-lived habitable zone planets—we’ve hit the jackpot, right?

This diagram shows the change in brightness of a flaring red dwarf star. Credit: NASA
This diagram shows the change in brightness of a flaring red dwarf star. Credit: NASA

Well. Probably not. Planets close enough to a red dwarf to be within its habitable zone are very likely tidally locked the way our Moon is, always showing the same face to the star at all times. That would mean one side of the planet constantly being bathed in daylight with the other in eternal night. This will lead to extreme temperature variations between the two sides.

This could actually cause the atmosphere (if the planet has one) to freeze out on the nighttime side, falling as snow the way Mars’s can at its poles in winter. That would eventually draw more atmosphere over from the daylit side where it would eventually cool and freeze which would draw more air over…with enough time (and red dwarf systems have nothing but time) you have a pile of frozen atmosphere on one side and no air anywhere.

The right kind of atmosphere could potentially mitigate this, thick enough with enough air currents to circulate heat from the daytime side to the nighttime side. If the atmosphere could remain stable that is. But these planets have an additional challenge on that front.

 Red dwarfs flare. A lot. Like…really big flares. Imagine if the Sun suddenly became hundreds of times brighter for a few minutes. We’ve seen red dwarfs do that. That sort of sudden energetic outburst is capable of stripping away planetary atmospheres, especially of planets close enough to be in the star’s habitable zone. And, you know, atmospheres are important, at least if you want to go to the trouble of being a living thing.

An artist’s illustration of a flaring red dwarf star. Credit: NASA
An artist’s illustration of a flaring red dwarf star. Credit: NASA

But for the sake of argument (and because we suspect it may be possible on certain parts of habitable zone planets around red dwarfs that flare less than most), let’s say life does form on a red dwarf planet. You’re probably not going to be getting a civilization eventually arising from it.

Red dwarfs are low-energy stars. Their peak output is in the infrared part of the spectrum, and they barely eke over into the visible, let alone higher energies (flaring tantrums aside). That’s okay for simple forms of life, but being complex, let alone intelligent, takes energy. At the base of nearly every food chain on Earth is light from the Sun. Now imagine that energy supply was cut by 90%. That’s the energy we’d be getting from a very energetic red dwarf, if one were to take the Sun’s place. The bacteria might survive, but we bigger critters aren’t going to fare well on that diet.

We don’t think life on red dwarf planets is impossible just…hard. 

 

X Factors

There are plenty of other things that could have screwed up our planet’s habitability chances. I just spent a lot of words explaining how important the type of star is for that, but can you imagine if we had more than one? Many stars are part of multi-star systems. That can make maintaining a stable planetary orbit tricky, never mind keeping up with long-term habitability! We lucked out with a loner star.

We also had a few things going for us elsewhere in the solar system. Having a big brother with the gravitational heft of Jupiter lurking in our backyard is thought to help protect the inner worlds from debris from the outer solar system. Anything that wants to get to the inner solar system needs to deal with Jupiter’s gravity first. Obviously stuff gets through, but Jupiter may be helping to thin the crowd. So maybe part of the reason we have an Earth is because we have a Jupiter.

But there was that time young Jupiter nearly pushed the entire inner solar system into the Sun and was only prevented from doing so by the gravity of the equally-young-and-still-growing Saturn. So maybe to get an Earth you don’t just need a Jupiter, you also need a Saturn as well. 

The Moon and Earth as seen from the DSCOVR spacecraft. Credit: NASA
The Moon and Earth as seen from the DSCOVR spacecraft. Credit: NASA

We also suspect that we might not be quite so lovely a patch of real estate if we didn’t happen to harbor an unusually large moon. Our Moon is the largest relative to its planet in any of solar system planet-moon pairs. Its pull helps gravitationally stabilize the Earth and greatly increases the tides of Earth’s oceans, which helps circulate nutrients and probably gave early life a boost (in addition to all modern life that still depends on a healthy ocean nutrient flow). 

Going even bigger picture, there’s our galactic location to consider! We think some parts of the Milky Way are much more likely to be able to harbor planets with life than others, and we think we’re in one of them. But according to a recent study we (or at least the baby Sun) didn’t start out in the nice part of the galaxy, but in the crowded inner portions where there are so many tightly-packed stars that keeping a planet habitable would be all but impossible. Fortunately, we think galactic shenanigans kicked our Sun out to the quieter neighborhood we find ourselves in today. 

This artist’s image of the Milky Way shows where we think our solar system can be found within it. Credit: Universitat Wien
This artist’s image of the Milky Way shows where we think our solar system can be found within it. Credit: Universitat Wien

And there are many other factors, some of which we think we know about and others that are still mysterious, that probably played a role in helping Earth become the relative paradise we enjoy. All of which brings me to a point you’ve no doubt heard before, but hopefully can appreciate with an even greater understanding.

Earth is precious. Earth is special. Earth is a diamond amidst pebbles. It is worth cherishing, and it is certainly worth protecting. Even if, as is increasingly the case, the thing it most needs protecting from is us.

And asteroids. We should also protect it from big asteroids.