# What kind of star should I use for my world?

If I want an Earth-like planet, what kind of star can I use to achieve that ?

According to the Harvard stellar classification, our Sun is a G-class star.

The best would be to have:

• Temperature similar to Earth's, but can be hotter or colder as long as Earth-like beings can live there.
• The life on the planet evolved naturally and is not the result of terraforming.
• A year is more or less made of 365 days. (Ideally)
• There is only one star. (Ideally)

The answer could be whether or not an Earth-like planet is possible around different kind of stars: A, F, K, M, white dwarfs and possibly other stars. It could be more exotic, such as a neutron star.

• Actually, should I make a specific question for each kind of stars? This question is kind of open ended as it is now. – Vincent Oct 6 '14 at 17:22
• Hotter or colder doesn't really matter, you just put your planet further or closer (respectively) to the star. If the length of your year is optional, then there's no criteria one way or the other to pick one star over another. (Well, color/spectrum might come into play, but IMHO that would only affect how photosynthesis evolved, rather than make it not possible.) – Kromey Oct 6 '14 at 17:24
• If the star is too large, it will not burn long enough for its planets to develop life. Oh, and the number of days per year can easily be set by appropriately choosing the rotation of the planet. – celtschk Oct 6 '14 at 19:04
• Related to the topic: Here you can find a overview of how would sky look like for planets orbiting various stars. – Irigi Oct 31 '14 at 10:55

I'll narrow down your list by talking about all the stars you shouldn't use. You'll find it gives you a pretty narrow range.

Let's start with the exciting ones: neutron stars. These are, technically, stellar remnants, leftovers of massive stars that blew themselves apart in supernovae. Supernovae are, in general, not a good thing for planets - or, in fact, anything that happens to be near the star. Given that planets would form early in a star's life (and hence pre-supernova), in many cases it's unlikely that a planet would survive. Just to mess with us, though (!), there are many cases in which planets somehow survived a supernova. Where the central neutron star is a pulsar, these planets are known as pulsar planets. So, actually, it is possible for planets to exist around neutron stars. If the neutron star is a pulsar, the planet may be bathed in enough radiation that life could not exist, but perhaps life would have a chance if the neutron star is not a pulsar.

Next up: A White dwarf. White dwarfs (or dwarves, depending on your personal preference) are also stellar remnants. They are the remains of stars like our Sun, who have cast off their outer layers as a planetary nebula and are now merely the small remnants of their former cores. Planets can exist around white dwarfs - in fact, it is thought that Mars and all the planets beyond it will continue orbiting the Sun for a period of time after it becomes a white dwarf (Earth, Venus and Mercury will likely be swallowed up). Life on Europa could be given a chance when the star expands into its red giant phase, before it becomes a white dwarf. As a white dwarf, there won't be a lot of light to help shine on Europa - in, fact the Sun will cool into a black dwarf - but Europa could temporarily harbor life.

Now I'll go to supergiants. These are the biggest of them all, the class O and B stars. They live short but exciting lives, often only ten million years or so (to put that in perspective, our Sun has been on the main sequence for about 4.5 billion years, and will live for a few billion more). They are extremely massive and very hot. Planets may or may not form here - it can be hard to detect them. At any rate, complex life will certainly not form on planets around supergiants, because of their short lifespan. Ten million years go by, and fft! You get a supernova.

Next on the table are stars more like the Sun - think G, F, or K stars (A stars are more massive, and giant-like throughout their lives). These are the stars that get people excited, because many are solar analogs - stars like our Sun. They have great potential for harboring life, and many think a star like this should be our first target for an interstellar voyage.

Another cool (pun intended) type of star are red dwarfs. These are low-mass stars. They are cool, small, and long-lived, with potential lifespans of trillions of years. They could have exoplanets - in fact, many that we have discovered do - and could thus support life, if the exoplanet is within the star's habitable zone. Proxima Centauri, the nearest star to our solar system, is a red dwarf.

So out of all the basic types of stars, I'd go with a Sun-like star or a red dwarf. They have the best chances to harbor life, when compared to other types of stars.

As @celtschk pointed out in a comment above, you can change the orbital period of the planet to whatever you like by simply changing how far it is from the star. This is the simplest answer you'll get to that question. To make it more complicated, though, I'll note that if you want life to develop on that planet, you do have some constraints. The planet's orbit must be within the star's habitable zone. For red dwarfs, that means the planet must be reasonably close, and so a year equivalent to 365 Earth days may not be possible in all cases.

I'll clear up the brown dwarf angle here, because there's an important distinction to be made between brown dwarfs and other stars. Brown dwarfs are "failed stars" - they aren't massive enough to sustain hydrogen-1 fusion. They are technically designated as "sub-stellar objects", and have been confused with large planets. Their masses can range anywhere from 13 Jupiter masses to 70-80 Jupiter masses. Because they don't undergo hydrogen-1 fusion, they don't emit a lot of light, and so would be poor choices to harbor life, unless a brown dwarf was orbiting another star.

• About red dwarfs: are the star flares very common ? chat.stackexchange.com/transcript/message/18050009#18050009 I can't find how common it is. Also, what about the brown dwarfs? – Vincent Oct 8 '14 at 15:01
• AVincent Brown dwarfs are a no-no because they do not undergo fusion, like other stars do. I'll add that. But Wikipedia seems to confirm what Chris said. – HDE 226868 Oct 8 '14 at 15:05
• I guess this mean no life around Proxima Centauri. If I understand correctly, most M stars are also flare stars ? – Vincent Oct 8 '14 at 15:09
• @Vincent I think the odds for that are slim. It is a flare star. I don't believe that most M stars are also flare stars, because many red giants are also M stars. But many lower-mass red dwarfs may be flare stars. – HDE 226868 Oct 8 '14 at 15:13
• Of course, about your edit in revision 2, technically you could pick a rotational period that gives you a 365-revolutions year at any desired distance from the star. In practice, if you get close enough to the star, I foresee tidal locking preventing that; closer still, the required rotational velocity for any reasonably-sized planet would likely cause prohibitive flattening even if tidal locking isn't a problem. Or, you used "day" as a shorthand not for "rotational period of the planet in question" but for "86400 seconds". Isn't discussing exoplanets fun? :-) – a CVn May 1 '17 at 12:22

The concept you should look into is popularly known as The Goldilocks Zone. What it boils down to (no pun intended) is a formula that tells you what is the possible range of orbital radii where liquid water can naturally occur, and thus a planet where water is present can support life. You can find this zone for most any class of star.

The only caveat here is that there are two parameters you need to keep an eye on (both can also be read from the Herzsprung-Russel diagram mentioned in the article you linked), and that is effective temperature and total mass.

Mass is important because it will let you calculate the orbital period (=the length of the planetary year) for a planet orbiting a given distance from the central star. A simple estimate can be obtained using a simple formula obtained by reversing Kepler's third law. Note however, that it only holds when the mass of the planet is much lower than the mass of the star. This will most likely be the case in your scenario, I'm just including this in case you decided to go for one of the extreme ends of the spectrum.

Effective temperature is important, because it tells you the spectral profile of the light emitted by the star, as well as the total energy. As a rule of thumb, hotter stars will tend towards higher frequencies, and higher total radiant energy output.

Since you need to get something in the neighbourhood 1000 W/m^2 of incoming radiation to get liquid water, the corollary to this rule is that hotter stars will appear smaller when viewed from the planet, since you need to be farther away from them to get the same incoming radiance.

Stars that are hotter, and thus radiating more in higher frequencies, will also have significantly higher UV output, making the starlight potentially hazardous for humans. This gets lots worse as you get to the superhot stars and neutron stars, which will be emitting significant amounts of X-ray and gamma radiation, probably making the surface inhospitable.

Curiously, there was actual serious research done into what sunlight would look like on extrasolar Earth-like planets. The primary paper is Predicting Sky Dome Appearance on Earth-like Extrasolar Worlds, Wilkie & Hošek, SCCG 2013. You can read the paper, or at least take a look at the pictures to get a visual idea here.

• Good answer, there are also theories about other worlds that might hold life though. For example Europa is outside the Goldilocks zone but the theory is that liquid water under the ice crust might contain life. – Tim B Oct 6 '14 at 20:15
• Well, you can theoretically disregard the Goldilocks zone and posit an internal heat source, like the one hypothesized on Europa, but at that point, it really doesn't matter what the star looks like, does it? – Mike L. Oct 6 '14 at 20:19
• One small comment though - if you can change "this formula" to give the name of the formula (Kepler's Third Law?) instead then that would be better. Generally we like links to supplement answers whereas in this case if the wikipedia page is down/edited/not available/etc your question loses value. If the name of the formula is there people can look it up elsewhere. – Tim B Oct 6 '14 at 23:15
• Right, well I tried changing that, but the problem is I can't rightly remember whether that formula has a name; it's just one of a set of basic orbital mechanics equations to me, and I tend to refer to it as just "the orbital period formula" (thanks, KSP). To take what you are saying a bit further, I would also like to make the link to the research paper persistent, but I am at a loss as for how to do that. – Mike L. Oct 6 '14 at 23:37
• If you just quote the paper name/date/authors/etc that's great. Basically the idea is that if the link breaks people can still search for and find the thing that it used to point to (or something equivalent). – Tim B Oct 7 '14 at 6:28

Regardless how hot or cold the star is, you can always place a planet into acceptable climate conditions by just putting it into the right distance. This will impact the duration of the year: for the small cold star, the planet must be closer so the year will be shorter (see orbital period). This depends much less on the mass of the planet as it is very light in comparison to the star anyway.

As the life takes long time to evolve, it should probably be a stable star from the main sequence, burning hydrogen, not helium or anything the like. Other stages of stellar evolution (like red giants or cooling dwarfs) may be too short for creating life "from scratch" but would still fit if the planet has been colonized later.

• Distance from the star only necessarily impacts the length of the year if you're trying to keep the planet's gravity to some constant (eg, 1g). If you're willing to vary the mass, you can change the orbit's radius and keep the same length of year. – Brian S Oct 6 '14 at 21:20
• @BrianS the year is the time taken to orbit the star, which is not significantly dependent on the mass of the planet. Changing the mass of the star will change the length of year for a given orbital radius. – trichoplax Oct 6 '14 at 22:07
• @githubphagocyte, In near-circular orbits, V = 2 pi R / T = sqrt(G M / R). V is the orbital velocity, R is the orbital radius, T is the orbital period, G is the gravitational constant, and M is the mass. Therefore, T = 2 pi R / sqrt(G M / R) and there is some R' and M' such that T = 2 pi R' / sqrt(G M' / R'). You can change the orbit and mass to achieve the same length of year. – Brian S Oct 6 '14 at 23:30
• @BrianS Yeah, the mass in that formula is actually the mass of the central body (ie. the star); Kepler's laws presuppose that the mass of the orbiting body (ie. the planet) is negligible in comparison. – Mike L. Oct 6 '14 at 23:40

Earth-sized planets can exist around any type of star; however, if what you mean by "Earth-like" is "a planet that we could live on," then the specifications get a lot more narrow.

A small star, like a red dwarf, lives for a long time but may be volatile and unpredictable on a year-to-year basis. And even if you found a particularly stable red dwarf, you would be limited by the fact that red dwarfs are colder than sun-like (F, G, or K) stars. To maintain Earth-like temperatures, the planet would either need a very thick atmosphere (resulting in high pressure at the surface) or would need to be very close to the star (resulting in tidal locking that would make one side of the planet very hot and the other side very cold). In short, it would be possible but very difficult for life to exist on a planet orbiting a red dwarf.

Larger stars, on the other hand, have short lifetimes. An Earth-like climate could exist (the year would be much longer than 365 days, since the planet would have to have a wider orbit to compensate for a hotter parent star), and the planet could certainly be habitable for a few million years, but that's the catch: it would only last a few million years. So if you're just talking about people visiting from Earth, a planet around a large star could be a good option. But don't expect it to have a full, rich history of life like we see on Earth.

The problem with using stars more massive than ours isn't the heat: as other answers show, just increase the distance. The trouble is the accelated lifespan. If a star doesn't sit in calm main sequence existance for billions of years, you won't have time for evolution.

You can find diagrams of the age at which various spectral types (or masses) run out of fuel. If you want something more exotic, look at the largest that provide a suitable age.

The trouble with small stars (red dwarfs), besides having to get very close up and deal with tidal locking, is that they flare all the time! However they are abundant so maybe that's the norm?

So for an Earth-like planet with a native complex biosphere, use a star like ours.