Building the ultimate Solar System part 1: choosing the right star
We are building the ultimate Solar System. Here is an introduction to the game.
What kind of star will anchor our ultimate Solar System?
It comes down to two choices: stars like the Sun or cooler, redder stars sometimes called “cool stars” or “red dwarfs”.
Why not stars bigger than the Sun? Because they don’t last that long. The Sun will become a red giant in about 5 billion years, putting its total lifetime as a normal “main sequence” star at about 10 billion years. Stars that are bigger than the Sun are also brighter and hotter and they burn out faster. A star two times as massive as the Sun only lives for about 1 billion years. That may seem like a long time, but it took about that long for life to appear on Earth! In contrast, a star one third as massive as the Sun lives for 100 billion years! That is seven times longer than the age of the Universe. Small red stars basically live forever. They are cool and relatively faint, but they are as constant as it gets.
Why not really really tiny “stars” that are even smaller? Those are called brown dwarfs. They are actually not bad candidates for having Earth-like planets in their habitable zones. In fact, they might be relatively easy to detect in some cases. BUT, brown dwarfs fade. They don’t burn hydrogen in their cores like normal stars do. So, they cool down and their habitable zones sweep inward in time. Any planet has a fixed, relatively short, time in the habitable zone. In some cases a planet can last upwards of a billion years in the habitable zone so I’m only calling this short when you compare it with cool stars that burn at the same brightness forever.
What to choose: stars like the Sun or cool red stars? On one hand, the Sun has been good to us. Earth is here and, let’s be honest, it kicks ass. But that doesn’t mean that Earth wouldn’t be even better off around a different kind of star. Let’s face it, the Sun is going to fry us in the not-all-that-distant future (say, in a billion years or so). And cool stars have a lot going for them. For example, the Earth-sized planet Kepler-186 f was recently discovered in the habitable zone of a cool star.
Let’s have a head-to-head. I’ll go through a few different factors and see who wins, cool stars or Sun-like stars.
STELLAR LIFETIME. Cool stars live basically forever. It’s hard to beat that. Although the Sun’s 10 billion year lifetime is not too shabby. This figure shows how long stars with different masses live. Cool stars are less than about half of the mass of the Sun.
Still, the Sun is slowly getting brighter and hotter. The Sun today is about 30% brighter than it was in its infancy. And in a billion years or so, the Sun will be so bright that Earth will probably get fried. Cool stars don’t change. They keep cranking away at the same brightness for eons. WINNER: Cool stars.
THE HABITABLE ZONE. In this game the habitable zone is our real estate. It’s where we want to build our ultimate planetary system. It is in the habitable zone that a planet can have liquid water and therefore life (as we know it) on its surface. [I have a series of posts about the habitable zone (written with Franck Selsis) coming soon. So, I won’t dwell on the details here.]
This diagram shows where the habitable zone is located for different types of stars.
The habitable zone is much closer-in for cool stars than for the Sun. This is because cool stars are fainter. To keep warm you need to stand a lot closer to a candle than to a bonfire!
The habitable zone is narrower for low-mass stars. It’s almost a full AU wide for the Sun but only a few tenths or hundredths of an AU wide for low-mass stars. Does this mean there is less space for planets? No! The orbits of a system of planets tend to be spaced in a logarithmic way (for example, at 1, 2, 4, 8, 16 rather than at 1, 2, 3, 4, 5) . There is about the same amount of “dynamical space” for planets in orbit around cool stars and Sun-like stars. WINNER: Tie.
RADIATION. All stars give off light with a wide spectrum of different energies. Our eyes only see at certain wavelengths, in what we call “visible” light. High-energy light such as ultraviolet (UV) and X-rays can be damaging. UV light causes sunburns. Strong X-ray and UV irradiation can act to strip a planet’s atmosphere or dry it out. This is a pretty complicated process: water is first broken into hydrogen and oxygen, then the hydrogen can be kicked off into space never to return.
Even though they are cooler and fainter, low-mass stars have proportionately larger high-energy light than Sun-like stars. This comes from the outer part of the star that are magnetically active, called the chromosphere. In a star like the Sun, the chromosophere is very active and produces a lot of X-rays and UV for just a small fraction of the star’s life. Then it quiets down. But for low-mass stars the chromosphere keep cranking out high-energy light for billions of years.
High-energy light is not all bad. Some UV irradiation may have been needed to kick-start life. It has also been proposed that UV may be a requisite for photosynthesis. In this game mutations from UV light sound good. Because we want our aliens to be as crazy and diverse as possible! But, too much UV and you’re fried. How much is too much? We don’t know. Hmmm…. WINNER: Slight edge to Sun-like stars. But cool-ish stars that get a little more but not too too much high-energy light might be okay.
TIDES. When you think of tides, you probably think of the Moon making the ocean slosh around. Tidal forces are simply differences in gravity. When a planet is close to a star (or the Moon to the Earth), the side facing the star feels a stronger gravity than the opposite side. This stretches the planet out. The amount of stretching changes if the planet moves a little closer or farther from the star. This causes a few things to happen. The planet tries to always show the same face to the star (like the Moon does to the Earth). This means the planet spins once every time it goes around the star. In this image it’s a moon that is “locked” to a planet.
Same idea for a planet locked to a star. Only one side of the planet ever sees the Sun. Permanent night on one side, permanent day on the other. And permanent sunset in between! [I’ll mention in passing that tides also make a planet’s orbit as circular as possible, but that’s not too important right now.]
Put simply, tides do two things that matter in this context. First, tides heat up the planet by dissipating energy inside it. Second, tides makes the planet always show the same face to the star.
Are these things good or bad? Some heat from tides could be helpful — it might stir things up in the mantle and help plate tectonics to occur. Too much is bad. Io, Jupiter’s closest big moon, is strongly heated by tides and is riddled with permanently active volcanos. Same goes for having the same side of the planet face the star. A planet with a thin atmosphere could end up getting fried on the dayside and frozen on the nightside. But on a planet with an Earth-like atmosphere, clouds can pile up on the dayside, smooth out the temperature across the whole planet, and even widen the habitable zone.
Tides are stronger for planets that are closer to their stars. We know that the habitable zone is closer-in for cooler stars. So tides are stronger in the habitable zones of cool stars.
WINNER: Sun-like stars, by a hair. There are enough “bads” to make me wary. But weakish tides are okay, even beneficial.
THE FINAL VERDICT
I am pretty torn. As I explained, we don’t know whether UV is good or bad for a planet or whether tides are good or bad. The head-to-head is a wash. Based on what we know right now there is no compelling argument in either direction.
So I’m going with my gut. I’m choosing lowish-mass stars. Stars about half the size/mass of the Sun. Toward the hotter end of the cool stars. These stars end up with most of the pluses of cool stars without the minuses. These stars live a lot longer than the Sun with a more gradual change in brightness. Their habitable zones are just close enough to their stars that tides are strong enough to possibly help with plate tectonics and widen the habitable zone but too weak for massive volcanism. These stars emit high-energy light (UV etc) for longer than the Sun but not forever. And the clincher is that we can find planets in the habitable zones of these stars! The star Kepler-186 is half the size and half the mass of the Sun. It is host to habitable zone Earth-sized planet Kepler-186f and possibly an additional yet-to-be-detected habitable zone planet. We can’t find Earth-sized planets in the habitable zones of Sun-like stars (yet).
Up next: which kind of planets do we want in our ultimate Solar System?
I like low mass stars too. Should we worry about flare activity? These stars can be temperamental when young, and we don’t want our young biospheres getting fried or poached. Could we solve this by crashing vast amounts of Ritalin into the star to calm it down? Any thoughts?
David, you’re right that the temperamental flaring period lasts longer for low-mass stars. Since low-mass stars evolve so slowly, it’s like they spend forever in their whiny teenage years. It’s easy to imagine that flares could pose a threat to these planets’ atmospheres and maybe biospheres. I just asked two atmosphere experts in my building (Franck Selsis and Philip von Paris). Their take on flares are that they pose little danger as long as the planet has an atmosphere, or even a thin layer of sand or water. Another way to look at this is that when life arose on Earth, the Sun was about 15-50 times more active than it is today (different values for different spectral bands; Ribas et al 2005). Although that number refers to sustained activity, not flares, which are basically just burps on top of this.
«Cool stars are smaller than about half of an Earth mass.» That can’t be right.
Good catch! That makes no sense at all! I just fixed it.