A new Biocomplexity article - "The Solar System: Favored for Space Travel"

That ignores the fact we have uncertainty about the precise function we are optimizing.

@PdotdQ, meet Project Pluto!

Also, if you really want to get into it, follow this link and search “Pluto”.


Atomic Rockets is a site maintained as a resource for science fiction writers. Or maybe Winchell Chung is an evil genius scheming to take over the world … hard to tell. :slight_smile:

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No because that objection cuts both ways.

Radioactive power can be used in lieu of chemical power for both liftoff and course correction. They are much more efficient and thus you don’t need as much tonnage to achieve liftoff. On Earth, we do not need them because the surface gravity is low enough for chemical liftoff. Note that we do use radioactive fuel in space exploration; ~half of humanity’s plutonium is inside New Horizon (though they are used as batteries instead of rockets).

The closer a planet is to Earth, the more habitable it is (at least, for life-forms as we know it, i.e., those that are like Earth’s lifeform).

There are two “definitions” of Super-Earths:

  1. planets that are larger than the mass of the Earth up to ~10 times the mass of the Earth
  2. planets that are in the lower end of the mass range in 1) (with the upper range called “Sub Neptune”)

The most common type of exoplanet is, according to current evidence, sub-Neptunes (not super-Earths according to definition 2, but super-Earths according to definition 1). Sub-Neptunes are likely not habitable by lifeforms as we know it (i.e., ones similar to life on Earth), while the “super-Earths” of definition 2) might be habitable.

I do know of Atomic Rockets (due to my interest in science fiction), but thank you for the reminder!


This was hard to follow. I think you mean,

The most common type of exoplanet is (1). But (1) are not likely to be habitable by lifeforms like we see on Earth. However, (2) are the ones that could be habitable. Of course (2) are much like planet earth, and are smaller, and space travel might be possible with them.

So, you are saying that by conflating these two categories, we are counting these 10x size worlds as “habitable” but they really are not. That is the egregious error you see, right?

Is there reason to think that intelligent life on (1, those ~10 times the mass of Earth) would not be possible? Of course it would not be Earth-like, but would they really not hospitable to any intelligent life? If we can’t rule that out, doesn’t that leave his argument intact?

Wikipedia to the rescue?

According to one hypothesis,[92] super-Earths of about two Earth masses may be conducive to life. The higher surface gravity would lead to a thicker atmosphere, increased surface erosion and hence a flatter topography. The end result could be an “archipelago planet” of shallow oceans dotted with island chains ideally suited for biodiversity. A more massive planet of two Earth masses would also retain more heat within its interior from its initial formation much longer, sustaining plate tectonics (which is vital for regulating the carbon cycle and hence the climate) for longer. The thicker atmosphere and stronger magnetic field would also shield life on the surface against harmful cosmic rays.[93]
Super-Earth - Wikipedia

This would imply that 10x are not potentially habitable?


We cannot say anything on whether they are hospitable/not for any intelligent life. We can only say that they are most likely not habitable, or at least less habitable than rocky worlds, to life that looks like those on Earth.

If one allows for any kind of intelligent life, then the parameters of the problem becomes too wide for this problem to be relevant. For example, perhaps these are gaseous life-forms floating in the weakly-bound upper atmospheres of their planet. These lifeforms do not require (and thus do not build) chemical rockets to go to space.

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Maybe I don’t have a reasonable picture in my head about these 10x worlds. Do they not have land? What are they like?

So what do you make of this article?

Super-Earth Kepler 20b, for example, is nearly double the size of Earth and is 10 times more massive. Making its surface gravity almost 3 times stronger. That stronger gravity means the planet can hold on to more air molecules forming a thicker atmosphere.

Which is great for protection against harmful space radiation. It also means mountains and hills would erode faster leaving a relatively flatter surface compared to Earth. Which might sound boring but scientists think this could actually spawn dozens of shallow islands across the planet.

Which, in turn, could be the perfect place for life to form and evolve. “Just as biodiversity in Earth’s oceans is richest in shallow waters near coastlines, such an ‘archipelago world’ might be enormously advantageous to life.”

There’s just one problem leaving this tropical paradise would be extremely difficult. The escape velocity on Kepler 20b is more than double compared to Earth’s. Which means either rockets would need more fuel to reach their destinations. For example, a mission similar to the Apollo moon landings would require twice the amount of fuel or, rockets could only carry a fraction of the payload.

Maybe this image is useful:

Note that according to definition 1) above, both the “Super-Earths” and “Mini-Neptunes” in this picture are classified as “Super-Earths”.

It really depends on the particular planet that one is talking about. Not all Super-Earths or Sub-Neptunes are alike (the same as Venus and Earth are both Earth-like planets, yet one is extremely habitable, and the other not so).

Edit: No one knows the answer for sure, but it is reasonable to think that the more Neptune-like a planet is, the less habitable it is for Earth-like-life (almost tautologically).


If God wanted to make it easy for us to explore space, he would have made a universe in which things were closer together,

There, that’s a one sentence article I could submit to “BIO-Complexity”, and it would be about as scientifically significant as this one.

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What is a gas envelope if not the atmosphere? Why doesn’t earth have an atmosphere in the diagram?

Gas envelope is an atmosphere, but those that are large in size. In the diagram, if they even bothered drawing it, the Earth’s atmosphere is maybe less than a pixel thick.

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But not, AFAIK, to Proxima Centauri - it would be a massive co-incidence if new Horizons was pointed in the right direction, That’s why I asked if it was. Gonzalez specified Proxima Centauri, so the New Horizons probe potential is probably irrelevant.

So there is not “serious consideration of sending an interstellar probe to the closest star, Proxima Centauri”.

They have enough fuel to choose. Look it up.

Yes there is. Don’t forget about the breakthrough project. Perhaps his reference was wrong, but be serious here. There really is consideration of precisely this.

Out of curiosity, how long would it take this “probe” to arrive at Proxima? Are we talking about several thousand years? If so, I don’t think I would consider it a serious probe of anything other than very near interstellar space.

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So I looked it up. They have enough fuel to choose new targets, possibly including a star. This does not mean they have enough fuel to choose any star. In fact, the New Horizons team aren’t even sure that they have enough fuel to manoeuvre towards any new Kuiper Belt objects they might find:

So as we look forward to the 2020s, our team is planning the next few years for New Horizons. Starting next summer, we plan to use some of the largest ground-based telescopes and possibly the Hubble to begin a search for new KBOs to explore, both up close and in the distance. (We can’t search until summer because that is when the part of the sky where New Horizons is going is in the darkness of the night time sky.) We don’t know how many KBOs we will discover or whether any will be within our fuel supply to reach for a final close flyby, but that’s what these searches, in 2020 and again in 2021, will reveal.

New Horizons is heading towards Sagittarius, which has an ascension of ~18.5h and a declination of -25˚. Proxima Centauri is in Centaurus, and has an ascension of 14.5h and a declination of -62˚. To point New Horizons at Proxima Centauri would require it make a sharp right turn, in an area where there are no large objects that could affect its trajectory. It’s not going to happen.

There are no serious considerations to send New Horizons to Proxima Centauri, for the simple reason that it’s headed in the wrong direction.

I’m being serious. Both his and your cited sources say that it is not currently possible. The Breakthrough Starshot project states: “Breakthrough Starshot aims to demonstrate proof of concept for ultra-fast light-driven nanocrafts, and lay the foundations for a first launch to Alpha Centauri within the next generation.” There is serious consideration of improving technology to the point where we might be able to send such a probe, but there is not yet “serious consideration of sending an interstellar probe to the closest star, Proxima Centauri”.

Gonzalez is overstating the current situation.

I’m being fair. Gonzalez’s claim is simply wrong. Super-Earths are not the most common type of exoplanet we have found, and we don’t know the distribution of exoplanets that we haven’t found.

Anyway, who says hot Jupiters aren’t habitable? Or that super-Earths are? Hot Jupiters may not be habitable by us, but nor are most of the super-Earths we’ve found.

Proxima is 270,000 AU away. New Horizons is travelling at about 3AU per year, so it’d take more than 90,000 years to reach another star. According to the NASA website, it’ll run out of power long before then.

NASA themselves aren’t saying anything about New Horizons being used to reach another star. Josh is completely wrong about that.

I agree that the New Horizons can’t be retargeted to Proxima Centuri specifically, and by the time it reached another star it would be a dead satellite.

I’m not sure how any of this is meaningfully relevant to the point. Interstellar travel is essentially impossible for everyone. The more salient point is that interplanetary travel is conceivably possible, and so are things like the Hubble telescope etc.