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

The abstract scores 0 on the standard scale:

Here, we compare Earth to the most common type of exoplanet, the super-Earth, …

It’s easy to check that the exoplanets found so far include more hot Jupiters than super-Earths

The main body scores just 1:

The discovery of over 4,000 exoplanets has changed the direction of scientific research and motivated scientific questions previously entertained only in science fiction. For instance, there is now serious consideration of sending an interstellar probe to the closest star, Proxima Centauri, in order to study its planet [1].

No, there isn’t serious consideration of sending such a probe. The cited article concludes that it isn’t currently possible:

The mission is based on technologies that are currently available or under development, but would need extensive improvements to actually build the required space infrastructure. In-orbit fabrication and assembly are crucial to build the required structures. Materials for the light sail and the magnetic sail need to reach the required characteristics and technical readiness level.

The rest of the article is no better. It spends far too long explaining that Earth-sized planets have lower escape velocity and atmospheric pressure than exoplanet super-earths, forgetting that larger exoplanets are easier to find so the mass distribution of ones we’ve found won’t match the mass distribution of ones that exist (rather like saying that there are more elephants that mice in Africa because only elephants show up on current satellite images), and forgetting that Mars-sized planets have even lower escape velocity.

It supports its clam that the Solar System is good for space travel with a graph showing that space travel would be easier if the Sun were more massive.

It even undermines its own title by admitting that the Solar System is not optimal for space travel: “However, it seems odd that the Earth is near the upper limit in mass for manned space travel.


The Solar System: Favored for Space Travel

Guillermo Gonzalez


Here, we compare Earth to the most common type of exoplanet, the super-Earth, with respect to interplanetary space travel. The typical super-Earth should have higher gravity and atmospheric pressure at its surface. These factors pose significant challenges to rocket launches and to reentering spacecraft. In addition, the Solar System is compared to exoplanetary systems with respect to interstellar travel. It is easier to launch an interstellar spacecraft from a planet in the circumstellar habitable zone of the Sun than from planets in the circumstellar habitable zones of less massive stars. In the larger context of the Milky Way galaxy, our Solar System is in the best location to initiate interstellar missions. In summary, we here confirm and expand upon recent studies that argue that the Earth and the Solar System are rare in the degree to which they facilitate space exploration.

Full Text: PDF

I do believe there is serious consideration.

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Serious consideration of whether a probe can be sent, or serious consideration of actually sending it? The cited articles are the former, Gonzalez claims the latter. He’s taking an article that talks about possibilities and representing them as plans.

Is New Horizons headed in the right direction for Proxima? That’d be a big coincidence. Your second link is more relevant, but again says that we can’t actually send them yet.

I wonder if Gonzalez ever entertained the notion of setting parameters for a particular “super-Earth” (mass, atmosphere, solar system properties, star properties) and predicting (or, to be more provocative, reverse engineering) the properties (such as masses, anatomies, physiologies, etc.) of space-faring intelligent life forms on such planets.

NASA is seriously considering sending New Horizons to another star to study its exoplanet. It appears that they are trying to decide which which one. I’m not sure the timeline involved with this.

With proper planning, New Horizons could mark humanity’s first encounter with a foreign planetary system.

I do not know how long it would take for New Horizon to reach.

Voyager 1 and 2 were already sent, more or less, decades ago too, though I don’ think they have instrumentation to study a planet. Certainly at the time, people did not know about exoplanets.

That’s already been done. Space travel might be essentially impossible from Super Earths.


They most likely would not have, for example, have GPS or spy satellites.

It seems the prediction is more boring, that life on Super-Earths would not be spacefaring.

I don’t see how one can think that. It seems as if most assume intelligent life with approximately the same properties as humans. But what if we first set the properties based on an ability to leave the “super-Earth” and then brainstorm as to the sorts of chemistry and physiology might fit into such a framework?

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Did you read the link I posted?

The issue is not our physiology, but the physics of rocket propulsion. To overcome gravity on a super earth and reach escape velocity might require a rocket with 400,000 tons of propellant, equivalent to the Pyramids of Giza.

To launch the equivalent of an Apollo moon mission, a rocket on a super-Earth would need to have a mass of about 440,000 tons (400,000 metric tons), due to fuel requirements, the study said. That’s on the order of the mass of the Great Pyramid of Giza in Egypt

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The probability of thicker atmospheres would not be conducive to astronomy either.

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I don’t think you are being fair here. He is talking about the distribution of the most likely habitable planets, which would exclude hot Jupiters.

I read over the article, and it looks like his basic point is correct. It does look like we lucked out. I am more curious to see what specifically in this article is new, as his point actually seems fairly non-controversial. What do the @physicists think?

I have not read the paper, so I will just give my 2 cents based on the somewhat contextless quotes in this thread.

Most of the exoplanets found so far are hot Jupiters, but this is because they are the easiest to find (c.f. your analogy with mice vs elephants in Africa). No one knows the actual distribution of exoplanetary types, but current evidence do point that something in the sub-Neptune range (i.e., technically high mass super-Earths) are the most abundant.

Although, it is a bit disingenuous to call the most abundant planets super-Earths, because the term super-Earth is also sometimes used to refer specifically to the lower end of the Earth-Neptune spectrum, and sub-Neptunes (or mini-Neptunes) specifically referring to the higher end. This is particularly egregious because there is a huge difference in terms of habitability between planets that are more “Earth-like” and planets that are more “Neptune-like”.

There are serious consideration of sending such a probe, from both the government (NASA) and private (e.g., Breakthrough Starshot). The first few stages of Breakthrough Starshot have been fully funded, which means that this project is now much more “seriously considered” than many other science projects that I would say have “serious consideration”.

Space travel is not impossible from super-Earths; chemical rockets are much more difficult to built, but there are many other ways to travel to space. Their planet might have an overabundance of radioactive fuel, for example.


They might not have the concept anything outside of their own planet, or it would be delayed until radio technology was invented. But then meteorites would be a tip-off to something from outside their spherical universe.

Doctor Pangloss would certainly agree with the article. So would Douglas Adams’s puddle.


An a supernatural resistance to nuclear fallout? I don’t follow your reasoning here.

It is short, and I don’t see anything particularly wrong with it (but I am not a physicist!). The idea is interesting. Perhaps we did luck out with earth, whether or not you want to call it “fine-tuning” or not. That might even be true.

Help me understand this. I don’t follow the egregiousness.

How does habitability align with the Earth-Neptune Spectrum?

I would not be inclined to conflate this with the fine-tuning problem, at least in the cosmological sense. For physical constants, we do not know if they are inherently fixed or follow a distribution, but it seems to be generally accepted that planetary sizes are pretty much just random and any given mass will likely be represented among some percentage of systems. Once our universe is granted, the existence of earth(s) is to be expected.

Is the solar system conducive to space travel? Sure. Space travel is very difficult, but possible.

I’d be more impressed if it was optimal for space travel. Interplanetary distances making for faster journeys, better or total lack of micrometeorites and ionizing radiation, perhaps even more and better opportunities for gravitational slingshots etc. etc.


I don’t think it’s possible to distinguish near optimal from optimal.

I think the two might be connected.

Could something along the lines described in the Breakthrough Starshot link be launched into orbit, or beyond, from a “super-Earth”? If so, then why not draw upon an understanding of biology, chemistry, and, if one is an ID proponent, engineering principles to try and predict the sorts of intelligent life that could develop and use such a technology for space flight? ( I am thinking - these craft need not be probes, but actual vehicles.)

And so on and so forth. To be sure, perhaps a fanciful venture. But more challenging and engaging than just presenting some equations and claiming “Privileged Planet”.

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I agree with you in the sense that if you’ve discovered some solution and you can’t seem to figure out any way to improve it, that doesn’t really prove no such improvement is possible. But as long as you can adjust the parameters and still find better solutions, you’ve definitely shown sub-optimality.