Multiverse Predictions for Habitability: Planetary Characteristics

Round 1

Reviewer 1 Report

The authors investigate conditions for planetary habitability and how they depend on fundamental physical parameters that may vary in a multiverse scenario, in order to understand better which features of our world are typical in this framework.  Particular parameters under investigation include the eccentricity of planetary orbits and the obliquity of axial tilt (which may require a large moon only for planets around Sun-sized or larger stars), as well as different hypothetical mechanisms for the delivery of water.

I think that the topic is interesting and the paper is overall readable and with adequate references.  It would be good to a have a little more clarity as to the assumptions that are going into the calculation of probabilities, as the uncertainty surrounding the prior distributions is perhaps the main unanswered question in discussions of the multiverse scenario.  Before I recommend the paper for publication I would like to give the authors an opportunity to further clarify the following points:

It would be helpful to have more of an explanation (or references) of how the probabilities in eq. (4) and elsewhere are being calculated – are there particular priors assumed on the fundamental constants (flat?  logarithmic? power law?), and what assumptions on the number of observers have been folded in?

A few minor points:

I would suggest reworking the last sentence of the first paragraph, or removing it entirely.  Sheer volume of arguments alone doesn’t make them more convincing if they’re not very convincing to begin with.  The paragraph after makes the logic clearer, but this one sentence sounds a bit too tongue-in-cheek.

Page 16: Halley is misspelled.

Reference 2 looks incomplete.

Author Response

We thank the reviewer for their comments.  We've added some discussion from line 89-111 detailing what goes into the probability calculations a bit more.  We've also incorporated their more minor suggestions, including deleting the last sentence of paragraph 1.

Reviewer 2 Report

As is clear from the introduction and references, this paper is the "sixth entry in a series of papers aimed at using the multiverse framework [...] to deduce testable predictions " for planetary habitability. I admit that I did not read the previous five papers by the same authors.

I acknowledge that, behind the submitted paper, there is a lot of work. It is well-organized, well-written, and equipped with many references. From a formal point of view, it is impeccable.

However, I believe that the main weakness is just in the approach. I believe that nothing meaningful can be obtained from the multiverse hypothesis. As far as I understand, almost every line of the paper is a untested assumption from which the authors want to derive reasonable conclusions.

The hardest part to digest is the easiness with which the authors connect planetary parameters to fundamental physical constants. There are tons of things we still do not know about planetary formation, dynamics, habitability conditions, the origin of life, etc., that believing that the derived equations make any physical sense is naive. I know that the authors' approach has been already published in this Journal, but still, I consider it too speculative and useless, if not dangerous. I accept that physicists spend time dwelling on things they know they do not know. However, with these authors' approach, we are in the realm of things we do not know we do not know, and therefore, in my opinion, there is nothing even barely meaningful (from a physics point of view) that can be said. 

That said, however, I suggest acceptance. As previously mentioned, this is the sixth paper by the same authors on the same subject, and at least four of them have already  been published (passing the peer-review process). Moreover, I acknowledge the time and effort invested by the authors in that research and the fact that the paper is formally unexceptionable. If I am wrong and the paper is correct and useful, time will tell.     

Author Response

We're very sympathetic to the reviewer's criticism, as we are aware that planetary systems are very complex systems with many features that are still not completely explained.  In addition, there is no way to directly check the extrapolations we make, and the multiverse literature contains numerous historic examples of reasonable sounding extrapolations that fell apart upon further scrutiny.  All of our results should be considered in this light, as initial forays into the questions being considered that should act as guideposts to be challenged and built upon in future work, rather than as infallible proclamations.

Reviewer 3 Report

An excellent paper on planetary characteristics for habitat. A fascinating read.

Author Response

We thank the reviewer for their favorable appraisal.