There are two things that upset me quite a bit about this business.
The role of moderators is to enact community consensus. When there isn't one, their role is to bring the question formally for community consideration on this meta site, so that a communal decision can be taken. A good example of what I consider successful moderator handling of such a situation is this question.
In this particular instance, five people voted to close and five people voted to reopen. Neither of these is an indication that the question is on- or off-topic here; rather, they must be taken together as an indication that there is no consensus. Unilateral moderator action in these conditions is, I feel, out of line. This is regardless of whether the question was, or turns out to be, off-topic.
I simply do not see the point of closing old questions, and particularly ones which already have answers. If the motivation for that is to keep them from cluttering the front page, or out of a broken-windows approach, I believe closing them is in fact detrimental, and just brings more attention to them.
This particular question is on two gray areas but I would strongly dispute that either of them merits closure.
It is an astrobiology question and as such it is best answered by biologists, or possibly chemists. Indeed, if it were asked today, I would recommend direct migration to biology.se. However, this question was asked on the old astronomy beta and, having been on topic there, it should be taken as on topic here. It is now too old to be migrated, and there are very good reasons for that age limit.
Further than that, the reason we close questions as off-topic is because they clutter and take up the attention of people who do not have the expertise to answer them. This question has two perfectly good answers that quite patently satisfied the OP and are good resources for anyone who lands on this question.
The other reason we close questions as off-topic is so that they don't invite new questions like them which, again, clutter the front page. I do not think this question does that. Anyone looking to ask such things would more naturally ask it in the new astronomy beta. If someone does ask it here, we can simply migrate to biology or astronomy as appropriate. If we ever get inundated with those then we can rethink closing old questions which create the impression that biology is on-topic here.
It is indeed slightly too broad for the SE model. However, it has answers that address essentially every point raised. The reason we close questions as too broad is that they are simply unlikely to attract good answers and their broadness undermines their future usefulness. This question has attracted good answers which are short, concise and to the point. I do not think it is even a broken window, and its broadness harms no one. To put it clearly, I do not think it should be closed as too broad.
I should also note that closing astronomy-related questions sends the message that any astronomy questions are off-topic here. This goes against (1) the discussions at the time of the merger, (2) the loud objections on this meta site to the opening of the new astronomy beta, and (3) the loud objections by many phycisists on the new astronomy beta's discussion page to the reopening of a parallel site.
Having addressed this particular question, I do think that astrobiology questions have much better venues than this site. Unless the question really does involve physics, at least on the side of the astrobiology, we should point the posters to the biology, astronomy or chemistry betas, and migrate the questions there after consultation with the poster and the target site moderators. We do not get anywhere near enough of these to make the process unfeasibly long or complicated, and this should really be done on a case-by-case basis.
As for the fate of this particular question, I think it is a perfectly valid question and should not be closed, though it is arguably on the wrong site and is too old to be migrated. To keep consistency, and to satiate the moderator thirst "to show new users that there are limits to what kinds of question we accept here", I propose that the question be reopened but that the following banner be added, as a quote, to the end of the question:
Please note that astrobiology questions are typically not on topic on this site unless they address specific physics issues. If you have a similar question, please consider posting it on our Biology, Chemistry or Astronomy sister sites. For more information, please refer to the discussion on Physics Meta about this question.