Why our nearest systems don't have aliens

For chuckles over the last week, I did a pulsecheck on every exo-system known within three-digit milliarcseconds, aka ten parsecs / 30ish lightyears. Oft-nicknamed "Sirius Sector" in SF.

This close to us: I assume if any sizable star in this sphere had large close-in planets, we'd know by now (cf. that hotneptune desert). For those we know, which skew toward Mercury-orbit superearths, I looked to the possibility of also something human-habitable up to one Sol AU. In light of TOI-4633 I'm not excluding binaries. I am excluding white dwarfs and any binary with those - so long, 40 Eridani and Sirius. I also figure that Proxima fills all our needs for irradiated eyeballs orbiting a red, directly; we're going green, not purple. The remainder, I sorted by stellar mass.

You could say I was fact-checking the Halo backstory, before the Covenant. This assumes a local answer to Drake Equation: that the Sirius Sector holds nothing worth the attention of what sentient aliens there be.

I started with εIndi, well-constrained by now. This remains a candidate, far as I know; I am unaware of it flaring up like, oh, Gliese 75. Over time, the "B" browndwarf pair has pulled A's 3jovian to a peri' now 4.576 UPDATE 7/25 17.2 AU (e=0.4 a=28.4). This system is young by the way: preVendian. I wouldn't rule out a planet in the habitable-zone (HZ), which is 0.4-0.8 AU; even with oxygen. My problem is that such a planet may well be orbiting from 0.4 to 0.8 AU, or outside those parameters. It's either a mercurylike or very cold; surface uninhabitable. I hereby declare this star's mass the absolute floor for HZ consideration.

5-Gould Capricorni = Gliese 785 now shining at 38+% hosts two Neptunelikes at 0.32 and 1.21 bracketing that HZ. Outer circular c carves a Hill of 0.033+ AU; inner b's effect is more on aphelion which rises to 0.355 AU. Between runs plenty of room for an Earthlike. Problem: factoring the system's age, we're looking at a Marslike without a spinning core.

GJ 892 = HD 219134 has many superearths of which g, in the Venus zone, is almost Uranian; it is 11Gy old so running low on essentials like phosphorus. Count this one out too. τCeti rotates edge-on. The planets claimed 2017 could be false-positives. Of these "f" was a superearth - but, who knows.

Over the edge of half Sol luminosity: εEridani is too young for free oxygen. 66-Gould Centauri = HD 102365 A, the furthest in our sample, has a Neptunelike in an eccentric orbit clearing out the HZ.

Closest to Sol mass at 82% luminosity is 61 Virginis, a bit on the far side, which we've looked at here. First up is that it's too old for any tectonics in Earthsized planets of its HZ. For those still reading I further note: outer d has a Marslike eccentricity which normally points to something large out there for *e. That debris disk from 30+ AU constrains mass and location for outer planets. Also by 2012 we'd ruled out anything large up to 6 AU, which range we can assuredly expand by now - although a long-period signal exists in the 55-year range (browndwarf at Saturn?). 1+ AU has mass for an Earthlike ... but will have to be superEarth to hold tectonics. Whatever's here has e=0.2 too, and I suspect further than that 1 AU. Futureproofing will prefer another cold dead Marslike.

We might post a watch on those "planet candidates". Alsafi / σDraconis, for one, may or (more likely) may not host a superEarth in its own HZ 0.7 AU. Weighing in heavier than Sol comes δPavonis (which that February 2023 poast looked at). This could own a Saturnlike at 11 AU; buuut... this star is old, so if it ever had an Earthlike at like 1.1 AU that planet is roasted. No tectonics after 6Gy either. Ditto β Hydri, μHerculis, 107 Piscium. I don't know about the core beneath, however. Check for ruins.

82-Gould Eridani is 3/4 L so habitable from 0.86+ AU; its inner planets b and c/d (tags vary in the literature) run well within that, if eccentric. I saved this system for "candidate" on account of the outer planets. Of these the most-likely option skates 0.816 AU perihelion. That ousts anything nice.

What did we learn? Basically, most stars are M dwarfs. Of those as are in our K/G range, one hot superEarth means probably more, and eccentric - sweeping planets like this one, out of the way. I get the feeling the astronomers already knew that and tried to warn us.

Any other nearby star could well have smaller close-in planets than we see, with larger planets out in their Saturn belt. Pretty much... like us! As to binaries like HU 918: ηCassiopeiae, 70 Ophiuchi, ξBoötis, χDraconis could have smaller planets in their HZ, but haven't turned up anything yet.