24 January 2023

A little planetary rhapsody

The constellation Cetus, the Whale or Sea Monster, can be seen around this time of year in much of the Northern Hemisphere, despite being a "southern constellation," and thanks to the Earth's axial tilt, which gives us a view of a good percentage of the sky at one time or another when it's clear throughout the year. I first picked out Cetus's stars about 50 years ago, and even then I was particularly intrigued with the relatively inconspicuous star Tau Ceti. It's not much to look at with the naked eye. Slightly orange yellow, once you accustom yourself to distinguishing the colors of stars. It is one of the "Bayer" stars, named after the astronomer Bayer who gave the brighter stars in the sky greek-letter  + constellation name designations (in Latin genitive case) back in the early 17th century. 

But TC was one of the subjects of Frank Drake's 1961 Project Ozma, where he first tried to listen for signals from possible extraterrestrial intelligence. This is because, the thinking went, such signals are likely to attenuate, so would be most likely detected, if at all, only from nearby sunlike stars, which might have planets and life much like Earth. (Others have since expanded this view, but no signals have yet been found, and some of us have already concluded that they may well never be found because they aren't there). Anyway, Tau Ceti was thought to be one of the better candidates. It's somewhat smaller than the Sun, but is the closest solitary G-type dwarf, albeit somewhat "later" in the parlance, meaning dimmer and smaller than the Sun, at about 78% solar mass. It's about 12 light years, which is close in stellar terms, although that's something like 75 trillion miles, so it's not close by any human standard. We will not have the technology to actually go there for a very long time, and will never be able to travel back and forth the way you go to Europe. Some will disagree with that, but they are engaging in wishful thinking; the speed of light is an absolute speed limit and it will never be possible to travel to nearby stars in less than years. Likely we will have the technology to send some kind of robot probe, on a long, long journey, long before we could actually try to travel to Tau Ceti or other nearby stars. But even that is not currently in the offing. Our knowledge of stars and star systems comes from ground based telescopes and satellites; currently the best investigation is from the James Webb Space Telescope and some other space based platforms.

Technically Tau Ceti is a G8 dwarf, which is dimmer and smaller than the Sun's G2 class. These classes don't tell you everything about a star. Tau Ceti is older than the Sun, at about 6 billion years by most estimates, and is lower in metallicity, meaning preponderance of elements heavier than helium. That would affect the chemistry of the planets it has. In 1961, it was thought that only solitary stars like the Sun would likely have planets, although it was known even then that the majority of stars are found in systems with at least two stars. (Not the majority of systems, but the majority of stars. Many smaller, red dwarf stars are solitary, so the slight majority of systems are solitary, but since by definition even a binary system has two stars, the stars in binary and multiple systems do outnumber the singletons. Turns out even stars in multiple systems almost always have at least some planets anyway). 

In 1973 when I spotted Tau Ceti in a dark winter sky and thought about what might be there, we really knew nothing. Now, thanks to huge advances in exoplanetology, it's fairly certain that Tau Ceti has at least five planets, and likely all of the ones so far detected are in a class of planets that, by chance perhaps or for unknown reasons, does not exist in the Solar System, namely so-called Super Earths. Super Earths are defined as rocky "terrestrial" planets larger than Earth in both diameter and mass, but enough smaller than so-called Ice Giants like Neptune and Uranus in the Solar System, which have sufficient gravity to retain hydrogen and helium in their atmospheres, to not have that kind of atmosphere. Hydrogen and helium escape from the atmospheres of planets like Earth, so they tend to have thinner atmospheres made up of other gases. Turns out, from the investigation of now thousands of planetary systems, Super Earths are very, very common. Most planetary systems have at least one, and frequently they are much closer to the star even than Mercury is to the Sun, especially in the cases of less massive stars. We have now realized that the Solar System is actually rather unusual, even for stars of the approximate mass of the Sun, and not even taking into account the extremely unusual planet Earth, with its huge moon and... highly developed life. Also, by far the majority of stars in the Galactic population are smaller...most of them much smaller... than the Sun, but almost all of them have planetary systems. Whether the Sun's higher than average metallicity has anything to do with its distribution of planets and what those planets are like is the subject of a lot of research but is not at all well understood at this point. 

So, Frank Drake's 1961 investigation now appears pretty quixotic. There is no particular reason that Super Earths could not have life; indeed it has been speculated that planets between 1.1 and 1.5 Earth Masses might actually be somewhat more hospitable to life than Earth, all other things being equal. But other things usually aren't equal, and in all the searching and investigation, an approximate endeavor all around in any case, we have yet to come across a really close analog to Earth, in terms of mass, distance from its star to give approximately equal "sunlight," etc. Not that these parameters are well known or easily confirmed, and equally important questions arising about composition of the planet and its potential oceans and atmospheres is mostly still in the realm of speculation. But planets with 3 and 4 times the mass of Earth, which is most of Tau Ceti's planets, seem unlikely to host life similar to Earth's, as their atmospheres are likely to be extremely dense, like Venus's. Whether high gravity alone would be a major obstacle or not is unknown. (TC may well have a "Jupiter" or "Super Jupiter" further out; planets distant from the star can be hard to detect so there's a selection bias). But, still, most systems just don't include a planet with all the "goldilocks" characteristics to have liquid water and stable temperatures for ultra-long periods of time, which seems to be what made complex life on Earth possible. 

There are other reasons for believing that complex life like ours is actually pretty rare. We don't know enough to rule out a range of biochemistries, but a good deal of work on the possible origin of life on Earth suggests that the range is far from infinite, and that conditions have to be fairly circumscribed by planetary conditions, and remain stable for long periods of time, for evolution to work its magic and produce complex macroscopic life. See any discussion of the Fermi Paradox for reasons other than direct observation to infer that the evolution of complex life and intelligence is very probably quite rare in the universe. 

So, you may ask, what is my point? Just this: let's not lose sight of the wonder of it all. Tau Ceti is a real place. It has real worlds. They're not just like our world, but they have existed for longer than the Earth, presumably, and we have no idea whether some form of life might have originated on one or even two of them, where liquid water could exist on their surfaces. And it is just one of countless stars... literally countless. There are about 400 billion stars in the Galaxy alone, and the observable universe (not the entire universe, just the part you can see from here) contains about that many entire galaxies, each with tens of millions to up to a trillion or so stars (like our nearest big galactic neighbor, M31 in Andromeda, which as about 1 trillion stars at roughly 2 million light years' distance). The universe is actually more diverse than most science fiction writers over the years have imagined. And it is out there, in a sense, waiting for us. Apparently, in terms of conscious living beings, mostly empty, but nonetheless are real as our world, made of the same stuff, governed by the same physics and chemistry. We as a species are like a petulant child, obsessed narcissistically with our own petty concerns and narrowly perceived world. If we could but look out at the stars and realize that our job for the present is to learn to live with each other in peace and sustainable technology on the only world anywhere near here that can sustain us, we can dream that one day we will actually begin the long future where the stars are real places, where our descendants and successors may go, and find a whole universe of wonder we can scarcely dream of. We can, if we can just get through this toddler phase of rampant stupidity and short-sightedness, emerge into a maturity of an almost limitless future, where what it means to be human and aware will evolve and grow in ways we can't imagine. 

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