Goldilocks next door

Springboarding off yesterday's post, about how easy it is to form organic compounds abiotically, today we have: our nearest neighbor might be a decent candidate for the search for extraterrestrial life.

At only 4.24 light years away, Proxima Centauri is the closest star to our own Sun.  It's captured the imagination ever since it was discovered how close it is; if you'll recall, the intrepid Robinson family of Lost in Space was heading toward Alpha Centauri, the brightest star in this triple-star system, which is a little father away (4.37 light years) but still more or less right next door, as these things go.

It was discovered in 2016 that Proxima Centauri has a planet in orbit around it -- and more exciting still, it's only a little larger than Earth (1.17 times Earth's mass, to be precise), and is in the star's "Goldilocks zone," where water can exist in liquid form.  The discovery of this exoplanet (Proxima Centauri b) was followed in 2020 by the discovery of Proxima Centauri c, thought to be a "mini-Neptune" at seven times Earth's mass, so probably not habitable by life as we know it.

And now, a paper in Nature has presented research indicating that Proxima Centauri has a third exoplanet -- somewhere between a quarter and three-quarters of the Earth's mass, and right in the middle of the Goldilocks zone as well.

"It is fascinating to know that our Sun’s nearest stellar neighbor is the host to three small planets," said Elisa Quintana, an astrophysicist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, who co-authored the paper.  "Their proximity make this a prime system for further study, to understand their nature and how they likely formed."

The newly-discovered planet was detected by observing shifts in the light spectrum emitted by the star as the planet's gravitational field interacted with it -- shifts in wavelength as little as 10 ^-5 ångströms, or one ten-thousandth the diameter of a hydrogen atom.  The device that accomplished this is the Echelle Spectrograph for Rocky Exoplanets and Stable Spectroscopic Observations (ESPRESSO -- because you can't have an astronomical device without a clever acronym) at the European Southern Observatory in Cerro Paranal, Chile.  

"It’s showing that the nearest star probably has a very rich planetary system," said co-author Guillem Anglada-Escudé, of the Institute of Space Sciences in Barcelona.  "It always has a little bit of mystique, being the closest one."

What this brings home to me is how incredibly common planets in the Goldilocks zone must be.  It's estimated that around two percent of spectral class F, G, and K stars -- the ones most like the Sun -- have planets in the habitable zone.  If this estimate is accurate -- and if anything, most astrophysicists think it's on the conservative side -- that means there's five hundred million habitable planets in the Milky Way alone.

Of course, "habitable" comes with several caveats.  Average temperature and proximity to the host star isn't the only thing that determines if a place is actually habitable.  Remember, for example, that Venus is technically in the Goldilocks zone, but because of its atmospheric composition it has a surface temperature hot enough to melt lead, and an atmosphere made mostly of carbon dioxide and sulfuric acid.  Being at the right distance to theoretically have liquid water doesn't mean it actually does.  Besides atmospheric composition, other things that could interfere with a planet having a clement climate are the eccentricity of the orbit (high eccentricity would result in wild temperature fluctuations between summer and winter), the planet being tidally locked (the same side always facing the star), and how stable the star itself is.  Some stars are prone to stellar storms that make the ones our Sun has seem like gentle breezes, and would irradiate the surface of any planets orbiting them in such a way as to damage or destroy anything unlucky enough to be exposed.

But still -- come back to the "life as we know it" part.  Yeah, a tidally-locked planet that gets fried by stellar storms would be uninhabitable for us, but perhaps there are life forms that evolved to avoid the dangers.  As I pointed out yesterday, the oxygen we depend on is actually a highly reactive toxin -- we use it to make our cellular respiration reactions highly efficient, but it's also destructive to tissues unless you have ways to mitigate the damage.  (Recall that burning is just rapid oxidation.)  My hunch -- and it is just a hunch -- is that just as we find life even in the most inhospitable places on Earth, it'll be pretty ubiquitous out in space.

After all, remember what we learned from Ian Malcolm in Jurassic Park:

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People made fun of Donald Rumsfeld for his statement that there are "known unknowns" -- things we know we don't know -- but a far larger number of "unknown unknowns," which are all the things we aren't even aware that we don't know.

While he certainly could have phrased it a little more clearly, and understand that I'm not in any way defending Donald Rumsfeld's other actions and statements, he certainly was right in this case.  It's profoundly humbling to find out how much we don't know, even about subjects about which we consider ourselves experts.  One of the most important things we need to do is to keep in mind not only that we might have things wrong, and that additional evidence may completely overturn what we thought we knew -- and more, that there are some things so far out of our ken that we may not even know they exist.

These ideas -- the perimeter of human knowledge, and the importance of being able to learn, relearn, change directions, and accept new information -- are the topic of psychologist Adam Grant's book Think Again: The Power of Knowing What You Don't Know.  In it, he explores not only how we are all riding around with blinders on, but how to take steps toward removing them, starting with not surrounding yourself with an echo chamber of like-minded people who might not even recognize that they have things wrong.  We should hold our own beliefs up to the light of scrutiny.  As Grant puts it, we should approach issues like scientists looking for the truth, not like a campaigning politician trying to convince an audience.

It's a book that challenges us to move past our stance of "clearly I'm right about this" to the more reasoned approach of "let me see if the evidence supports this."  In this era of media spin, fake news, and propaganda, it's a critical message -- and Think Again should be on everyone's to-read list.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]

Chasing Goldilocks

Given my fascination with the possibilities of life in other star systems, I was thrilled to read two papers that came out last week detailing our efforts to narrow down where to look.

After all, that's the problem, isn't it?  There are billions of stars in our galaxy alone, and it's impossible to study all of them with any kind of thoroughness.  It seems pretty certain that most stars have some kind of planetary system, but trying to find Earth-like planets is another thing entirely.  Most of the exoplanets that have been identified are gas giants, and a good many of those are very close to their parent star (and so are extremely hot).  The reason these were identified first is not necessarily because they're more common; being more massive, and (for the close-in ones) having a stronger gravitational pull on their stars because of their proximity, makes them easier to see by both of the common methods used -- occlusion (seeing them pass in front of their stars) and Doppler spectroscopy (massive planets cause a wobble in the position of their stars as they orbit).

But there's no reason to believe that Earth-sized planets are uncommon, and indeed, we're now finding that they're plentiful.  The trick, of course, is not only locating one that's the right size, but one in the "Goldilocks zone" -- the distance from the star that is neither too hot nor too cold, but just right.  (Since we're concentrating on "life as we know it, Jim," we're most interested in planets where water can be in liquid form during at least part of its orbit.)

[Image licensed under the Creative Commons ESO/L. Calçada, Artist’s impression of the exoplanet Tau Bootis b, CC BY 4.0]

The first paper, called "Habitable Zones and How to Predict Them," by a team led by Ramses M. Ramirez of the Tokyo Institute of Technology, takes a purely practical approach of not only estimating habitability based upon a planet's size and distance from its star, but looks at composition -- quantity of water, presence of carbonate and silicate minerals, percentage of the atmosphere that is carbon dioxide or methane (both greenhouse gases that considerably raise the heat-trapping ability of the air), and the presence of tectonic activity.  The authors conclude with a cautionary note, however, about not concluding too much based upon partial evidence:

[W]e should be careful about using our Earth to extrapolate about life on other planets, particularly those around other stars.  The future of habitability studies will require first principles approaches where the temporal, spatial, geological, astronomical, atmospheric, and biological aspects of a planet’s evolution are dynamically coupled.  This, together with improved observations, is the key to making more informed assessments.  In turn, only through better observations can we improve such theoretical models.

The second paper, published last week in Astrophysical Journal Letters, describes a study by a team of astronomers from Cornell University, Lehigh University, and Vanderbilt University, in which TESS -- the Transiting Exoplanet Survey Satellite -- will examine 400,000 stars considered good candidates for hosting planets in the habitable zone.

"Life could exist on all sorts of worlds, but the kind we know can support life is our own, so it makes sense to first look for Earth-like planets," said Cornell astronomer Lisa Kaltenegger, who was the study's lead author.  "This catalog is important for TESS because anyone working with the data wants to know around which stars we can find the closest Earth-analogs."

Even the scientists who study this stuff on a daily basis recognize what a leap forward this is.  TESS has already identified over 1,800 stars that have planets up to 1.4 times the mass of the Earth -- considered an upper limit for habitability -- and 408 for which TESS could recognize a planet as small as, or a little smaller than, the Earth from one transit alone.

"I have 408 new favorite stars," Kaltenegger said.  "It is amazing that I don't have to pick just one; I now get to search hundreds of stars."

Unlike the old look-everywhere-and-hope-for-the-best approach, this study starts out by examining the most likely candidates, making the hopes for positive results much stronger.  "We don't know how many planets TESS will find around the hundreds of stars in our catalog or whether they will be habitable," Kaltenegger said, "but the odds are in our favor.  Some studies indicate that there are many rocky planets in the habitable zone of cool stars, like the ones in our catalog.  We're excited to see what worlds we'll find."

So am I.  It's long been my dearest hope to have unequivocal proof of extraterrestrial life in my lifetime.  (Intelligent life would be even better, but I'm trying to keep a modest goal, here.)  The idea that we are now devoting significant time and effort into locating good candidates for hosting life is tremendously exciting.  While it's still not likely that we'll find neighbors to talk to, at least knowing they're out there is cool enough for now.

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This week's Skeptophilia book recommendation combines science with biography and high drama.  It's the story of the discovery of oxygen, through the work of the sometimes friends, sometimes bitter rivals Joseph Priestley and Antoine Lavoisier.   A World on Fire: A Heretic, an Aristocrat, and the Race to Discover Oxygen is a fascinating read, both for the science and for the very different personalities of the two men involved.  Priestley was determined, serious, and a bit of a recluse; Lavoisier a pampered nobleman who was as often making the rounds of the social upper-crust in 18th century Paris as he was in his laboratory.  But despite their differences, their contributions were both essential -- and each of them ended up running afoul of the conventional powers-that-be, with tragic results.

The story of how their combined efforts led to a complete overturning of our understanding of that most ubiquitous of substances -- air -- will keep you engaged until the very last page.

[Note:  If you purchase this book by clicking on the image/link below, part of the proceeds will go to support Skeptophilia!]