Requiem for a dead planet

If I had to pick my favorite episode of Star Trek: The Next Generation, the clear winner would be "The Inner Light."  Some classic episodes like "Darmok," "Frames of Mind," "Remember Me," "Time's Arrow," "The Chase," and "Best of Both Worlds" would be up there in the top ten, but "The Inner Light" not only has a beautiful story, but a deep, heartwrenching bittersweetness, made even more poignant by a tour-de-force performance by Patrick Stewart as Captain Jean-Luc Picard.

If you've not seen it, the plot revolves around the Enterprise encountering a huge space station of some kind, of apparent antiquity, and in the course of examining it, it zaps Captain Picard and renders him unconscious.  What his crew doesn't know is that it's dropped him into a dream where he's not a spaceship captain but an ordinary guy named Kamin, who has a wife and children and a job as a scientist trying to figure out what to do about the effect of his planet's sun, which has increased in intensity and is threatening devastating drought and famine.

As Kamin, he lives for forty years, watching his children grow up, living through the grief of his wife's death and the death of a dear friend, and ultimately grows old without ever finding a solution to his planet's dire circumstances.  All the while, the real Captain Picard is being subjected to ongoing interventions by Dr. Crusher to determine what's keeping him unconscious, and ultimately unsuccessful attempts to bring him out of it.  In the end, which makes me ugly cry every damn time I watch it, Kamin lives to watch the launch of an archive of his race's combined knowledge, realizing that the sun's increase in intensity is leading up to a nova that will destroy the planet, and that their civilization is doomed.  It is, in fact, the same archive that the Enterprise happened upon, and which captured Picard's consciousness, so that someone at least would understand what the civilization was like before it was wiped out tens of thousands of years earlier.

"Live now," Kamin says.  "Make now always the most precious time.  Now will never come again."

And with that, Picard awakens, to find he has accumulated four decades of memories in the space of about a half-hour, an experience that leaves a permanent mark not only on his mind, but his heart.

*brief pause to stop bawling into my handkerchief*

I was immediately reminded of "The Inner Light" by a paper this week in Nature Astronomy, called, "Alkali Metals in White Dwarf Atmospheres as Tracers of Ancient Planetary Crusts."  This study, led by astrophysicist Mark Hollands of the University of Warwick, did spectroscopic analysis of the light from four white dwarf stars, which are the remnants of stellar cores left behind when Sun-like stars go nova as their hydrogen fuel runs out at the end of their lives.  In the process, they vaporize any planets that were in orbit around them, and the dust and debris from those planets accretes into the white dwarf's atmosphere, where it's detectable by its specific spectral lines.

In other words: the four white dwarfs in the study had rocky, Earth-like planets at some point in their past.

"In one case, we are looking at planet formation around a star that was formed in the Galactic halo, 11-12.5 billion years ago, hence it must be one of the oldest planetary systems known so far," said study co-author Pier-Emmanuel Tremblay, in an interview in Science Daily.  "Another of these systems formed around a short-lived star that was initially more than four times the mass of the Sun, a record-breaking discovery delivering important constraints on how fast planets can form around their host stars."

This brings up a few considerations, one of which has to do with the number of Earth-like planets out there.  (Nota bene: by "Earth-like" I'm not referring to temperature and surface conditions, but simply that they're relatively small, with a rocky crust and a metallic core.  Whether they have Earth-like conditions is another consideration entirely, which has to do with the host star's intrinsic luminosity and the distance at which the planet revolves around it.)  In the famous Drake equation, which is a way to come up with an estimate of the number of intelligent civilizations in the universe, one of the big unknowns until recently was how many stars hosted Earth-like planets; in the last ten years, we've come to understand that the answer seems to be "most of them."  Planets are the rule, not the exception, and as we've become better and better at detecting exoplanets, we find them pretty much everywhere we look.

When I read the Hollands et al. paper, I immediately began wondering what the planets around the white dwarfs had been like before they got flash-fried as their suns went nova.  Did they harbor life?  It's possible, although considering that these started out as larger stars than our Sun, they had shorter lives and therefore less time for life to form, much less to develop into a complex and intelligent civilization.  And, of course, at this point there's no way to tell.  Any living thing on one of those planets is long since vaporized along with most of the planet it resided on, lost forever to the ongoing evolution of the cosmos.

If that's not gloomy enough, it bears mention that this is the Earth's ultimate fate, as well.  It's not anything to worry about (not that worry would help in any case) -- this eventuality is billions of years in the future.  But once the Sun exhausts its supply of hydrogen, it will balloon out into a red giant, engulfing the inner three planets and possibly Mars as well, then blow off its outer atmosphere (that explosion is the "nova" part), leaving its exposed core as a white dwarf, slowly cooling as it radiates its heat out into space.

Whether by that time we'll have decided to send our collective knowledge out into space as an interstellar archive, I don't know.  In a way, we already have, albeit on a smaller scale than Kamin's people did; Voyager 2 carries the famous "golden record" that contains information about humanity, our scientific knowledge, and recordings of human voices, languages, and music, there to be decoded by any technological civilization that stumbles upon it.  (It's a little mind-boggling to realize that in the 43 years since Voyager 2 was launched, it has traveled about 20,000,000,000 kilometers, so is well outside the perimeter of the Solar System; and that sounds impressive until you realize that's only 16.6 light hours away, and the nearest star is 4.3 light years from us.)

So anyhow, those are my elegiac thoughts on this February morning.  Dead planets, dying stars, and the remnants of lost civilizations.  Sorry to be a downer.  If all this makes you feel low, watch "The Inner Light" and have yourself a good cry.  It'll make you feel better.

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Science writer Elizabeth Kolbert established her reputation as a cutting-edge observer of the human global impact in her wonderful book The Sixth Extinction (which was a Skeptophilia Book of the Week a while back).  This week's book recommendation is her latest, which looks forward to where humanity might be going.

Under a White Sky: The Nature of the Future is an analysis of what Kolbert calls "our ten-thousand-year-long exercise in defying nature," something that immediately made me think of another book I've recommended -- the amazing The Control of Nature by John McPhee, the message of which was generally "when humans pit themselves against nature, nature always wins."  Kolbert takes a more nuanced view, and considers some of the efforts scientists are making to reverse the damage we've done, from conservation of severely endangered species to dealing with anthropogenic climate change.

It's a book that's always engaging and occasionally alarming, but overall, deeply optimistic about humanity's potential for making good choices.  Whether we turn that potential into reality is largely a function of educating ourselves regarding the precarious position into which we've placed ourselves -- and Kolbert's latest book is an excellent place to start.

[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!]