The Lithium Problem: Why Our Sun is Anomalously Unique

Added: 2026-05-22

[00:00:00]Well, Hugh, why don't you go ahead and start us off with these you mentioned to me off air that you have three papers that you want to discuss that highlight the uniqueness of our solar system.

[00:00:12]>> papers all published in the last oh seven months and one quite recently published, which is the one we're featuring in the show notes.

[00:00:21]And it has to do with trying to explain uh the what's called the lithium problem.

[00:00:27]>> Mhm. [clears throat] And uh one of the papers I looked at actually did a survey of the scientific literature and said, "Oh, 240,000 scientific papers published on the lithium problem. This is really getting a lot of attention."

[00:00:41]But I'm also guessing that not too many lay people have even heard of the lithium problem. Yeah. So, let me begin by discussing what the problem is and how it's linked to another big problem, what's called the refractory element problem. But to begin with lithium, uh there's only one place where lithium is manufactured.

[00:01:02]And that's in the first three and a half minutes of the Big Bang.

[00:01:06]So, the universe starts off with nothing but hydrogen.

[00:01:09]But the universe starts off infinitesimally small and nearly infinitely hot. But as it expands, it cools.

[00:01:17]And it spends a few seconds in a temperature window where nuclear fusion can occur. Mhm. Roughly between 150 billion Kelvin and about 17 billion Kelvin.

[00:01:30]And uh during that few seconds, uh the about 25% of the primordial hydrogen is fused into helium. So, that explains why virtually all stars seem to be nothing but hydrogen and helium. Mhm. Now, they do make a trace amount of lithium. And by trace, it's like 1/10 millionth of uh If you compare the abundance of lithium to the hydrogen, it's like 1 10 millionth. So, it's just a very trace amount.

[00:02:01]However, um the stars, when they fuse uh elements of hydrogen to helium to heavier elements, don't make any lithium.

[00:02:10]>> Oh, I didn't realize that.

[00:02:12]>> Yeah, all the lithium, so your lithium batteries, all that lithium came from the first but between 3 minutes and 3 and 1/2 minutes after the cosmic creation event. So, it's like, hey, we burned through all that lithium, well, there's nowhere else to get it.

[00:02:27]So, uh So, that's where it all comes from, but we also know that once stars form, uh the stars basically destroy lithium.

[00:02:38]Oh. And so, as stars get older and older, they wind up becoming more and more deficient in the lithium.

[00:02:45]>> becomes part of the the the the fuel that the stars are using to burn? Well, what happens is that uh lithium uh gets degraded at about uh 10 million or 2 million degrees uh 2 million Kelvin. Okay. So, in the interior stars, if you get the interior structure stars uh where you got mixing going on, uh where the upper layers get mixed down the lower layers, the lithium in those upper layers uh will get uh destroyed.

[00:03:16]And so, it explains why when you look at the surfaces of stars, uh they're lithium depleted. Oh. And what we now know is that uh as stars slow down in their rotation rate, uh they wind up depleting more and more lithium. So, this is what's called uh a rotation lithium depletion correlation.

[00:03:38]Mhm. Uh there's a correlation between the rotation rate, the slower the rotation, the more mixing you get, and the more depletion of lithium occurs.

[00:03:49]And as stars get older and older, they rotate more and more slowly. So, right now the equatorial regions of the sun, their rotation is about 25 days, 25 Earth days.

[00:04:01]>> Uh but stars older than the sun will be rotating more slowly.

[00:04:06]And uh what's been helping with this lithium problem? So, what's kind of the lithium problem is that we look at the sun, it's way more depleted in lithium than it should be. Okay. And so, well, literally for 70 years, astronomers have been on a quest to find a twin of the sun, a star that's sufficiently like the sun that it could be a candidate to have a planet orbiting it in which advanced life exists. After 70 years, still no twins.

[00:04:37]Now, what they've accumulated are several thousand what they call solar analogs, stars that are similar to the sun, Right. but not enough that they could be a candidate to have a planet orbiting it in which advanced life takes place.

[00:04:52]I've often shared this with my friends that are interested in searching for extraterrestrial intelligence signals.

[00:04:59]It's like you're trying to find intelligence signals from another civilization beyond our solar system, the planet has to be orbiting a star that's virtually identical to the sun, and we've yet to find that star. And I've argued we shouldn't be spending time and telescope telescope time and money uh searching for extraterrestrial intelligence signals until we first find a star that would be a candidate. Uh I'd rather the telescope time be used to study the physics of quasars, galaxies, and stars. Yeah. But that's just my little uh political soapbox there. Yeah.

[00:05:37]So, uh but what we notice is that when you have this these several thousand solar analog stars, they beautifully beautifully fit the relationship. The older the star Right.

[00:05:49]>> the less the lithium. The slower rotates, the less the lithium. So, it all fits. Right. The sun stands out like a sore thumb.

[00:05:58]>> Right. And this is the hence the lithium problem.

[00:06:01]>> problem. Why is the sun so anomalously lithium deficient? Mhm. And it's deficient by about a factor of a hundred. Mhm. So, it's not a trivial deficiency.

[00:06:12]So, and the other thing we notice is that when you look at lithium and what we call um primitive meteorites, these are meteorites that formed when the solar system formed, is that uh they have about a hundred times more lithium than the sun has.

[00:06:29]>> Mhm. Uh the sun's photosphere. So, that implies something's driving the the loss of lithium. Well, yes, and it means it has to be after the pre-main sequence phase. Mhm. You know, it takes place after the formation of planets, asteroids, comets, and meteorites. Because look at these primitive meteorites, they got the lithium that you'd expect them to have, uh but the present sun does not. So, it's like, how do you explain that factor of a hundred?