In “Not Quite Stars,” by Katelyn Allers, the diagram “A Guide to Brown Dwarfs” states that these objects are “at least” 13 Jupiter masses. Yet it shows one brown dwarf that is eight Jupiter masses and another that is between three and 10. Meanwhile the main text refers to “planetary-mass brown dwarfs” that are less than 13 Jupiter masses.

I thought the definition of brown dwarfs precluded Jupiter masses below about 13—and that the process of their formation would not produce objects below a certain mass. So what, exactly, is the lower limit? Do we know?

ROBERT WALTY Stephens City, Va.

ALLERS REPLIES: Astronomers haven’t settled on an accepted name for objects with planetary masses lower than 13 times that of Jupiter that don’t orbit a host star, and there’s still some good-spirited debate about the dividing line between exoplanets and brown dwarfs. Ideally, we would define extrasolar planets as objects that formed from the disk of a host star and brown dwarfs as objects that formed like scaled-down stars. Unfortunately, we can’t easily observe how an individual object formed. I personally prefer the term “planetary-mass brown dwarf,” but that can be a bit of a mouthful.

We use the same techniques to discover and characterize free-floating objects whether they have masses above or below 13 times that of Jupiter. So for most purposes, lumping them together as brown dwarfs makes sense. In the end, I’m much more interested in what we can learn about and from these objects than what name we label them with.


I noticed an unexpected connection between “Why Animals Play,” by Caitlin O’Connell, and the gem that was “The World’s First Trans Clinic,” by Brandy Schillace. O’Connell stated that animals, from pachyderms to primates, engage in play as practice for hunting, fighting, fleeing or mating. Growing up, I learned an additional function of play: identity formation. I viscerally cringed when I heard my female name, so I reinvented myself as a dog or dinosaur to whom names meant nothing. It should be noted that sexual differences in those species are far more subtle than in humans. For the hours in which I was fantasizing, I could escape the body that abraded me, as well as any roles of “daughter” and “girl.”

Now, four years after realizing my identity as a gay transgender man, I gaze down at the black-and-white image of a costume party in the early 20th century at the Institute for Sexual Research in Germany that opens Schillace’s article. If I had lacked the privilege to live in a time and location where social and medical transition is safe, you can bet I would be attending all sorts of “costume parties.” Sometimes the guise of mere play is what we need to align ourselves with instinctual ipseity. Thank you for giving visibility to transgender and other LGBTQ+ people.

STEPHEN HUITING Grass Valley, Calif.


Counting Birds,” by Clara Moskowitz and Jen Christiansen [Graphic Science], claims that no single avian species has an estimated population of more than 1.6 billion individuals, according to an analysis of 92 percent of extant bird species. This is a remarkable number for two reasons. First, it implies that humans outnumber every single bird species, despite weighing far more than common birds. Second, it ignores domesticated avian species. In 2019 there were 25.9 billion farmed chickens worldwide, making them by far the most common avian species on Earth.

The article questions what effect humanity has on avian populations. Any such inquiry would be incomplete without considering farmed species.

DAVID LEPPIK via e-mail

MOSKOWITZ REPLIES: It’s sobering and true that the most numerous avian species on Earth is the domesticated chicken rather than any of the species in the wild examined by the researchers. This fact is all the more alarming when we remember that humans are also the reason that so many of the populations of birds in nature are decreasing, just as our consumption of farmed chickens rises every year.


In the concluding paragraph of “Overhyped,” David Wright and Cameron Tracy note that there are shrinking resources for impartial research on the abilities and impacts of novel weapons. Although this is far from my own research area of music history, and as a Quaker, I am a pacifist, I recognize the importance of that work: it was my father’s.

In the 1950s and early 1960s Laurence B. Dean, Jr., was a member of the Weapons Systems Evaluation Group and its successor, the Institute for Defense Analyses. Among his colleagues (and indeed his closest friend) was the late political scientist George W. Rathjens, who wrote for Scientific American on national-security matters in April 1969, January 1970 and February 1993.

Although the work they were doing was focused on weaponry, they construed it as directed toward peace. I see the work of Wright and Tracy and their colleagues in a similar light, and I fervently hope it can be sustained at a level strong enough to inform policy- and decision-making in an area that affects the peace and security of us all.

Royal Birmingham Conservatoire, Birmingham City University, England


Thank you for “The Stuttering Mind,” by Lydia Denworth. Now pushing 80, I have stuttered since early childhood. I endured countless trips to “therapists,” one of whom squirted me in the face with a water pistol when I stuttered. My father was convinced that I just didn’t try hard enough to use the various “techniques” that were the standard of the day. I was never able to convince him that trying harder often made things worse.

Even at my age, learning from Denworth’s article that my brain is apparently wired differently brought a tremendous sense of relief—and tears to my eyes. We now have scientific evidence that this is not a personal failing or a character weakness. Despite my stuttering, I enjoyed a 30-year career as a college professor. Now retired, I still recall the pride and sense of accomplishment I experienced when marching into the auditorium at commencement with my faculty colleagues.

via e-mail


As the primary caregiver for an elderly Alzheimer’s patient, I eagerly read “A New Understanding of Alzheimer’s,” by Jason Ulrich and David M. Holtzman. Unfortunately, I ultimately found it disappointing. The insight that microglia can modulate the course of the disease is interesting, and immune system seasonality might explain some phenomena, such as the seasonal dependence of Alzheimer’s symptoms, which I have observed in my own patient.

But the research does nothing to elucidate the underlying causes of the disease. Based on the article, my impression is that therapies aimed at microglia might ease symptoms but not be curative.

Alzheimer’s disease victims and families are desperate for real cures. Sadly, we still have a long way to go. I’ll keep looking for breakthroughs in the science, although it is likely already too late to help my own loved one.

Kaneohe, Hawaii


Not Quite Stars,” by Katelyn Allers, should have said that astronomers Rafael Rebolo López, María Rosa Zapatero-Osorio and Eduardo L. Martín observed elements in Teide 1’s atmosphere, not molecules.