Thoughts from a Planetarian: Universe Update for March 2020

The third Thursday of every month, the Morrison Planetarium hosts “Universe Update” as the 6:30 planetarium show during NightLife—with the California Academy of Sciences currently closed, that won’t be happening tonight, so I decided to share some of my favorite astronomy stories from the past month.

Normally, I take NightLife audiences on a guided tour of the Universe while I’m sharing these astronomical highlights from the previous month. (As you may or may not know, the planetarium sports a three-dimensional atlas of the Universe, so we can take you places virtually while talking about the latest astronomy news. It provides context to the remarkable discoveries astronomers are making on a daily basis.) I always start at Earth and work my way out to cosmological distances, so I’ll list the news stories in the same order—from closest to farthest from home.

Let’s start with a nearby destination, the asteroid Ryugu! The Japanese spacecraft Hayabusa2 has spent a couple years visiting this small rubble pile in space, whose orbit occasionally brings it fairly close to Earth. Recently published infrared imagery of Ryugu shows that its surface is highly porous material with low density, and the asteroid seems to have formed from leftover fragments created by impacts. A recent announcement from the Deutsches Zentrum für Luft- und Raumfahrt (DLR) calls asteroid Ryugu a “likely link in planetary formation” because its porous structure might be similar to planetesimals, the building blocks of planets.

That’s one bit of news from our own solar system (all the stuff that orbits our own star, the Sun), but as you’ve almost certainly heard, astronomers have been discovering planets around other stars for nearly a quarter century. We now know of more than 4,000 of these exoplanets, and we’re finding more all the time. One student from the University of British Columbia even made news this month for using data from NASA’s Kepler mission to discover no less than 17 new planets—including an Earth-sized world in the habitable zone of its parent star!

Among those thousands of exoplanets, we’ve identified a few so-called “Tatooine” planets, which orbit a pair of stars, a little like Luke Skywalker’s fictional homeworld in Star Wars. Much as we’re interested in understanding how our solar system formed, astronomers are also trying to figure out how these planets take shape around a pair of central stars, rather than a lone star like the Sun.

Newly-released results announced by the National Radio Astronomy Observatory (NRAO) shed some light on this question. Researchers looked at rings of dusty material around binary stars using the Atacama Large Millimeter/Submillimeter Array (ALMA). These so-called protoplanetary disks are the environments in which planets form, and ALMA has already observed a whole gallery of such disks around lone stars, but how might the disks around binary stars look different?

Astronomers ended up finding a strong correlation between the orbital period of the binary stars and the alignment of the protoplanetary disks. First off, it’s important to realize that binary stars don’t just hang out next to one another in space; instead, they orbit one another due to their mutual gravitational attraction. The binary stars’ orbit defines one plane, and the protoplanetary disk defines another one. It turns out that the shorter the orbital period of the binary stars (the faster and closer they orbit one another), the more likely the protoplanetary disk lines up with the binary stars’ orbital plane. But binary stars with periods longer than a month typically host protoplanetary disks that are misaligned. (Well, that sounds judgmental—differently aligned might be a more polite way of putting it.)

For a first-person perspective on this result, check out this video featuring Ian Czekala of the University of California at Berkeley, who explains his research on “Tatooine” protoplanetary disks. Or there’s a shorter video that shows a visualization contrasting the two circumbinary disks around the stars HD 98800 B and AK Scorpii.

Astronomers combine observations of objects in our solar system (like little Ryugu) with observations of distant planetary systems in the process of taking shape (like those ALMA targets) to refine their computational models that use the laws of physics to reconstruct the history of planetary formation. Observational results help refine the theories and computations, while theories and computational models help us interpret the observational results. A virtuous cycle!

By the way, the Morrison Planetarium team actually created a video on this topic a few years ago. Called “Simulating Solar System Formation,” it shows 3D models of asteroids and comets (not Ryugu, unfortunately) and describes how they can help us understand how Earth and other planets took shape… We round out the video with a series of computer simulations that recreate various points in the history of the Solar System. So it’s a nice way to connect those two stories about planetary formation that have come out in the last month!

We posted this next story on our Facebook page last week, about iron rain falling from the skies of exoplanet WASP-76b. What’s up with that? Well, we know of several tidally-locked planets that orbit with the same side of the planet always facing their star, and some of them show evidence for high-speed winds that travel from the daytime side to the nighttime side (a consequence of the atmosphere heating up and having basically nowhere else to go). So, as it turns out, the daytime side of this crazy hot exoplanet has temperatures that climb above 2,400°C (more than 4,300°F), hot enough to vaporize metals! Hurricane-force winds might carry vaporized iron to the cooler nighttime side, where it could condense into droplets and fall as rain. Researchers surmised this by observing a strong signature of iron vapor signature at the “evening” border between day and night but no such signature on the “morning” side. Where did the iron go? Iron rain, iron rain… (Sung to the tune of Purple Rain, perhaps?)

At about the same distance from Earth as the Prince Planet—er, WASP-76b—we find the quite famous star Betelgeuse. Unlike WASP-76 or HD 98800 B or AK Scorpii, which are all very young stars, Betelgeuse is an old timer. It’s been in the news since late last fall because the star, normally one of the top 10 brightest stars in our night sky, dimmed quite noticeably, by about 40% or so, dropping to about 25th in brightness. That caused a flurry of speculation because Betelgeuse is a red supergiant, poised to go supernova at “any minute” in astronomical terms (although that means in the next 100,000 years or so for us). Could it be preparing to go kaboom?

Luckily, there’s a way to test that hypothesis. We would expect the surface temperature of Betelgeuse to decrease if it were in its final stages of life, so a group of astronomers at the University of Washington and Lowell Observatory took a look! On Valentine’s Day, they made observations that would allow them to determine the temperature of the star. And Betelgeuse, it turns out, is much warmer than we’d expect if it were about to explode. So their results support an alternative hypothesis—that Betelgeuse simply burped up some dust that obscured some of its light as seen from Earth, making it appear dimmer. “We see this all the time in red supergiants, and it’s a normal part of their life cycle,” said Emily Levesque, from the University of Washington. “Red supergiants will occasionally shed material from their surfaces, which will condense around the star as dust. As it cools and dissipates, the dust grains will absorb some of the light heading toward us and block our view.”

We’ve traveled way beyond our solar system at this point, voyaging through interstellar space to visit planets forming and stars dying. For our next story, we’ll leave our Milky Way Galaxy altogether…

The photo accompanying this article came from a Hubble press release, revealing the birth of young, massive stars in LHA 120-N 150 (which the press release itself describes as an “uninspiring name”). The nebula lies more than 160,000 light years away in the Large Magellanic Cloud, where stars (and almost certainly planets) are taking shape. The gaseous envelope is excited by radiation emitted by the young stars, glowing pink, while dark lanes show where dust is absorbing light from the stars and gas. This image tells the story of star birth on a much larger scale than the ALMA images, and astronomers are studying this region to understand how massive stars (much bigger than our own Sun, more like Betelgeuse) form. In theory, all stars should form in clusters, but observations suggest that about one in ten massive stars form all on their own. We can find stars in LHA 120-N 150 that both reside in clusters and live in isolation, so it’s a great place to learn more about their formation history.

Some of the newborn massive stars in LHA 120-N 150 may eventually end their lives (in a few million years or so) as neutron stars. These objects make for great factoids—they’re incredibly dense, with more mass than our sun crammed into a sphere that would fit neatly inside San Francisco Bay. But exactly how big is a neutron star? Our previous estimates for their sizes came primarily from theoretical considerations, unconstrained by observations…

But in the last couple years, we’ve observed what happens when two neutron stars collide! On August 17, 2017, gravitational waves were detected from just such an event—and follow up observations by a host of telescopes operating at different wavelengths provided an unprecedented wealth of data about what happened. An international team of researchers combined these observations with a heapin’ helpin’ of nuclear physics to determine the diameter of a neutron star with far greater precision than previously possible. A typical neutron star, with a mass of about 1.4 times that of the Sun, ends up having a diameter of about 22 kilometers (a little less than 14 miles). Or to be more precise, a diameter between 20.8 and 23.8 kilometers (between 12.9 and 14.8 miles). That’s impressive precision when you consider that the event took place 120 million years ago at a distance of about 130 million light years.

That’s quite a ways, but our last story comes from even farther away…

Earlier today, a result from the Hubble Space Telescope described quasar tsunamis ripping across galaxies—which sure sounds exciting! (Although it turns out to be less visually appealing than the boring-sounding LHA 120-N 150 star-forming region.) We see quasars at tremendous distances (like, billions of light years away) because they shine brightly, fueled by black holes at the centers of young, massive galaxies. The research that inspired the tidal wave metaphor was looking at how quasars generate such impressive brightness. Astronomers found that the region around the black hole emits enough radiation to accelerate material to a few percent the speed of light. (That’s fast enough to make the trip from Earth to the Moon in a few minutes—a trip that took Apollo astronauts about four days.) That’s a lot of energy! And an amazing observation to make from a telescope orbiting our modest planet billions of light years away.

Thanks for spending some time catching up with astronomy news stories from the last month. I’m sorry I couldn’t share these stories with you in Morrison Planetarium, but I’ll look forward to future audiences hearing about future discoveries once we reopen.