Season Recap: Four Big Questions from the 2026 Astronomy Lecture Series

What lit up the early universe? Did Earth make its own water? What is the invisible substance holding galaxies together? And how can astronomers peer inside a planet hundreds of light-years away?

Carnegie Science researchers took on all four questions during the 2026 Astronomy Lecture Series. If you missed the talks—or want to revisit them—the complete season is now available online.

Carnegie Science Observatories has brought current astronomical research to the public through this annual series since 2002. The 2026 season was the first led by Observatories Director Michael Blanton, who succeeded John Mulchaey—now Carnegie’s president as well as the series’ founder—in January.

Each evening opens with a live musical performance before the science begins, a tradition Observatories Strategic Initiatives Coordinator Erica Clark has shepherded for the past decade. The talks are free, geared toward a general audience, and streamed live from Rothenberg Hall at The Huntington in San Marino—allowing a lecture given in Southern California to reach viewers around the world.

Here’s what we learned this spring.

Ultraviolet Suspects: Using Galaxies to Shed Light on the Early Universe

Tony Pahl, Postdoctoral Fellow, Observatories 

The universe started as a hot, murky soup of extremely energetic particles. As it expanded, this material began to cool, and the particles coalesced into opaque neutral hydrogen gas. Within the first billion years, however, nearly all that hydrogen had been ionized, filling the universe with plasma. But how? Something had to supply the ultraviolet light that did it: either the first massive stars or the supermassive black holes at the centers of galaxies.

The trouble, Pahl explained, is that neutral hydrogen absorbs the ultraviolet light astronomers would need to trace. He compared it to driving up to Mount Wilson Observatory at night when the marine layer rolls in: You know there are headlights ahead, but you can’t make them out through the fog. In the early universe, neutral hydrogen is the fog and ultraviolet light is the headlights. Only after reionization ended did the universe become transparent to that light.

Pahl studies galaxies from just after that transition using JWST, the Magellan telescopes at Carnegie Science’s Las Campanas Observatory in Chile, and the telescopes at W.M. Keck Observatory in Hawaii to peer billions of years into the past—and then work backward.

The evidence so far points to massive stars, rather than black holes, as the primary source of the ultraviolet light. The transition may have unfolded over roughly 500 million years, less than five percent of the universe’s current age.

What remains unanswered is which galaxies did most of the work: rare, massive galaxies or the far more numerous small, faint ones. Pahl is pursuing that question with an instrument at Magellan that splits a single galaxy into 16 separate spectra. Instead of averaging the galaxy into one point of light, he can see how the signal changes across it.

There’s a nice bit of local—and Carnegie Science—history in all this. Evidence that the space between galaxies is ionized came from observations at Carnegie’s Mount Wilson and Caltech’s Palomar observatories in the mid-1960s—the same observatory in Pahl’s fog analogy. The questions he is working on now were first raised about 60 years ago, just a few miles from the hall where he was standing.

Planetary Habitability: The Origin of Water

Anat Shahar, Vice President for Research 

Where did Earth’s water come from? One longstanding explanation is delivery: Comets and asteroids formed beyond the snow line, then carried water-bearing material with the right chemical signatures to Earth at the right moment.

Shahar’s objection is that this scenario makes Earth lucky. Perhaps too lucky. 

A great deal has changed since scientists developed that explanation. In 1990, we knew of no planets outside our Solar System. We now know of more than 6,000, including many rocky worlds somewhat larger than Earth. Many may have spent their youths wrapped in thick hydrogen atmospheres, some of which were lost as the planets aged. If that is a common way to build a planet, Shahar asked, why assume Earth formed differently?

Her alternative explanation looks inward, suggesting that planets may be capable of manufacturing their own water.

Young rocky worlds can spend their early lives as magma oceans beneath thick hydrogen atmospheres. Shahar calls this the “teenage stage” of planet formation because it is extremely dramatic—and because everything that happens next depends on it. The atmosphere acts like a blanket, keeping the planet molten for tens of millions of years.

Models suggest that when hydrogen reacts with molten rock, it can produce water. A lot of it. Shahar’s model generated roughly two oceans’ worth.

Then her lab had to test the idea.

Researchers began with bubble-free silicate glass made in a laser levitation furnace in Paris. They squeezed it between diamonds at 350,000 atmospheres—roughly equivalent to 40 elephants standing on a fingernail—and heated it beyond 4,000 kelvins.

The experiment produced iron metal that should not have been present unless hydrogen had reacted with the rock, along with other evidence of water formation. The samples contained as much as six percent water by weight, far more than the researchers expected. The result nearly saturated the instrument.

What does that mean? If water can form as a natural part of building rocky planets, then water—and worlds with the potential to support life—could be much more common in the cosmos than scientists once thought!

Shining a Light on Dark Matter

Andrew Robertson, Theoretical Astrophysics Center, Observatories 
 
In the 1970s, Carnegie Science astronomer Vera Rubin and instrument-maker Kent Ford produced the first compelling observational evidence for dark matter. They found that stars near the edges of galaxies were orbiting much faster than the galaxies’ visible mass could explain.

But, as Robertson pointed out, stars moving too fast could mean one of two things: There is unseen matter out there, or our theory of gravity is wrong. And, to be fair, astronomy has landed on both answers before.

For Robertson, one of the strongest pieces of evidence for dark matter comes from the Bullet Cluster, where two clusters of galaxies collided.

Most of a galaxy cluster’s ordinary matter isn’t in its galaxies. It is in hot gas between them, with roughly 10 times the mass of all the cluster’s stars. During a collision, that gas behaves almost like something sticky, slowing down and collecting near the middle. The galaxies, which are mostly empty space, pass through to either side.

The collision gave Robertson a kind of natural experiment.

If the cluster contained only the matter we can see, most of its mass should have remained in the middle with the gas. Instead, observations enabled by a phenomenon called gravitational lensing showed that most of the mass had traveled outward with the galaxies—to places where very little ordinary matter ended up.

Something invisible made the crossing, and it outweighed everything visible.

Robertson’s team won 25 hours on JWST to map the cluster more precisely. The resulting data more than doubled the number of background galaxies the researchers could use to trace its mass. They also revealed that the main cluster is actually composed of two overlapping structures that are still merging, which makes the picture more complicated rather than less.

Side note: What may have been the best audience question of the season came during this lecture: Why call it dark matter instead of invisible matter? Robertson conceded the point immediately. He didn’t name it, he said, and dark matter sounds slightly cooler.

How to Look Inside an Exoplanet

Shreyas Vissapragada, George Ellery Hale Distinguished Scholar, Observatories 

Even though we continue to improve the resolution of our images of distant worlds, Vissapragada pointed out, we still see only their outermost layers. Nearly everything worth knowing is underneath.

Within our own Solar System, scientists have improvised. Earthquakes map our planet’s interior. Volcanoes occasionally hand us pieces of the mantle. Juno measures lumps in Jupiter’s gravitational field. Saturn’s rings register the planet’s internal sloshing well enough to act as a seismograph.

Even so, whether Jupiter has a compact core remains an open question.

When it comes to exoplanets, the problem is even more challenging. Astronomers can generally measure a planet’s mass and size, which together reveal its density.

Using call-and-response, Vissapragada taught the room a quick numerical guide for differentiating iron-rich, rocky, and gas-rich worlds by their densities.

Then he described his search for puffy, Neptune-sized planets whose atmospheres would be relatively easy to study. But they weren’t easy to find. His team kept finding the opposite.

Rock. Iron. Iron again.

Three in a row stopped looking like a coincidence and started looking like a pattern. These planets could be remnant cores—the dense remains of gas giants that swung too close to their stars and had their outer envelopes stripped away.

That is the payoff. If these unusually dense planets really are stripped cores, their properties could preserve evidence about the internal structures of the gas giants they once were. They may offer astronomers a way to infer something about distant gas giants that remains difficult to determine even for our own neighbors, Jupiter and Saturn, despite having spacecraft orbiting them.

Nature ran the experiment for us hundreds of light-years away.

As Vissapragada put it, you can’t go to Jupiter with a shovel and get rid of all the gas—but you sure can get a star to do it!

What's next? 

The questions explored this season are far from settled. The Giant Magellan Telescope, now under construction at Carnegie Science’s Las Campanas Observatory, will give astronomers powerful new tools for studying early galaxies, dark matter, distant planetary systems, and other phenomena examined throughout this year’s series.

The 2027 season will be announced early next year. Sign up for our newsletter to hear about it and other public events first.