Up until the early 1990s, we didn't know what sorts of planets lived around stars other than our Sun. Were they like our own Solar System, with inner, rocky planets close to our star and large, giant worlds farther away? It turned out that exoplanetary systems come in a great variety of configurations: with planets of all sizes, masses, and distances from their parent stars. But some configurations are more common than others.

There are lots of hot Earth-sized planets and lots of hot Jupiter-sized planets, but precious few "hot Neptune" worlds out there. Furthermore, there appear to be lots of Earth-sized and super-Earth-sized worlds at greater distances, as well as many Neptune-sized and mini-Neptune-sized worlds. However, there's a gap there, too: between the large super-Earths and the small mini-Neptunes. Where are these missing exoplanets? Or, rather, why are these classes of exoplanets so uncommon?

That's what we're exploring on this episode of the Starts With a Bang podcast, featuring Ph.D. candidate Dakotah Tyler as our guest this month. By looking at how a hot (but low-mass) Jupiter-sized planet is being photoevaporated by its parent star, we can learn so much about not only the classes of objects we see out there, but even the ones we don't!

(Around the star WASP-69, a "hot Jupiter" exoplanet has its outer layers of atmosphere photoevaporated away, creating a comet-like tail whose extent and mass were recently measured for the first time. Credit: W. M. Keck Observatory/Adam Makarenko)

Starts With A Bang #102 - The Missing Exoplanets

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All throughout the Universe, galaxies exist in a great variety of shapes, ages, and states. Today's galaxies come in spirals, ellipticals, irregulars, and rings, all ranging in size from behemoths hundreds or even thousands of times larger than the Milky Way to dwarf galaxies with fewer than 0.1% of the stars present here in our cosmic home. But at the centers of practically all galaxies, particularly the large ones, lie supermassive black holes.

When matter falls in towards these black holes, it doesn't just get swallowed, but accelerates and heats up, leading to phenomena like accretion disks, jets, and emitted radiation all across the electromagnetic spectrum. When these conditions exist, we know we have what's called an active galaxy, and it isn't just the rest of the galaxy that's impacted by that central activity, but far larger structures in the Universe beyond. 

Here to help us explore these objects and their impact this month is Skylar Grayson, a PhD candidate at the School of Earth and Space Exploration at Arizona State University. Skylar works at the intersection of theory and computational astrophysics, and helps simulate the Universe while focusing on the inclusion and modeling of this type of galactic activity, and is one of the people helping uncover just how profound of a role these galaxies play in shaping the Universe around them. Buckle up for another exciting 90 minute episode; you won't want to miss it!

The powerful radio galaxy Hercules A, shown above, is a stunning example of how central activity from the galaxy's active black hole influences not only the host galaxy, but a large region of space extending far outside the galaxy itself, as visible from the extent of the radio lobes highlighted visually. (Credit: NASA, ESA, S. Baum and C. O'Dea (RIT), R. Perley and W. Cotton (NRAO/AUI/NSF), and the Hubble Heritage Team (STScI/AURA))

Starts With A Bang #103 - Active galaxies and the universe

Have you ever wondered what the full story with the galactic center is? Sure, we have stars, gas, and an all-important supermassive black hole, but for hundreds of light-years around the center, there's a remarkable story going on that's traced out in a variety of elements at a whole slew of different temperatures. Imprinted in that material is a remarkable set of features that reveals the magnetic fields generated in our galaxy's core, with some of them spanning much greater distances than have ever been seen elsewhere.

It's a testament to the power of multiwavelength astronomy, and in particular to the long wavelengths like the far-infrared, the microwave, and the radio portions of the spectrum that shows us these features of the Universe that simply can't be revealed in any other way. To help bring this story to all of you, I'm so pleased to welcome Dr. Natalie Butterfield, a scientist at the National Radio Astronomy Observatory (NRAO), to join us on this episode of the Starts With A Bang podcast.

Natalie is the discoverer of a giant magnetized ring some 30 light-years in diameter located in the galactic center, and is one of the leaders of the FIREPLACE survey: the Far-Infrared Polarimetric Large-Area CMZ Exploration survey that used the (sadly, now-defunct) SOFIA telescope to image the galactic center as never before. Strap in and have a listen, because you just might never think about the core of the Milky Way in the same way again!

(This image shows the magnetized galactic center, with various features highlighted, as imaged by the SOFIA/HAWC+ FIREPLACE survey team. The giant bubble at the left of the image is some 30 light-years wide, several times larger than any other supernova-blown bubble ever discovered. Credit: D. Paré et al., arXiv:2401.05317v2, 2024)

Starts With A Bang #104 - The Magnetized Galactic Center

Starts With A Bang #102 - The Missing Exoplanets

Ethan Siegel

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