Milky Way's Strangest Exoplanets: From Iron Rain to Rogue Worlds

Overview

In this Ashram video, Alex McColgan explores the Milky Way's strangest exoplanets, from planets in complex multi-star systems to rogue worlds roaming free of a star. The content covers detection techniques such as transits and gravitational microlensing, and highlights extreme worlds like WASP 76B with iron rain and Celt 9B in an ultrahot system. It also discusses the role of binary and higher order star configurations on planetary orbits and potential habitability, and introduces notable objects such as Kepler 16B and J1407B with rings.

Introduction to Exoplanet Diversity

The video opens by framing the galaxy as a laboratory of strange worlds, far more varied than our own solar system. While thousands of exoplanets have been confirmed, many push the boundaries of what we imagine a planet to be, including planets in systems with multiple suns, planets that orbit pulsars, and worlds with extreme temperatures. The presenter explains that exoplanets are discovered primarily via transit observations and radial velocity measurements, and that detection biases favor short orbital periods, which is why many known exoplanets orbit their stars in a matter of days or weeks.

Mercury serves as a local example with the quirk that a Mercurian day is longer than a Mercurian year, illustrating how orbital dynamics and rotation can complicate time concepts on other worlds. The piece then transitions to exotic planets that challenge simple one-star, one-planet intuition, including worlds in multi-star environments, rogue planets, and ultra-hot gas giants close to their stars.

Extreme Orbits and Timescales

The discussion surveys the range of planetary years, from the ultra-short to the extreme. Notable cases include a pulsar planet around a neutron star with an orbital period of about 49 minutes, and K2137B, a planet with a year of roughly 4.31 hours that orbits inside its star's atmosphere, threatening tidal disruption. On the other end, a planetary-mass object can have a year lasting around a million years, illustrating the breadth of possibilities in the cosmos.

Multi-Star Systems and Planetary Orbits

The video explains that most stars are in binary or multi-star systems, and it describes hierarchical configurations where planets can form and stabilize in various tiers. It outlines how planets can orbit a single star in a binary, or orbit all stars in the system, or lie in complex arrangements with different orbital centers of mass. The presenter notes that while many bright stars in the sky are binaries, single stars are also common, and planetary formation can still proceed in such diverse environments.

Kepler 16B, often called the Tattooine planet, is highlighted as the first confirmed exoplanet in a circumbinary orbit. Kepler 16B is a gas giant about 0.3 times the mass of Jupiter, showing how a planet can stably orbit a binary pair. The talk also addresses the potential habitability in multi-star systems, including how stellar separation and stellar activity, such as red dwarf flares, could impact climate, seasons, and the viability of life as we know it.

Rogue Planets and the Microlensing Frontier

The concept of rogue planets—planetary-mass objects not bound to any star—takes center stage. The video explains gravitational microlensing as a detection method capable of revealing rogue planets and even small exomoons by watching background stars brighten as a massive object passes by. It covers current ground-based surveys like OGLE, MOA, and SuperMacho, which have found rogue planet candidates, and highlights the planned role of the Nancy Grace Roman Space Telescope in tracking microlensing events across large sky swaths, particularly toward the galactic bulge where events are more frequent.

Notable Extreme Exoplanets and Ring Worlds

Specific planets are discussed to illustrate the extremes. WASP 76B, a very hot gas giant in a close orbit, shows iron rain on its intensely heated day side, as iron vapor condenses near the terminator and precipitates on the morning side. Celt 9B orbits Celt 9, a star of about 9700 degrees Celsius, with day-side temperatures around 4300 degrees Celsius, a hydrogen atmosphere boiling away and a risk of rapid atmospheric loss. The discussion then covers the possibility of planets larger than their stars, the mass thresholds that separate planets from brown dwarfs and true stars, and the intriguing VHS 1256-1257 system, where a directly imaged planet may be paired with a faint companion that could be a brown dwarf rather than a planet.

The video also touches on WD 1856B, a planet in a close orbit around a white dwarf, as evidence that planets can survive late stellar evolution. The closing sections consider the biggest exoplanet candidates reported so far, including GQ Lup B and others with large radii, while also describing J1407B, a candidate with rings an order of magnitude larger than Saturn’s rings, inferred from dramatic light dips when transiting its star.

Closing Thoughts

The host invites viewers to share favorites and acknowledges the channel’s funding model, reinforcing the platform’s aim to make space science accessible and engaging while maintaining rigorous standards. The segment ends with a call to support the channel through Patreon and continue exploring the cosmos together.