TOI-561 b
An ultra-short period super-Earth with a volatile-rich atmosphere.
TOI-561 b is an ultra-short period super-Earth exoplanet orbiting the Sun-like star TOI-561. As the innermost planet in its system, it completes an orbit in just 11 hours, resulting in a dayside temperature exceeding 2000 Kelvin. Its low density for an ultra-short period planet, combined with the detection of a volatile-rich atmosphere, makes it a notable subject for studying planetary composition and atmospheric retention under extreme irradiation.
Quick Facts
- Discoverer
- Transiting Exoplanet Survey Satellite
- Discovered
- 2020
- Discovery Method
- Transit
- Star
- TOI-561
- Density
- 4.1 · 0.5 g/cm / 3
- Albedo
- 0.74 · 0.08 · 0.10
Facts from the source article.
Lore & Background
TOI-561 b orbits an old, metal-poor star with lower iron abundance relative to silicon and magnesium than the Sun. This stellar composition may result in a smaller dense iron core for the planet, contributing to its lower bulk density. The planet's low density also suggests the presence of an envelope of low-density materials, supported by the detection of a volatile-rich atmosphere that transports heat from the dayside to the nightside.
James Webb Space Telescope measurements of the dayside emission spectrum and full-orbit phase curve reveal moderately efficient global heat transfer, inconsistent with a bare rock planet or one with an atmosphere of evaporated rock. This confirms a thick atmosphere of volatiles. The existence of such an atmosphere is surprising, as highly irradiated super-Earths are expected to have lost volatiles; possible explanations include outgassing from a magma ocean or volatile-rich formation.
TOI-561 b's high Bond albedo suggests reflective clouds in its atmosphere, possibly mineral clouds condensed from rock vapor evaporating from the planet's magma ocean. Given its small mass and intense irradiation, it is probably unable to hold onto a primordial hydrogen and helium envelope, so the atmosphere is expected to consist of volatiles like water or carbon dioxide.
Reader's Guide
TOI-561 b is significant as an ultra-short period super-Earth that challenges expectations about volatile retention under extreme stellar irradiation. Its low density, despite being so close to its star, points to an iron-poor composition or a thick envelope of low-density materials. The detection of a volatile-rich atmosphere via JWST phase curve and emission spectroscopy provides direct evidence that such planets can maintain atmospheres, possibly through outgassing from a magma ocean. This finding has implications for understanding the diversity of exoplanet compositions and the processes that govern atmospheric loss and replenishment. The planet's high Bond albedo, likely due to reflective mineral clouds, adds another layer of complexity to models of cloud formation on hot exoplanets. As the innermost world in its system, TOI-561 b serves as a benchmark for studying the interplay between stellar metallicity, planetary interior structure, and atmospheric dynamics in the ultra-short period regime.
Did You Know?
- TOI-561 b completes an orbit in just 11 hours.
- Its dayside temperature exceeds 2000 Kelvin.
- The planet has a volatile-rich atmosphere that transports heat from dayside to nightside.
- Its high Bond albedo suggests reflective mineral clouds in the atmosphere.
Frequently Asked Questions
What is TOI-561 b?
TOI-561 b is a super-Earth exoplanet that serves as the innermost world in the TOI-561 system, circling a Sun-like star. It measures roughly 1.4 Earth radii in size and carries about twice Earth's mass, making it a compact yet dense rocky world.
How long does a single orbit of TOI-561 b take?
A full lap around its host star clocks in at a mere 11 hours, which earns it the ultra-short period label. That frenetic pace slams the dayside with relentless stellar radiation, driving temperatures well above 2000 Kelvin.
Does TOI-561 b actually have an atmosphere?
Yes—spectroscopic work has revealed a volatile-rich envelope likely composed of species such as water vapor or carbon dioxide. That is a striking find, because most ultra-short period planets are expected to have been stripped bare by their scorching stellar proximity.
Why do astronomers care so much about TOI-561 b?
Its surprisingly low density for a planet this close to its star, combined with a retained volatile atmosphere, offers a rare natural laboratory for probing how composition and atmospheric survival interact under extreme irradiation. It pushes back against the assumption that every ultra-short period super-Earth must be a naked, airless rock.
What kind of star does TOI-561 b orbit?
Its parent, TOI-561, is an older, metal-poor star that still broadly resembles the Sun in spectral type. The low metallicity of the host adds an extra puzzle piece when researchers try to explain the planet's bulk composition and the origin of its volatile inventory.
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