TOI-561

Key takeaways

TOI-561 belongs to astronomy; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect TOI-561 to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of TOI-561 from memory before moving on to harder problems.

Reference excerpt

TOI-561 is an old, metal-poor, Sun-like star, known to have multiple small exoplanets orbiting it. It is an orange dwarf, estimated to be 10.5 billion years old, and about 79% the mass and 85% the radius of the Sun. It is located in the constellation of Sextans, near the border with Leo. In January 2021, a team led by Lauren Weiss of the University of Hawaii at Manoa announced that, using data from NASA's Transiting Exoplanet Survey Satellite, they had found a Super-Earth in a very close orbit, as well as two outer Sub-Neptunes. The innermost planet, TOI-561 b, orbits in under one Earth day. Another team led by Gaia Lacedelli of the University of Padua independently announced the discovery in a paper published in December 2020. However, the two papers disagree on the structure of the system. While the innermost two planets were confirmed from data by both papers, Weiss proposes only a single third planet in a 16.3-day orbit, while Lacedelli argues that the system instead contains two further planets, in wider orbits of 25.6 and 77 days.

Discovery and nomenclature TOI-561 is also designated 2MASS J09524454+0612589 in the 2MASS catalog and TIC 377064495 in the TESS Input Catalog. When its planets were first identified, it was renamed TOI-561, with TOI standing for "TESS Object of Interest". The planetary system was independently confirmed and characterized by Lacedelli et al. (2020) and Weiss et al. (2021). Lacedelli et al. found evidence for four exoplanets: the Ultra-Short-Period (USP) Super-Earth TOI-561 b, and three Sub-Neptunes designated TOI-561 c, d, and e. The two planets TOI-561 d and TOI-561 e were originally listed as a single planet with a period of 16 days on ExoFOP, but Lacedelli could not detect a planet in that orbit using radial velocity data from HARPS, and instead interpreted it as two separate transits coming from planets correlating with periods of 25.6 and 77.2 days found by HARPS. In January 2021, Lauren Weiss and her team's study on TOI-561 was published. Unlike Lacedelli, they kept the 16-day signal and designated it TOI-561 d; it is referred to as TOI-561 f on NASA's Exoplanet Archive to avoid confusion with the TOI-561 d from Lacedelli's paper.

Characteristics TOI-561 is a yellow or orange star approximately 80% the size of the Sun. According to Lacedelli, it is 85% the radius and 79% the mass of the Sun, with a temperature of 5455 Kelvin. Weiss found the star to be 83.2% the radius and 80.5% the mass of the Sun, with a temperature of 5326 K and a luminosity just over half that of the Sun. Both teams found that TOI-561 has an extremely low abundance of metals, or any element heavier than hydrogen or helium, and is very old; Weiss calculates an age of roughly 10 billion years. It is also a part of the Galactic thick disk and is the first of those stars to have confirmed transiting exoplanets.

Planetary system Depending on the study, TOI-561 has either 3 (Weiss) or 4 (Lacedelli) planets. The discrepancy comes from different interpretations of the two transit events associated with TOI-561 d in Weiss 2020. Only two transits were observed by TESS, and a third transit for a 16-day period would have occurred in the middle of a data gap. Weiss attributes the two transits to that of a single Sub-Neptune sized planet. However, in the radial velocity analysis by Lacedelli 2020, the 16-day signal is not recovered, but there are two additional signals of 26 and 77 days that they attribute to one of the two transits each. The follow-up study in 2022 has confirmed the architecture of four-planet system. Additional, fifth planet on the 473+36−25 days orbit is suspected. The orbital parameters were refined with additional observations from CHEOPS and TESS in 2024, further confirming four transiting planets and a fifth non-transiting candidate.

TOI-561 b

TOI-561 b is an ultra-short period Super-Earth with a radius of roughly 1.4 Earths. It has an extremely short orbital period of under 11 hours, less than half of an Earth day, resulting in an equilibrium temperature of 2,480 ± 200 K (2,207 ± 200 °C; 4,004 ± 360 °F). The planet is believed to be far too small and irradiated to hold onto its primordial Hydrogen and Helium envelope. However, the composition of the planet varies greatly between the two studies. Weiss 2020 found a mass of around 3.2 Earths and a density of 5.5 grams per cubic centimetre, around the same as Earth and implying a rocky but iron-poor composition. Lacedelli 2020, on the other hand, found a mass of only 1.59 Earths and a density of 3.0 grams per cubic centimetre, abnormally low for a planet of its size and suggesting a composition made of 50% or more of water. Even their higher mass estimate of 1.83 Earths is still consistent with a water-world. With an insolation 5,100 times greater than Earth, TOI-561 b should have lost its gaseous layer and have little volatiles, so the authors believe if the planet has a significant amount of water, it has been evaporated into a puffy steam atmosphere that makes the planet seem larger, less dense, and more water-rich. If it is an extremely water-rich world, TOI-561 b would prove formation scenarios about Super-Earths forming beyond the "Snow Line" and migrating inwards.

TOI-561 c TOI-561 c is a Mini-Neptune orbiting every 10.7 days with an equilibrium temperature of 860 ± 70 K (586.9 ± 70.0 °C; 1,088.3 ± 126.0 °F). With a radius of 2.9 Earths and a mass of 5.4 to 7.0 Earths, the planet has a Neptune-like density of 1.3 to 1.6 grams per cubic centimetre, implying that it is a small gas planet with a similar composition, albeit far hotter and closer to its star than our system's ice giants.

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with TOI-561

Start with the simplest possible case. Write down what TOI-561 claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In astronomy, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to TOI-561 before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about TOI-561 ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of TOI-561

In research: TOI-561 appears in astronomy research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses TOI-561 in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: TOI-561 is common in secondary-school and first-year university syllabi. It links to neighbouring topics G-type main-sequence stars, Planetary systems with four confirmed planets, Sextans, so understanding it makes those chapters shorter.
In everyday life: Look for TOI-561 outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study TOI-561 in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what TOI-561 means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain TOI-561 out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is TOI-561 in simple terms?

TOI-561 is an old, metal-poor, Sun-like star, known to have multiple small exoplanets orbiting it. It is an orange dwarf, estimated to be 10.5 billion years old, and about 79% the mass and 85% the radius of the Sun.

Why does TOI-561 matter?

Because it connects several astronomy ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study TOI-561?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on TOI-561.