HD 100546 b

**HD 100546**, also known as **KR Muscae**, is a pre-main sequence star of spectral type B8 to A0 located 353 light-years (108 parsecs) from Earth in the southern constellation of Musca. The star is surrounded by a circumstellar disk from a distance of 0.2 to 4 AU, and again from 13 AU out to a few hundred AU, with evidence for a protoplanet forming at a distance of around 47 AU.

Estimated to be less than 10 million years old, it belongs to Herbig Ae/Be stars, and also the nearest example to the Solar System.

## Planetary system

The HD 100546 system as a whole has one confirmed protoplanet candidate and there is evidence for 1–2 others (2 fanon) , thus it is considered an important evolutionary precursor to intermediate-mass stars with multiple super-jovian planets at moderate/wide separations like HR 8799. While other hypothetical planets have been claimed to exist around the star, none of the discoveries have been confirmed.

## Planet

In 2013, researchers reported that they had found what seems to be a planet in the process of being formed, embedded in the star's large disc of gas and dust. If confirmed, it would represent the first opportunity to study the early stages of planet formation observationally. The flux from HD 100546 b and its circumplanetary disk (CPD) are superimposed, leaving its properties such the radius and temperature thus very uncertain.

Various estimates for the mass of HD 100546 b has been varying between 1 and 25 MJ. Although standard hot-start models imply a mass of approximately 15 *M*J, other models and HD 100546 b's H-band photometry implies masses below 10 *M*J for a 1-million-years-old newly born planet or if made visible by its CPD, while older ages suggest higher masses.

More recently, a 2019 study placed an upper limit for the planetary mass was given to be as low as 1.65 *M*J based on the relation between the planet, CPD, and circumstellar disk (CSD) masses derived from numerical simulation. The CPD has been assumed to be optically thin with derived upper mass and radius limits of 1.44 times as massive as Earth (MEarth) and 0.44 astronomical unit (AU), while the mass of CSD was given to be 50 *M*J. While gas-starved models are also still compatible, this would suggest that HD 100546 b is inconsistent with several planet accretion models.

Fitting a single temperature blackbody to the observed fluxes of the point source component, a very large radius of 6.9+2.7

−2.9 times that of Jupiter (RJup) and an effective temperature of 932+193

−202 K has been derived for the emitting area surrounding the embedded protoplanet respectively, although those estimates were once mistakenly used as a single planetary radius and effective temperature by NASA Exoplanet Archive. A best-fit luminosity was also found by the same study to be 2.3+0.6

−0.4×10−4 times as luminous as the Sun (L☉).

Despite the uncertainty of the planet's properties, a 2017 study calculated HD 100546 b as a very highly reddened substellar object with a good-fit effective temperature of 2,630 K and a planetary mass and radius of 25 MJup and 3.4 RJup, making it still nonetheless one of the largest exoplanets discovered by size. However, it has been predicted by planetary evolutionary theory that gas giants including hot Jupiters typically cannot exceed 2.2 *R*J and that more massive gas giants would have lower radius at the time of their formation. This mass would put the planet beyond the border between a large planet and a brown dwarf.

## See also

* KR Muscae c
* KR Muscae d
* KR Muscae e
* TYC 8998-760-1 b

-Wikipedia