HD 21749

HD 21749 is a K-type main sequence star (orange dwarf) with a spectral type of K4.5V,[3] indicating it is smaller and cooler than the Sun. It is estimated to have a mass of 0.73 M_☉, a radius of 0.70 R_☉, and a luminosity of 0.20 L_☉. Its effective temperature is 4,640 K, which gives the star an orange color typical of K-type stars. Its metallicity—the proportion of elements other than hydrogen and helium—is approximately equal to the Sun's.[4]

This star is moderately active, as shown by its spectral activity indicators and photometric data. These measurements indicate a rotation period of around 30 to 40 days, with a most likely value of 34 days. Stellar activity also creates radial velocity variations, which complicates the measurement of the mass of the planets in the system.[5]

In January 2019, the discovery of an exoplanet around HD 21749 was published. The planet was identified from a single transit event detected by the TESS spacecraft, using data from the first two observation sectors of the mission. Since a single transit is insufficient to determine the orbit of a planet, astronomers used archival radial velocity data from the HARPS spectrograph to detect the planet's signal, which allowed the determination of its orbital period and mass.[6] In April 2019, with two additional months of data from the TESS spacecraft, the orbital period of the planet was confirmed with the observation of new transits, and a second planet was discovered.[4]

The inner planet, HD 21749 c (the second in order of discovery), is orbiting the star at a distance of 0.08 AU with a period of just 7.8 days. A terrestrial planet, it has a radius of 1.1 R_🜨 and was the first Earth-sized planet found by TESS.[5][4] Its mass is too low to be calculated with current radial velocity data, with an upper limit of 3.5 M_🜨; a probabilistic model estimates it is most likely between 1 and 2 M_🜨.[5]

The outer planet, HD 21749 b, orbits the star at a distance of 0.21 AU with a period of 35.6 days. With a mass of 20 M_🜨 and a radius of 2.9 R_🜨, it is similar to Neptune but much denser. Its density of 4.7 g/cm³ suggests it is composed of a substantial rocky core, with a radius of approximately 2.1 R_🜨, plus a relatively thick gaseous layer.[5]

• Staff (2019). "Stellar Overview Page - HD 21749". NASA Exoplanet Archive. Retrieved 8 January 2019.
• Overbye, Dennis (7 January 2019). "Another Day, Another Exoplanet: NASA's TESS Keeps Counting More". The New York Times. Retrieved 8 January 2019.
• Massachusetts Institute of Technology (16 April 2019). "TESS discovers its first Earth-sized planet - Orbiting a nearby star, the new planet is the smallest identified so far by the TESS mission". EurekAlert!. Retrieved 16 April 2019.