How were the two planets detected?

HIP 11952 b and HIP 11952 c were found using the radial-velocity technique, a common method for detecting exoplanets that has been in use for more than two decades. A star and its planets move around a common center of mass  - and the motion of the star, a tiny wobble back and forth, towards the observer and away again, caused by the presence of unseen low-mass companions, can be measured by observing how certain features in the star’s light (spectral lines) shift over time.

Which telescopes and instruments were used?

The star HIP 11952 was monitored from 2009 until 2011 with the spectrograph FEROS (Fibre-fed Extended Range Optical Spectrograph), mounted on  the 2.2 meter Max-Planck-Gesellschaft/European Southern Observatory (MPG/ESO)- telescope, located at ESO's La Silla observatory in Chile.

Why is the star called “HIP 11952”? And what about its planets’ names?

HIP 11952 is the star’s catalogue number (11952) in the catalogue of stars observed by the Hipparcos astrometry satellite (hence the HIP). A star’s planets are named, in order of discovery, by appending lower-case letters to the star’s name, alphabetically, starting with the letter b.

What are the properties of HIP 11952?

The parent star has what astronomers call very low stellar metallicity – its atmosphere contains very few chemical elements heavier than hydrogen or helium. A common indicator for metallicity is the presence of iron. HIP 11952 has an iron abundance of only about 1%  that of the Sun. HIP 11952 is said to belong to “(stellar) population II”, a class of stars that includes the oldest and metal-poorest stars that have been observed.

Old and metal-poor stars such as HIP 11952 are very rare in the solar neighbourhood, which is relatively metal-rich and mostly contains stars younger than 10 billion years. This makes it interesting to speculate about the birthplace of HIP 11952: It is possible that, like its predecessor HIP 13044, HIP 11952 is part of a stellar stream – the remnant of another galaxy that was swallowed by our own Milky Way galaxy billions of years ago.

How was the age of HIP 11952 determined?

Astronomers have fairly solid models of how stars evolve over time; during their evolution (which, in turn, depends on the star’s initial metal content), properties such as the star’s intrinsic brightness and its surface temperature change systematically over time. This, in turn, allows astronomers to give age estimates for stars for which those properties can be determined with reasonable accuracy. By this measure, HIP 11952 is about 12.8 billion years old.

The precise evolutionary status of the star is, however, unclear: At that age, the star could still be in the so-called main-sequence phase, where stars spend most of their lives, their radiation sustained by the fusion of hydrogen in their cores. The Sun, for instance, is a main sequence star; HIP 11952 could be either that or an “evolved star” that has left the main sequence.


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