Object 3: HD 85512
Podcast release date: 09 September 2019
Right ascension: 09:51:07.1
Corresponding Earth location: Just off the southeastern coast of Tasmania
HD 85512 is a hydrogen-burning star like the Sun, but it is smaller, cooler, and more orange-coloured . It is also very close to the Earth; the distance is about 36 light years, or 11.3 parsecs . This places the star within the top 300 closest star systems to the Solar System. However, HD 85512's proximity to Earth is only a small factor in what makes this star truly interesting. The most exciting thing about this star is that an exoplanet was found in orbit around it.
One of the techniques for finding planets orbiting other stars is to look for slight changes in the velocities of the stars that would be caused by gravitational interactions between the star and its planets. This type of shifting can be observed by measuring the spectra of the stars and looking to see if the spectra shift in frequency over time. These frequency shifts can then be used to measure the velocity of the star; this phenomenon is called Doppler shifting. In 2003, the European Sourthern Observatory set up the creatively-named High Accuracy Radial velocity Planet Searcher or HARPS instrument on the uncreatively-named European Sourthern Observatory 3.6 Meter Telescope with a few specific goals including the identification of exoplanets by using this technique . It had a lot of early successes, but the people working with HARPS wanted to find an Earth-sized planet orbiting a Sun-like star in an orbital region called the habitable zone. These habital zones are places where the planet is at just the right distance for water to exist in liquid form. Finding such a planet would be a potential step towards identifying extraterrestrial life and could be the inspiration for many science fiction stories and scientific funding proposals.
HD 85512 was one of ten nearby, Sun-like stars selected for a very intense observational survey with the HARPS instrument that was started in 2009. The first observations revealed the presence of planets orbiting three different stars, including HD 85512 . However, what was special about the planet orbiting HD 85512 was that it was very close to Earth-like, unlike the exoplanets around the other two stars or most other exoplanets detected at that time. If the star had not been so close to the Earth, the exoplanet may not have been detectable with current technology.
The planet, which was called HD 85512b because astronomers are not allowed to make up their own names for exoplanets, has a mass that is estimated to be a minimum of 3.6 times the mass of the Earth , which would make it larger than Earth, but it would still be a rocky planet and not a gas giant like Jupiter, Saturn, Uranus, or Neptune. This is a lower limit because it is hard to determine the exact angle at which HD 85512b is orbiting its star, and the geometry of the orbit affects the mass measurement, but it is likely that the planet is not much larger than 3.6 Earth masses.
The average distance from the exoplanet to HD 85512 is a minimum of 0.26 Astronomical Units or AU . 1 AU is defined as the distance from the Earth to the Sun. Mercury's distance from the Sun is 0.39 AU, so if HD 85512 and its exoplanet are closer than this, people's first impression might be that the exoplanet is so hot that some metals like lead and tin would melt. However, recall that I mentioned that HD 85512 is smaller and cooler than the Sun, and it is also less luminous. This means that planets can get closer to HD 85512 without getting as hot as Mercury does. The average surface temperature of HD 85512b is estimated to be about 25 degrees Celsius or 77 degrees Fahrenheit , which is only slightly warmer than the Earth's average temperature.
HD 85512b was big news when it was first found, as finding Earth-like planets in the habitable zones around stars has been very difficult. It also went on to inspire many scientific funding proposals, although I don't think anyone has yet written any science fiction stories about the exoplanet.
 Pepe, F. et al., The HARPS search for Earth-like planets in the habitable zone. I. Very low-mass planets around
 Gaia Collaboration et al., Gaia Data Release 2. Summary of the contents and survey properties, 2018, Astronomy and Astrophysics, 616, A1
 Mayor, M. et al., Setting New Standards with HARPS, 2003, The Messenger, 114, 20