Object 52: HD 154345
Podcast release date: 26 July 2021
Right ascension: 17:02:36.4
Corresponding Earth location: Prarie in Carter County, Montana, in the United States
In large part, HD 154345 is a relatively ordinary star in the constellation Hercules (and yes, that's the same Hercules that appears in Greek mythology fighting lions and hydras). It's slightly hotter than but otherwise very similar to the Sun , it's somewhere between 2 and 10 billion years old, and it's located at a distance of about 59.59 light years (or 18.27 pc) [2,3], which means that I get to use my new Star Wars themed sound effects. Although less than 20 pc seems relatively close, there are actually quite a few stars, including Sun-like stars, that are closer than HD 154345. Nonetheless, some astronomers were interested in this star specifically because it was a kind-of close Sun-like star. They were specifically interested in trying to detect exoplanets around the star.
Astronomers who search for exoplanets have a few different strategies for selecting stars for their search. Some astronomers like to look at the brightest stars in the sky because they are the easiest to observe. This is a strategy that has been used, for example, by the Transiting Exoplanet Survey Satellite and the Kilodegree Extremely Little Telescope (and yes, that's the name of a real telescope) [4,5,6]. A variant on this strategy would be to look at the closest stars, but the closest stars in the sky are not always the brightest, so they could be more challenging to observe. Some people like to monitor lots of stars but just in one part of the sky. This is the strategy used specifically by the Kepler spacecraft . Finally, other people like to look at Sun-like stars because they are hoping to find planets that are analogous to the planets in our Solar System and in particular are hoping to find Earth-like planets orbiting Sun-like stars.
Astronomers also use a few different techniques to find planets. Although directly imaging exoplanets is possible, it's also really hard, so astronomers don't normally try that. Instead, astronomers prefer to use two other techniques. One method is to look for the slight dimming of light from individual exoplanets passing in front of stars. The other method is to try to observe the slight redshifting and blueshifting of light caused by the gravitational pull of exoplanets in orbit around stars.
The Next 2000 (N2K) project, which consists of a group of astronomers who have been working with the Keck Observatory, the Magellan Telescope, and the Subaru Telescope, selected a set of close, Sun-like stars to observe for exoplanets by applying the redshifting and blueshifting technique . In terms of the objects that they selected to observe, they combine the "nearby stars" strategy with the "Sun-like stars" strategy. Their sample of stars included HD 154345, and eventually, a planet was found in orbit around that star. This would be kind of a boring podcast episode if they didn't find anything. However, can you guess what type of planet they found? I will give you a hint: it is like one of the planets in our Solar System.
If you guessed that the astronomers found an Earth-like planet orbiting HD 154345, then you are absolutely wrong. If you guessed that the astronomers found a Uranus-like planet orbiting HD 154345, well, that would be kind of an unusual guess, but it would still be wrong. If you guessed that the astronomers found a Jupiter-like planet orbiting HD 154345, then you are absolutely right.
The planet is named HD 154345b, and it is surprisingly similar to Jupiter. It has a mass around 0.95 times the mass of Jupiter (or, more accurately, the minimum possible mass of the planet has been calculated to be 0.95 times the mass of Jupiter) . Given this, it's very likely to be a gas giant made out of hydrogen and helium just like Jupiter. HD 154345b orbits its star at a distance of 4.2 astronomical units (AU) , where 1 AU is the distance from the Earth to the Sun, while Jupiter orbits the Sun at a distance of 5.2 AU. And HD 154345b orbits its star once every 9.2 years , whereas Jupiter goes around the Sun every 11.9 years. Also, it's worth reminding ourselves that both planets are orbiting similar yellowish stars, so the planets really are quite similar.
Because it takes HD 154345b so long to orbit its star, it had to be observed for 10 years so that astronomers could conclusively determine that the star was moving back and forth in such a way that would be consistent with a planet orbiting it. The N2K project started observing the star in 1997, but they didn't publish the paper identifying the planet until 2008 . Rather interestingly, the group got really impatient and actually published a paper in 2007 saying that they think that they found an exoplanet but they need a little more time to prove that it's there .
One of the important points about this discovery is that it is a Jupiter-like exoplanet orbiting a star at roughly the same distance that Jupiter orbits our Sun. While this may not sound quite so astonishing, one of the problems with early exoplanet observations was that astronomers were finding a lot of gas giants orbiting closer to their stars than Mercury orbits our Sun. I discussed one of these types of exoplanets in Episode 14. Planets that orbit close to stars are easy to identify because they go around the stars so quickly that astronomers can see them complete an orbit in just a few days, whereas planets that are further away can take a decade or more to identify. This meant that, when astronomers began finding exoplanets, they first found planetary systems with massive planets orbiting close to their host stars that looked nothing like our Solar System, which was really confusing. It almost seemed like it was normal for gas giant planets to be located really close to their host stars, and it also seemed like the Solar System was a rather peculiar planetary system with its gas giant planets unusually located far away from the Sun. However, the discovery of HD 154345b as well as the subsequent discoveries of similar gas giants that orbit at large radii have demonstrated that the Solar System is not actually that unusual after all.
 Wright, J. T. et al., The Jupiter Twin HD 154345b, 2008, Astrophysical Journal Letters, 683, L63
 Gaia Collaboration et al., The Gaia mission, 2016, Astronomy & Astrophysics, 595, A1
 Gaia Collaboration et al., Gaia Early Data Release 3: Summary of the contents and survey properties, 2020, arXiv e-prints, arXiv:2012.01533
 Ricker, George R. et al., Transiting Exoplanet Survey Satellite (TESS), 2015, Journal of Astronomical Telescopes, Instruments, and Systems, 1, 014003
 Pepper, Joshua et al., The Kilodegree Extremely Little Telescope (KELT): A Small Robotic Telescope for Large-Area Synoptic Surveys, 2007, Publications of the Astronomical Society of the Pacific, 119, 923
 Pepper, Joshua et al., The KELT-South Telescope, 2012, Publications of the Astronomical Society of the Pacific, 124, 230
 Borucki, William J. et al., Kepler Planet-Detection Mission: Introduction and First Results, 2010, Science, 327, 977
 Fischer, Debra A. et al., The N2K Consortium. I. A Hot Saturn Planet Orbiting HD 88133, 2005, Astrophysical Journal, 620, 481
 Wright, J. T. et al., Four New Exoplanets and Hints of Additional Substellar Companions to Exoplanet Host Stars, 2007, Astrophysical Journal, 657, 533