Object 86: HD 260655
Podcast release date: 14 November 2022
Right ascension: 06:37:10.8
Corresponding Earth location: Mae Wa National Park in Thailand
HD 260655 is a red dwarf amout 0.44 times the mass of the Sun located in the constellation Gemini. The distance to the star is 32.609 light years (9.998 pc) [1, 2]. The level of precision in this measurement is 0.02% [1, 2], which is equivalent to being able to measure the height of a typical adult to about 0.35 mm (0.014 inches). While the distance measurement is mind-bogglingly precise, it does not place HD 260655 within the list of the closest 100 stars to Earth, but it is close enough that I can use my Star Wars sound effects. In and of itself, HD 260655 isn't a very remarkable red dwarf. However, earlier in the year 2022, a science paper written by a collaboration led by Rafael Luque was published announcing the discovery of two exoplanets in orbit around this star , which is certain to pique everyone's interest.
As a quick review, exoplanets are very difficult to directly image, so they are generally discovered using one of two techniques. The first technique is to look at how the exoplanets exert gravitational forces on the stars that they are orbiting. These gravitational forces will cause the stars to oscillate slightly, so when astronomers see that the light from an individual star is slightly blueshifted and then redshifted in a periodic way, they can infer that an exoplanet is causing these oscillations. The second technique is to look for the slight dimming that occurs when an exoplanet passes (or transits) between us and the star that the exoplanet is orbiting. This relies on the exoplanet's orbit being aligned in a very specific way, but it is a very effective way for spotting exoplanets.
The two planets in orbit around HD 260655 were discovered using this second method by a spacecraft called the Transiting Exoplanet Survey Satellite . This spacecraft was launched in 2018, and it was designed to make observations every 30 minutes of chunks of the sky equivalent to about 1/18 of the entire celestial sphere . This allows the spacecraft to monitor lots and lots of stars, looking for any stars that may dim in brightness periodically from planets passing in front of them. It was during such observations in late 2021 that people identified HD 260655 as having two exoplanets orbiting it . The one closer to the red dwarf has been designated HD 260655b, while the one further away has been designated HD 260655c.
Both planets orbit the red dwarf really rapidly and really close to the star. I'm going to use Astronomical Units (AU) to describe these orbits, where 1 AU is defined as the distance from the Earth to the Sun. The closer planet orbits once every 2.77 days at a distance from the red dwarf of 0.029 AU, while the outer one orbits every 5.71 days at a distance of 0.047 AU . For reference, Mercury orbits the Sun one every 88 days at a distance of 0.39 AU , so in comparison, the exoplanets HD 260655b and HD 260655c are really close to their host star and moving really rapidly.
This might leave the impression that these exoplanets should be extremely hot, but remember that the exoplanets are orbiting a relatively faint red dwarf and not a brighter star like our Sun. This means that the exoplanets are not ultra hot. While they are still hotter than Mercury, they are actually cooler than Venus [3, 5]. The temperature on HD 260655b is estimated to be 436 C, while the temperature on HD 260655c is estimated to be 284 C . These temperatures are high enough to melt a couple of metals like tin and, on the inner planet, lead, but they aren't high enough to melt most other metals or to melt silicates (i.e., sand) [6, 7]. In other words, the surfaces of the exoplanets should look like solid hellscapes and not like molten hellscapes.
Careful modelling of the observational data of these exoplanets have shown that both are rocky planets like the planets in our inner Solar System. Both are also larger than the Earth as well, with the inner one being slightly more than twice the mass of the Earth and the outer one being slightly more than three times the mass of the Earth . Both have diameters slightly larger than the Earth's as well . Interestingly, HD 260655c appears to be less dense than expected for its size, falling below the theoretical mathematical functions relating planets' masses and densities . It's actually less dense than Earth. This may indicate that HD 260655c is made mostly of silicate and is largely devoid of iron, which would be a little weird. HD 260655b, meanwhile, seems to have a density slightly higher than Earth's, which matches what is predicted theoretically and which indicates that the exoplanet contains a mixture of iron and silicates .
The discovery of the two exoplanets orbiting HD 260655 quickly made it a potential target to be observed with the James Webb Space Telescope (JWST) . This isn't just because this is a recent exoplanet discovery, as people have been finding exoplanets for a couple of decades at this point, and it isn't just because this is a discovery of rocky exoplanets orbiting a red dwarf, as it has actually become very common to find rocky exoplanets orbiting red dwarfs [8, 9, 10]. HD 260655 is a particularly interesting target for JWST observations for two other reasons.
First, theoretical calculations have indicated that this star's exoplanets are both very likely to have atmospheres . At first, this seems like it would be a little unlikely given how hot the two exoplanets are estimated to be. I initially would have expected the atmospheres of these exoplanets to be heated up so much that they basically blow away from these planets' surfaces. However, these exoplanets are not as hot as Venus, and Venus has the densest atmosphere of the rocky planets within our inner Solar System . Given that HD 260655b and HD 260655c are more massive than Venus, I would expect them to be better than Venus at retaining hot atmospheres.
The second reason why HD 260655 is a potential target for JWST observations is that it is fourth closest star known at this time with multiple planets that have been detected using the transiting technique (i.e., by watching the star dim slightly when the exoplanets pass in front of it) . This means that astronomers could use observations of the spectrum of the starlight passing through the exoplanets' atmospheres to determine their chemical compositions. This would not be possible for exoplanets detected using other techniques, and this is why people are really interested in aiming the JWST at HD 260655 specifically.
I recorded this episode in November 2022. If the random number generator had given me the coordinates for HD 260655 a year earlier, I would not have created an episode about the star system, or if I did, it would be a very boring episode stating that HD 260655 was a rather ordinary-looking red dwarf and that's it. Recording this in November 2022, I can say something very interesting about the nature of the star's exoplanets. However, I worry that, in a year or two from recording this episode, someone's going to publish something about discovering these exoplanets' atmospheres and making interesting measurements of the atmospheres' compositions and characteristics, which is going to make this episode seem a little out-of-date when people listen to it in the future. Still, I thought that this was a good story to share in this podcast now regardless of what happens in the future.
 Gaia Collaboration et al., The Gaia mission, 2016, Astronomy & Astrophysics, 595, A1
 Gaia Collaboration et al., Gaia Data Release 3: Summary of the content and survey properties, 2022, arXiv e-prints, arXiv:2208.00211
 Luque, R. et al., The HD 260655 system: Two rocky worlds transiting a bright M dwarf at 10 pc, 2022, Astronomy & Astrophysics, 664, A199
 Ricker, George R. et al., Transiting Exoplanet Survey Satellite (TESS), 2015, Journal of Astronomical Telescopes, Instruments, and Systems, 1, 014003
 Howard, Andrew W. et al., Planet Occurrence within 0.25 AU of Solar-type Stars from Kepler, 2012, Astrophysical Journal Supplement Series, 201, 15
 Mulders, Gijs D. et al., A Stellar-mass-dependent Drop in Planet Occurrence Rates, 2015, Astrophysical Journal, 798, 112
 Sabotta, S. et al., The CARMENES search for exoplanets around M dwarfs. Planet occurrence rates from a subsample of 71 stars, 2021, Astronomy & Astrophysics, 653, A114