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HD 184010 Exoplanet System

Trio of gas giants orbiting an ageing orange star //

HD 184010 is a real exoplanet system 200.2 light-years away in the constellation Vulpecula. It consists of an evolved star and three Saturn-mass gas giants. The stellar host HD 184010 is younger than the Sun at 2.76 billion years old but 1.35 times as massive. Its appetite has already consumed all of the hydrogen fuel in its core, causing it to balloon to 4.86 solar radii. It possesses a cooler temperature of 4971 K (4698 °C) but is also 13 times more luminous due to the greater surface area. The star’s spectral type K0III–IV indicates that it is an orange-hued subgiant at the cusp of the red giant branch and nearing the end of its life.

Three gas giants with intermediate-length orbits were discovered around HD 184010 in 2022. From innermost to outermost, they are HD 184010 b, c and d. The trio weigh a minimum of 0.31, 0.30 and 0.45 Jupiter masses respectively. They orbit their host every 286.6, 484.3 and 836.4 days with average distances of 0.940, 1.334 and 1.920 AU. These are fairly close separations for such big gas giants, so their orbits must be near-circular for the system to remain stable. Indeed, simulations have shown that the planets must have eccentricities below 0.1, else one or more will be ejected within 1 billion years. Further, their true masses cannot be much greater than the minimum values as previously described to avoid any destabilising interactions.

The architecture of the HD 184010 system is somewhat unusual. Planets tend to occur around evolved giant stars in pairs rather than threes and the majority of these display shorter orbits or orbits that are spaced further apart. Neither resemble the layout seen in HD 184010. Another curiosity is that systems with multiple giant planets seem to be preferentially found around these ageing stars more massive than the Sun. This may be down to observational bias as detection tends to become easier for stars of this type, or because they have more leftover material to allow for bigger companions. We may need to revisit formation/migration theories, which could be challenged by these systems.

In this simulation, features of the HD 184010 system are replicated from existing data wherever possible or inferred in cases of knowledge gaps:

• The sizes of HD 184010 b, c and d are not yet known but are given as 1.08, 1.06 and 1.28 Jupiter radii in this simulation. This is based on knowledge that close-in gas giants can be “puffed up” by the heat of their star, causing them to inflate to a larger size. Consequently, the planets have low densities below half that of Saturn.
  
• The objects likely have similar compositions with hydrogen and helium making up the majority of their mass. In consideration of aesthetics, I have designed each with a unique colour palette.
  
• HD 184010 b is a hot Jupiter with average temperatures at a toasty 502 K (229 °C). It features a prominent sky-blue band in its southern hemisphere amid a backdrop of white. A day on this planet lasts 14.0 hours and it is slightly tilted by 3.57° to the orbital plane.
  
• HD 184010 c is a warm Jupiter with an average temperature of 425 K (152 °C). It features swirling yellow-green clouds and intense polar vortices that expose a purplish atmosphere below. The planet has a 13.7-hour rotation period and obliquity of 9.85°.
  
• HD 184010 d, the largest and outermost of the triplet, is also a warm Jupiter at a slightly more manageable 335 K (62 °C). It is dominated by an orange equatorial belt that churns around the planet every 15.9 hours. The rest of the atmosphere is a largely featureless mass of grey and white, but no less turbulent than its neighbours.

HD 184010, GSC 02133-02965, PPM 109158, UBV M 23858, AG+26 2058, HIC 96016, SAO 87314, WEB 16821, BD+26 3573, HIP 96016, SKY# 36381, YZ 26 10062, GC 26968, HR 7421, TIC 113329742, Gaia DR2 2024845946447691264, GCRV 11945, IRAS 19293+2630, TYC 2133-2965-1, Gaia DR3 2024845946447691264, GEN# +1.00184010, 2MASS J19312163+2637018, UBV 16568

• Teng, H.Y., Sato, B.E., Takarada, T., Omiya, M., Harakawa, H., Nagasawa, M., Hasegawa, R., Izumiura, H., Kambe, E., Yoshida, M. and Itoh, Y., 2022. A trio of giant planets orbiting evolved star HD 184010. Publications of the Astronomical Society of Japan, 74(6), pp.1309-1328.