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HIP 9618 Exoplanet System

Yellow dwarf with two sub-Neptunes in a possible binary star system //

HIP 9618, also known as TOI-1471, is a real star hosting two known exoplanets 220.3 light-years away in the constellation Aries. It is a G5 yellow dwarf of 0.93 solar masses and 0.95 solar radii with a 5550 K (5277 °C) surface temperature. HIP 9618 is marginally older than the Sun at 5.2 billion years in age and has a slightly longer 29-day rotation period. Like the Sun, it displays a low level of stellar activity and can be considered a close twin, albeit slightly smaller and dimmer. The object may be the primary (i.e. more massive) component of a binary star system, but as will be discussed, the nature of its potential companion is not clear.

Two exoplanets were discovered around HIP 9618 in 2023, designated HIP 9618 b and c. They orbit the star every 20.8 and 52.6 days at an average distance of 0.144 and 0.268 AU—too close to be in the habitable zone. HIP 9618 b is likely the more prominent of the pair—its mass was first estimated to be 10 times greater than Earth before being revised down to 8.4 Earth masses. The parameters of HIP 9618 c are less well-defined, but it most likely weighs less than 7.9 Earths. They have a measured size of 3.75 and 3.34 Earth radii respectively, placing them in the territory of sub-Neptunes with significant gaseous envelopes. Assuming a rocky core, preliminary modelling suggests that a hydrogen/helium layer would comprise 9.2% of HIP 9618 b and 7.0% of HIP 9618 c by mass.

Alongside the planets, astronomers also detected a bigger long-term trend in HIP 9618’s motion. This was attributed to the gravitational pull from a massive outer companion in the system. This candidate is at least 5.0 AU away, but precise parameters are highly uncertain. Preliminary estimates loosely constrain a 0.08-solar-mass body on a 26.0 AU orbit—a little less than the distance between the Sun and Neptune. Such a mass would correspond to a high-end brown dwarf or very low-mass star just able to sustain hydrogen fusion. All databases currently list HIP 9618 as a solitary star but if the companion does exist, it would be given the designation HIP 9618 B, making for a binary system.

This simulation replicates the HIP 9618 system from real data wherever possible, though fictional elements may be incorporated due to knowledge gaps:

• I’ve imagined HIP 9618 b as a tidally-locked hot sub-Neptune with a deep blue hue—even richer than that of Neptune itself. The heat from its star energises its atmosphere, generating powerful vortices and mid-latitude storms. It has an average temperature of ~533 K (259 °C)
  
• I’ve imagined HIP 9618 c as a tidally-locked warm sub-Neptune with alternating bands of white and purple. Like its inner neighbour, this planet is also a turbulent engine of clouds. It has an average temperature of ~409 K (135 °C).
  
• The eccentricity of the planets' orbits is not well constrained and only upper limits are known at <0.13 for HIP 9618 b and <0.12 for HIP 9618 c. I have chosen to use these in the simulation, but the true eccentricity could be lower (i.e. the orbits would be closer to a perfect circle).
  
• The potential massive companion HIP 9618 B is included in this simulation. At 0.08 solar masses, Universe Sandbox produces a very small star. I assume it has a circular orbit at a distance of 26 AU, with the orbital plane closely aligned to the two planets.

LTT 10696, HD 12572, SAO 75115, WISEA J020337.14+211652.2, AG+21 189, HIC 9618, TIC 306263608, Wolf 111, ASCC 744451, HIP 9618, TOI-1471, YZ 21 617, BD+20 328, LSPM J0203+2116, TYC 1213-1071-1, Gaia DR2 94468978202180352, GC 2478, 2MASS J02033700+2116508, UCAC2 39254959, Gaia DR3 94468978202180352, GEN# +1.00012572, NLTT 6867, USNO-B1.0 1112-00024234, Gaia DR1 94468973906643712, GSC 01213-01071, PPM 91320, uvby98 100012572

• Akana Murphy, J.M., Batalha, N.M., Scarsdale, N., Isaacson, H., Ciardi, D.R., Gonzales, E.J., Giacalone, S., Twicken, J.D., Dattilo, A., Fetherolf, T. and Rubenzahl, R.A., 2023. The TESS-Keck Survey. XVI. Mass Measurements for 12 Planets in Eight Systems. arXiv e-prints, pp.arXiv-2306.
  
• Osborn, H.P., Nowak, G., Hébrard, G., Masseron, T., Lillo-Box, J., Pallé, E., Bekkelien, A., Florén, H.G., Guterman, P., Simon, A.E. and Adibekyan, V., 2023. Two warm Neptunes transiting HIP 9618 revealed by TESS and Cheops. Monthly Notices of the Royal Astronomical Society, 523(2), pp.3069-3089.