HD 176693#Planetary system
{{Short description|Star in the constellation Draco}}
{{Starbox begin}}
{{Starbox observe
| epoch = J2000
| constell = Draco
}}
{{Starbox character
| type = Main sequence
| variable =
}}
{{Starbox astrometry
| prop_mo_ra = 4.704{{±|0.013}}
| prop_mo_dec = 9.263{{±|0.016}}
| parallax = 11.2176
| p_error = 0.0126
| absmag_v =
}}
{{Starbox detail
| mass = 1.05{{±|0.04}}
| radius = 1.253{{±|0.051}}
| gravity = 4.318{{±|0.08|0.089}}
| temperature = 6,080{{±|65}}
| metal_fe = −0.138{{±|0.043|0.042}}
| rotation = 12.89{{±|0.19}} d
| rotational_velocity = 2.8{{±|1.0}}
}}
{{Starbox catalog
| names = {{odlist | BD=+48 2806 | GSC=03545-01227 | HD=176693 | Kepler=408 | KOI=1612 | 2MASS=J18590868+4825236 | TYC=3545-1227-1 | KIC=10963065}}, Gaia EDR3 2131593785132997632
}}
{{Starbox reference
| Simbad = HD+176693
}}
{{Starbox end}}
HD 176693, also known as Kepler-408, is a star with a close orbiting exoplanet in the northern constellation of Draco. It is located at a distance of 291 light years from the Sun based on parallax measurements, but it is drifting closer with a radial velocity of −55 km/s. The star is predicted to come as close as {{convert|7.0957|pc|ly|1|disp=out|abbr=off}} in 1.6 million years. It has an apparent visual magnitude of 8.83, which is too faint to be viewed with the naked eye.
The spectrum of HD 176693 matches an F-type main-sequence star with a stellar classification of F8V. The star is older than the Sun, at {{val|7.15|u=billion years}}. It is slightly and uniformly depleted in heavy elements compared to the Sun, having about 75% of the solar abundance of iron and other heavy elements. HD 176693 is a chromospherically inactive star, although there is weak evidence for tidal spin-up due to star-planet interaction.
HD 176693 is 5% more massive than the Sun and has a 25% larger radius. It is radiating 1.9 times the luminosity of the Sun from its photosphere at an effective temperature of 6,080 K. The star is spinning with a rotation period of 12.89 days. As of 2016, multiplicity surveys have not detect any stellar companions to HD 176693.
Planetary system
In 2014, a transiting Sub-Earth planet b was detected on a tight 2.5 day orbit. Initially reported with a relatively low confidence of 97.9%, it was confirmed in 2016.
The planetary orbit is inclined to the equatorial plane of the star by 41.7{{±|5.1|3.5}}°. Such strong spin-orbit misalignment is unique for a sub-Earth transiting planet, and needs either additional giant planets in the system or a history of close stellar encounters to explain it. The planet may also be a captured body originating from elsewhere.
{{OrbitboxPlanet begin
| name = Kepler-408
}}
{{OrbitboxPlanet
| exoplanet = b
| radius_earth = 0.86{{±|0.04}}
| semimajor =
| period = 2.465024{{±|0.000005}}
| eccentricity =
| inclination = 81.85{{±|0.10}}
}}
{{Orbitbox end}}
References
{{Reflist|refs=
| title=Kepler-408 | mode=cs2 | access-date=1 July 2021 }}
{{Cite Gaia DR3|2131593785132997632}}
| title=Gaia DR3 Part 1. Main source
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| title=Masses, Radii, and Orbits of Small Kepler Planets: The Transition from Gaseous to Rocky Planets
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| title= Atmospheric parameters of 169 F-, G-, K- and M-type stars in the Kepler field
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| bibcode= 2013MNRAS.434.1422M | s2cid=59269553
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| arxiv=1306.6011 | display-authors=1 }}
| title=Spin–Orbit Alignment of Exoplanet Systems: Ensemble Analysis Using Asteroseismology
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| title=The Misaligned Orbit of the Earth-sized Planet Kepler-408b
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| title=A Disk-driven Resonance as the Origin of High Inclinations of Close-in Planets
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| doi=10.3847/2041-8213/abb952 | doi-access=free }}
| title=New stellar encounters discovered in the second Gaia data release
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| display-authors=1 | journal=Astronomy & Astrophysics
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| arxiv=1805.07581 | s2cid=56269929 }}
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{{Cite constellation|Kepler-408}}
}}
{{Draco (constellation)}}
{{Sky|18|59|08.6847|+|48|25|23.5990}}
{{Portal bar|Astronomy|Stars|Outer space}}
{{DEFAULTSORT:HD 176693}}
Category:F-type main-sequence stars
Category:Planetary systems with one confirmed planet
Category:Planetary transit variables