Key parameters at a glance

A radius of 1.87 Earth radii
A mass of 4.15 Earth masses
Surface gravity around 1.19 g
An orbital period of 4.646 days
Semi-major axis 0.0500 AU
Distance from Earth 79.36 light-years
Earth Similarity Index 0.810
Travel time at Voyager 1 speed 1,399,536 years
Almost certainly tidally locked to its host star given the close orbit

Context from the literature

HD 156668 b is an extrasolar planet orbiting the star HD 156668 78.5 light-years away in the constellation Hercules. It has a minimum mass of 3.1 Earth masses. At the time of discovery it was the second least massive planet discovered by the radial velocity method after Gliese 581 e, subject to the mass/inclination degeneracy that affects radial velocity measurements. In addition to this, it has the lowest semi-amplitude, or the speed of the stellar wobble caused by planet's gravity tugging on the star determined by radial velocity, at 2.2 m/s. This planet was discovered on January 6, 2010; it is the 8th planet discovered in 2010 after the first five planets detected by Kepler on January 4 and two planets around HD 9446 on January 5.

Excerpted from Wikipedia · full article

1 sibling around HD 156668

HD 156668 b shares its host star with 1 other confirmed planet. Side-by-side measurements below.

Planet	Type	Radius (R⊕)	Mass (M⊕)	Period (d)	Eq. T (K)	Year
HD 156668 b this	Super-Earth	1.87	4.15	4.646	—	2010
HD 156668 c	Neptune-like	6.17	31.50	811.300	—	2021

HD 156668 b Planet Profile

Physical Specs

Radius

1.87 R⊕

Radius (Jupiter)

0.167 R♃

Mass

4.15 M⊕

Mass (Jupiter)

0.013 M♃

Density

3.49 g/cm³

Surface Gravity

1.19 g (Earth)

Classification

Planet Type

Super-Earth

ESI Score 0.810

HZ Position Unknown

Controversial No

Discovery

Year 2010

Method Radial Velocity

Facility W. M. Keck Observatory

Telescope 10 m Keck I Telescope

Size rank in cohort

Rank by radius

#169of 1176

top 14.3%

This planet

1.87R⊕

Super-Earth median

1.60R⊕

Nearest-size peers

HD 23472 c 1.87 R⊕ Kepler-299 e 1.87 R⊕ EPIC 201238110 b 1.87 R⊕

Metric	Earth	HD 156668 b	Jupiter
Radius (R⊕)	1.00	1.87	11.21
Mass (M⊕)	1.00	4.15	317.83
Density (g/cm³)	5.51	3.49	1.33
Surface Gravity (g)	1.00	1.19	2.53
Insolation (S⊕)	1.00	—	0.037

Alternative Mass Estimates

Method	Value	Interpretation
Minimum mass (M sin i)	4.150 M⊕	Lower bound from radial-velocity fitting.

Star Catalogue Identifiers

HD 156668

HIP

HIP 84607

TIC

TIC 257715493

Gaia DR2

Gaia DR2 4575374857376398976

Gaia DR3

Gaia DR3 4575374857376398976

System

HD 156668

Percentile among Super-Earth cohort

Radius 1.870 R⊕ · percentile 85 / cohort 1176

Mass 4.150 M⊕ · percentile 78 / cohort 1176

Orbital period 4.65 d · percentile 38 / cohort 1164

Distance 24.33 pc · percentile 5 / cohort 1171

ESI 0.810 · percentile 90 / cohort 1176

Orbit & Habitability

Orbital Dynamics

Period

4.646 days

Semi-major axis

0.0500 AU

Eccentricity

0.000

Inclination

— °

Year Length

A year here lasts approximately 4.65 Earth days (1.3% of a terrestrial year) on a nearly circular orbit at a mean orbital distance of 0.0500 AU.

Extended Orbital Architecture

RV semi-amplitude (K)

1.890 m/s

Transit mid-time (BJD)

2,455,201.6000

Long. of periastron (ω)

36.00°

Angular separation (arcsec)

2.05000

Eq. Temperature

—

Insolation (S⊕)

—

Earth = 1.00

Earth Similarity (ESI)

0.810

0 = alien, 1 = Earth-identical

Insufficient insolation data. Close-orbiting tidally-locked around a cool host.

Discovery Paper

Primary reference

Howard et al. 2011

Instrument

HIRES Spectrometer

Publication

2011-01

Observation locale

Ground

Discovery cohort

Planets confirmed in 2010 at W. M. Keck Observatory (12 shown).

HD 1461 b · Sub-Neptune GJ 876 e · Sub-Neptune HAT-P-17 c · Gas Giant HD 97658 b · Sub-Neptune HD 163607 c · Gas Giant HD 177830 c · Neptune-like HD 218566 b · Neptune-like HD 102956 b · Gas Giant HD 163607 b · Gas Giant GJ 1148 b · Gas Giant HD 31253 b · Gas Giant HD 164509 b · Gas Giant

Host System: HD 156668

Spectral Class

K-type orange

Effective Temperature

4,850 K — cooler than the Sun (5,778 K)

Estimated Age

8.60 Gyr — older than the Sun (4.6 Gyr)

Stellar Radius

0.720 R☉

Stellar Mass

0.772 M☉

Metallicity [Fe/H]

0.05

Extended Stellar Properties

Surface gravity (log g)

4.598 dex

Stellar density

2.533 g/cm³

Systemic radial velocity

-44.57 km/s

Rotational v·sin i

0.50 km/s

Metallicity ratio

[Fe/H]

Distance

24.33 parsec

Light-years 79.36 ly

V-band magnitude

8.43 mag

Voyager-speed travel 1,399,536 yr

Multi-band Host Photometry — 11 bands

Astrometric Data

Parallax

41.069 mas

Total Proper Motion

228.612 mas/yr

PM Right Ascension

-72.63 mas/yr

PM Declination

216.77 mas/yr

Galactic Cartesian (pc)

x = -0.160 · y = -0.858 · z = 0.488

Equatorial (J2000)

RA 259.41835° · Dec 29.22816°

Galactic ℓ, b

51.969° · 32.115°

Ecliptic λ, β

254.869° · 52.113°

HTM-20 index

-1072161331

Observation Record

Photometric series

Stellar spectra

Archive notes

Wikipedia →

Similar Worlds

Kepler-1032 b

Super-Earth · K-type orange

Radius 1.87 R⊕ · 2,195.1 ly

HD 23472 c

Super-Earth · K-type orange

Radius 1.87 R⊕ · 127.3 ly

TOI-2322 c

Super-Earth · K-type orange

Radius 1.87 R⊕ · 195.2 ly

Kepler-1221 b

Super-Earth · K-type orange

Radius 1.88 R⊕ · 2,979.9 ly

Kepler-440 b

Super-Earth · K-type orange

Radius 1.86 R⊕ · 981.8 ly

Kepler-48 b

Super-Earth · K-type orange

Radius 1.88 R⊕ · 1,000.4 ly

Explore Related Categories

Super-Earth Planets Radial Velocity Discoveries W. M. Keck Observatory Mission K-type orange Hosts