An artist's impression about what Diomedes may look like with rings invisible
Date July 15, 2006
Discoverers Butler et al.
Detection method Radial velocity
Site Keck Observatory
Name & designations
Pronunciation /'dī•(y)ō•mē•dēs/
Adjective Diomedian
Planet numbers P182, HD 164922 P1,
Hercules P6,Tarandus P18,
2006 P17, 2006 Her-3,
2006 Tar-5
Star designations PH 144 b, P5 Herculis b,
P16 Tarandi b, HD 164922 b,
HIP 88348 b, Gliese 700.2 b,
GJ 9613 b, SAO 85678 b
System HD 164922
Constellation Hercules
Caelregio Tarandus
Right ascension 18h 02m 30.86s (270.628 59°)
Declination +26° 18' 46.8" (+26.313 00°)
Distance 22.120 pc (72.145 ly)
Orbital characteristics
Semimajor axis 2.109 362 AU (315.556 0 Gm)
Periastron 1.998 878 AU (299.027 9 Gm)
Apastron 2.219 845 AU (332.084 1 Gm)
Eccentricity 0.052 377 8
Orbital circumference 13.246 86 AU (1 981.703 Gm)
Orbital area 13.959 03 AU² (312 396.5 Gm²)
Orbital period 1154.734 584 d (3.161 490 99 yr)
Avg. velocity 19.930 km/s (4.190 AU/yr)
Max. velocity 20.446 km/s (4.298 AU/yr)
Min. velocity 19.401 km/s (4.079 AU/yr)
Direction of orbit
relative to star's rotation
Inclination 76.288° to line of sight
0.854° to star's equator
0.577° to invariable plane
Argument of periastron 194.996°
Longitude of ascending node 85.699°
Longitude of periastron 280.694
Angular separation 95.365 mas
Observing the parent star
Mean angular star size 0.228 71° (13.722')
Max. angular star size 0.241 35° (14.481')
Min. angular star size 0.217 32° (13.039')
Mean star magnitude −24.663
Max. star magnitude −24.780
Min. star magnitude −24.553
Bulk characteristics
Mean radius 0.770 17 RJ (53.868 Mm)
Equatorial radius 0.769 06 EJ (54.953 Mm)
Polar radius 0.772 53 PJ (51.696 Mm)
Mean circumference 338.461 Mm
Equatorial circumference 345.283 Mm
Polar circumference 324.817 Mm
Surface area 0.593 24 SJ (36 434 Mm²)
Volume 0.456 92 VJ (653 940 Mm³)
Flattening 0.060 47 (1:16.54)
Angular diameter 32.459 μas
Mass 0.370 920 MJ
Reciprocal mass
relative to star
2 652.0
Density 1.077 g/cm³
Gravitational influence
Surface gravity 1.650 g (16.19 m/s²)
Weight on Diomedes
(150 lb on Earth)
248 lb
Standard gravitational parameter 4.699 × 107 km³/s²
Escape velocity 41.77 km/s
Hill radius 40.528 LD (15.578 8 Gm)
Roche limit
(3 g/cm3 satellite)
0.125 48 LD (48.236 Mm)
Stationary orbit 0.233 72 LD (89.843 Mm)
Stationary velocity 18.044 km/s (4.056 LD/d)
Rotation characteristics
Rotation period 14.005 83 h (0.583 576 d)
Rotation velocity 6.848 km/s (25.70°/h)
Direction of rotation
relative to orbit
Axial tilt 3.772°
Longitude of vernal equinox 300.216°
North pole right ascension 18h 01m 30s (270.375°)
North pole declination −57° 33' 39" (−57.561°)
North polar constellation Ara
North polar caelregio Simianus
South pole right ascension 06h 01m 30s (90.375°)
South pole declination +57° 33' 39" (+57.561°)
South polar constellation Camelopardalis
South polar caelregio Avis
Thermal characteristics
Surface temperature 196 K (−78°C, −108°F, 352°R)
Mean irradiance 188 W/m² (0.138 I)
Irradiance at periastron 210 W/m² (0.153 I)
Irradiance at apastron 170 W/m² (0.124 I)
Albedo 0.477 (bond), 0.575 (geom.)
Scale height 68.76 km
Surface density 0.174 g/m³
Molar mass 2.49 g/mol
Composition 91.204% hydrogen (H2)
6.543% helium (He)
0.918% water (H2O)
0.687% ammonia (NH3)
0.605% methane (CH4)
324 ppm neon (Ne)
68.6 ppm hydrogen deuteride (HD)
39.7 ppm hydrogen cyanide (HCN)
177 ppb ethane (C2H6)
1.23 ppb propane (C3H8)
Ammonia (NH3)
Water (H2O)
Ammonium hydrosulfide (NH4SH)
Dipole strength 36.2 μT (0.362 G)
Magnetic moment 7.10 × 1018 T•m³
Dipole tilt 8.40°
Satellite system
Number of moons 86
Number of rings 983

Diomedes (HD 164922 b, P183) is an exoplanet which orbits the yellow-orange K-type main sequence star HD 164922, meaning the star is smaller, cooler and thus dimmer than our Sun. It is approximately 72 light-years or 22 parsecs away towards the constellation Hercules in the caelregio Tarandus.

Diomedes is similar to Saturn in terms of mass, size, interior structure, and orbital eccentricity, but it orbits much closer to its sun, only 22% the distance between Saturn and the Sun, but still it orbits over twice the distance of Earth to the Sun. But like Saturn, this planet boasts a ring system and dozens of moons. Diomedes is a sub-Jupiter with a mass of 38 that of Jupiter, and the planet is surrounded by bands of ammonium hydrosulfide and water clouds.

Diomedes is named after the hero in Greek mythology who got involved in the Trojan War.

Discovery and chronology Edit

Diomedes was detected on July 15, 2006 by a team of astronomers led by Paul Butler who used the radial velocity method to carefully watch the star wobble caused by the planet's gravity. On that same day, four other planets were also announced.

Diomedes is the 174th extrasolar planet discovered overall, 148th since 2000, and 17th in 2006. However it is the 182nd planet in the list, because the first eight belong to the planets in our solar system. It is also the 6th exoplanet discovered in the constellation Hercules (3rd in 2006) and 18th in the caelregio Tarandus (5th in 2006). Diomedes is the first and only planet discovered in the HD 164922 system, hence the designations HD 164922 b (a is not used because the parent star uses this letter to reduce confusion) and HD 164922 P1. Note that the chronology does not include speculative brown dwarfs (objects with minimum masses below 13 MJ but with speculative true masses above 13 MJ).

Orbit and rotation Edit

Orbit Edit

Diomedes is found in the martian orbit (in this sense, it is an orbit class with semimajor axis ranging from 187–374 Gm (10.4–20.8 lmin)) around the star HD 164922 at a semimajor axis of 316 Gm (17.5 lmin). The orbital distance of this planet ranges from 299 Gm (16.6 lmin) to 332 Gm (18.5 lmin). This corresponds to an orbital eccentricity of 0.0524, which is considerably more circular than most of the detected planets with semimajor axes over 300 Gm. Like the planets in our solar system, Diomedes orbit in the same direction as the rotation of the star, so-called prograde orbit. This planet takes 100 megaseconds or 3.16 years to revolve around HD 164922 with an average velocity of nearly 20 km/s or over 12 mi/s. The inclination of the orbit, which is important for determining its true mass, is unknown. However its inclination is speculated to be 76.3° to line of sight, which is almost edge-on to the observers. Using the sine of its inclination, the true mass of this planet is only 2.9% greater than the minimum mass determined by radial velocity method. The argument of periastron is 195° while the longitude of ascending node is 86°.

Parent star observation and irradiance Edit

As viewed from the orbit of Diomedes at 2.11 AU from the parent star, the apparent magnitude of that star is −24.66, which is seven times fainter than the Sun as seen from Earth, due to its greater distance from its star and being less luminous. The angular diameter of a star as viewed from Diomedes is about 45% the angular diameter of the full moon and sun as seen from Earth.

Diomedes receives an irradiance of 2.11−2 times that of Earth, calculating to be 0.225 I, but the actual irradiance is 0.138 I. Dividing between 0.138 and 0.225 is 0.613, that quotient is the star's luminosity compared to our Sun.

Rotation Edit

This planet tilts 3.77° to the plane of its orbit, meaning that Diomedes does not have a considerable seasonal cycle like we have on Earth. This planet takes 14 hours to rotate once on its axis, which is longer than Jupiter and Saturn (10 hours) but not as long as Uranus and Neptune (16–17 hours). The tilt of this planet is such that its north pole points to the Earth's southern constellation Ara (in Simianus), while the south pole points to the Earth's northern constellation Camelopardalis (in Avis).

Structure and composition Edit

Mass and size Edit

Diomedes is 24% more massive than Saturn, but its radius is 7% smaller than Saturn, making the planet 54% denser than Saturn and 7.7% denser than water. This planet has no solid surface, like Jupiter and Saturn in our solar system, so it is impossible for spaceship to land on this planet.

However, Diomedes is not a perfect sphere as the planet is 6.05% wider at the equator than between the poles. The equatorial radius is 54,953 kilometers compared to 51,696 kilometers for the polar radius.

Gravitational influence Edit

The planet's gravity is 1.65 g and falls at 16.2 m/s². So the minimum speed needed to escape the planet's gravitational pull (so-called escape velocity) is 41.77 km/s, 3¾ times higher than the speed required to escape from Earth. If you weigh 150 pounds on Earth, then you'll weigh 248 pounds on Diomedes. So a person standing on Diomedes would, on average, weigh as much as the average weight of a professional football player on Earth!

The maximum distance from the planet where a 3 g/cm³ moon tear apart by tidal forces, called its roche limit, is 0.125 lunar distances (48.2 megameters), which is 0.90 planetary radii. The orbital distance where the gravitational influence of the planet is identical to the star, called its hill radius, is 40.5 lunar distances (15.6 gigameters), which is 289 times the radius of the planet. The orbit where its satellite orbital period is identical to the rotation period of the planet, called its stationary orbit, is 0.234 LD (89.8 Mm), which is 1.63 planetary radii or 14 times more distant from the planet than its roche limit. The orbital velocity at stationary orbit, called its stationary velocity, is 18.0 km/s or 11.2 mi/s. Since the planet takes 14 hours to rotate, then a moon would take 14 hours to orbit the planet at stationary orbit.

Interior Edit

Its interior is similar to Saturn because of its similar mass and size to Saturn. As we get deeper into the planet, the temperature and pressure increase. Underneath its thick outer envelope, Diomedes has mantles of liquid hydrogen and helium, despite the temperature is really hot. On Earth, liquid hydrogen can only be produced when we cool the hydrogen gas to below 20 K (−253°C or −423°F) and liquid helium can only be produced when we cool the helium gas to below 4 K (−269°C or −452°F). But inside Diomedes, the reason why there is liquid hydrogen and helium because the pressure is so great that it forces hydrogen molecules and helium atoms closer together despite their increasing temperature. Deeper below, liquid hydrogen can conduct electricity, known as metallic hydrogen. Below that layer lies a rocky core with a mass of 29 Earth masses (roughly 25% the total mass of its planet) and composes primarly of iron and silicates. The temperature of its core is about 12,100°C or 21,900°F.

Atmosphere Edit

This planet has a "surface" temperature of −78°C or −108°F. This temperature allows for the banded formation of ammonium hydrosulfide and water clouds. With that chemical makeup of the clouds, rays from the star reflects about 48% of its radiation striking the planet back into space.

Like all gas giant planets, the atmosphere of Diomedes composes mainly of hydrogen and helium. It also has some amounts of water, ammonia, and methane, as well as trace amounts of water ice crystals, neon, hydrogen deuteride, ethane, and propane. These gases are unique that these gases are not found on giant planets in our solar system. Hydrogen cyanide (HCN) forms when ammonia (NH3) reacts with methane (CH4) with the aid of ultraviolet radiation from the star. Some of this gas converts back into ammonia and methane.

NH3 + CH4 + UV rays ↔ HCN + 3H2

Also unlike the giant planets in our solar system, there is propane in the atmosphere instead of ethane.

Storm features Edit

Diomedes' storm systems are similar to Saturn, but winds on this planet are faster than Saturn since the planet orbits lot closer to HD 164922 than Saturn is to the Sun. There is a "Great White Spot" on Diomedes, like the Great White Spot on Saturn. At the poles, a hexagonal hurricane-like storm can be found like there is Saturn discovered by the Voyager 1 spacecraft. However unlike Saturn, all storms on Diomedes are powered by the star as the planet receives four times more heat from the star than Diomedes radiating heat from the interior.

Magnetic field Edit

Like most planets in our solar system, Diomedes has a magnetic field. The strength of magnetic field is about 17% stronger than Earth's, over 70% stronger than Saturn's, and one-twelve of Jupiter's.

This magnetic field is produced by the movements of metallic hydrogen in its interior caused by the planet's rotation. This mechanism is well known as dynamo effect. The magnetic field blocks most of stellar and cosmic radiation from reaching the planet, but occasionally it can produce auroras when the stellar radiation got caught in the magnetic field lines and move towards their poles where it interact with the planet's upper atmosphere (ionosphere).

Moons and rings Edit

Diomedes has 86 moons with at least 1 km across, but objects smaller than 1 km that orbits the planet are called moonlets. The largest moon has about the size of Titan, the largest moon of Saturn. This moon has lakes of ammonia, analogous to lakes of methane on Titan. Other moons are cratered, barren, or covered in ice with subsurface oceans.

Diomedes has a ring system about 83% as wide as Saturn's, but dark. The ring particles are grains of dust and bits of rocks and boulders but little ice. This ring system formed when an asteroid impacted the tiny moon close to its planet or flunged too close that tidal forces of the planet broke the moon apart into bits of rocks and boulders along with grains of dust. The ring system has divisions similar to Saturn's Cassini division. The ring particles orbit within the roche limit of Diomedes.

Future studies Edit

Even though Diomedes has an inclination of 76.3°, this planet is not edge-on enough to transit its star. If Diomedes does transit its star, then its inclination, radius, and other parameters can be constrained as well as studying its atmosphere and looking for moons and rings. If Diomedes does not transit its star, then Diomedes can be studied by direct imaging instead. Finding planets with direct imaging orbiting as close as 2 AU from the star is difficult because the glare of its star prevents astronomers from seeing close-orbiting planets. But in 2010, there is a new direct imaging instrument, called vortex coronagraph, that can detect and characterize planets as close to their stars as 1 AU.