Diomedes

Diomedes (HD 164922 b, P183) is an which orbits the yellow-orange    HD 164922, meaning the star is smaller, cooler and thus dimmer than our. It is approximately 72 s or 22 s away towards the  in the caelregio Tarandus.

Diomedes is similar to 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 and dozens of. Diomedes is a sub-Jupiter with a mass of that of Jupiter, and the planet is surrounded by bands of ammonium hydrosulfide and water clouds.

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Discovery and chronology
Diomedes was detected on July 15, 2006 by a team of astronomers led by who used the  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 174$J$ extrasolar planet discovered overall, 148$J$ since 2000, and 17$J$ in 2006. However it is the 182$J$ planet in the list, because the first eight belong to the planets in our solar system. It is also the 6$J$ exoplanet discovered in the constellation Hercules (3$J$ in 2006) and 18$⊕$ in the caelregio Tarandus (5$⊕$ 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 s (objects with minimum masses below 13 M$⊕$ but with speculative true masses above 13 M$2$).

Orbit
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 of 316  (17.5 ). The orbital distance of this planet ranges from 299 Gm (16.6 lmin) to 332 Gm (18.5 lmin). This corresponds to an 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. This planet takes 100 s or 3.16 s to revolve around HD 164922 with an of nearly 20 km/s or over 12 mi/s. The of the orbit, which is important for determining its true mass, is unknown. However its inclination is speculated to be 76.3° to, which is almost edge-on to the observers. Using the of its inclination, the  of this planet is only 2.9% greater than the  determined by radial velocity method. The is 195° while the  is 86°.

Parent star observation and irradiance
As viewed from the orbit of Diomedes at 2.11 AU from the parent star, the 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 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 of 2.11$2$ times that of Earth, calculating to be 0.225 I$3$, but the actual irradiance is 0.138 I$4$. Dividing between 0.138 and 0.225 is 0.613, that quotient is the star's luminosity compared to our Sun.

Rotation
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. This planet takes 14 hours to rotate once on its axis, which is longer than and Saturn (10 hours) but not as long as  and  (16–17 hours). The tilt of this planet is such that its points to the Earth's southern constellation  (in Simianus), while the  points to the Earth's northern constellation  (in Avis).

Mass and size
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, 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
The planet's gravity is 1.65 and falls at 16.2 m/s². So the minimum speed needed to escape the planet's gravitational pull (so-called ) is 41.77 km/s, 3¾ times higher than the speed required to escape from Earth. If you weigh 150 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, is 0.125 s (48.2 s), which is 0.90 planetary radii. The orbital distance where the gravitational influence of the planet is identical to the star, called its, 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, 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, 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
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 and, 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. 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
This planet has a "surface" temperature of −78°C or −108°F. This temperature allows for the banded formation of 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 planets, the atmosphere of Diomedes composes mainly of  and. It also has some amounts of, , and , as well as trace amounts of water ice crystals, , , , and. These gases are unique that these gases are not found on giant planets in our solar system. Hydrogen cyanide (HCN) forms when ammonia (NH$2$) reacts with methane (CH$6$) with the aid of from the star. Some of this gas converts back into ammonia and methane.


 * NH$3$ + CH$8$ + UV rays ↔ HCN + 3H$3$

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

Storm features
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 on Saturn. At the poles, a hexagonal -like storm can be found like there is Saturn discovered by the 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
Like most planets in our solar system, Diomedes has a. The strength of magnetic field is about 17% stronger than, over 70% stronger than , and one-twelve of.

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. The magnetic field blocks most of stellar and from reaching the planet, but occasionally it can produce s when the stellar radiation got caught in the magnetic field lines and move towards their  where it interact with the planet's upper atmosphere.

Moons and rings
Diomedes has 86 moons with at least 1 km across, but objects smaller than 1 km that orbits the planet are called s. The largest moon has about the size of, 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, 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. The ring particles orbit within the roche limit of Diomedes.

Future studies
Even though Diomedes has an inclination of 76.3°, this planet is not edge-on enough to transit its star. If Diomedes does 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 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, that can detect and characterize planets as close to their stars as 1 AU.