Alpheus

Alpheus (Gliese 651 b, P170) is a which orbits the yellow    Gliese 651 (usually referred as ). The star is slightly smaller, cooler and thus dimmer than our. It is approximately 61 s or 19 s from towards the   in the caelregio Tarandus.

Alpheus orbits at a distance of 4.2 AU (1 AU closer to the star than 's distance from our Sun) in an oval path (e=0.26). Alpheus is very slightly less massive and slightly bigger than Jupiter.

is named after the in.

Discovery and chronology
Alpheus was discovered on March 12, 2006 by a team of astronomers led by. The team used the mounted on the telescope in  and found that this star wobble caused by an orbiting planet. However, Alpheus wasn't confirmed until May 27, 2007. The planet was discovered to have 2.03 M$J$ and orbited at a distance of 9.21 AU (but it has since been revised to 0.963 M$J$ and 4.19 AU respectively).

Alpheus is the 163$J$ exoplanet discovered overall, 137$J$ exoplanet discovered since 2000, and 6$J$ exoplanet discovered in 2006. Alpheus is also the 4$J$ exoplanet discovered in the constellation Hercules (1$⊕$ in 2006) and 15$⊕$ in the caelregio Tarandus (2$⊕$ in 2006). Alpheus is the first and only planet discovered in the Gliese 651 system, receiving the designations Gliese 651 b (a is not used because the parent star uses this letter to reduce confusion) and Gliese 651 P1. Note that the chronology does not include speculative s (objects with minimum masses below 13 M$2$ but with speculative true masses above 13 M$3$).

Orbit
Alpheus takes nearly a decade to orbit around the star at an of 652 s (4.36 ). Light from its parent star would take more than four times longer to reach Alpheus than light from our Sun to reach Earth at more than 36 minutes. Unlike the gas giants in our solar system, Alpheus orbits in an oval path with an of 0.26. However, previous estimate put an eccentricity of 0.04. The planet moves at an of 13.3 km/s (2.79 AU/yr). The plane of its orbit is edge-on with an of 87.5°.

This planet is in the prototype orbit from which this orbit class originated from, called Alphian orbit (A orbit), which ranges from 2.5 to 5 AU. But due to its eccentricity, Alpheus would spend some time outside of the Alphian orbit range, in this case farther out to 5.5 AU in the Jovian orbit (J orbit) territory.

Parent star observation and irradiance
Viewed from Alpheus, the parent star would appear to be just 3% as bright as the Sun seen from Earth or 32 times fainter. The parent star has a −22.98 compared to −26.74 for the Sun's magnitude viewed from Earth. Viewed from Alpheus, the parent star would have an of 6.', which is  the angular diameter of the  we see every month.

Alpheus receives 3.5% of the Earth's from the sun because it orbits over 4 times farther away from the star whose luminosity is  that of our Sun.

Rotation
Alpheus takes 18 hours to rotate once on its axis, which is three quarters of an Earth day. A year on Alpheus is 4454 days compared to 366 days on Earth. The planet tilts 23.1° to the plane of its orbit, very similar to the 23.4° tilt of Earth. The planet's points to the constellation  (in Tarandus), while the  points to the constellation  (in Malus).

Mass and size
Alpheus' mass is almost identical to that of Jupiter, the most massive planet in our solar system. It is classified as mid-Jupiter in the planetary mass classification scheme. It is slightly larger than Jupiter, meaning that Alpheus is a bit less dense and weaker gravity than Jupiter. These correspond that Alpheus' is slightly lower than Jupiter's. It is a  with no solid surface with mean density slightly greater than water.

Like all other planets, Alpheus is not a perfect sphere like a soccer ball, but an. Its equatorial diameter is 6384 miles wider than its polar diameter. Its (ellipticity) is 0.0679, similar to Jupiter's.

Gravitational influence
Alpheus' is well over twice as strong as Earth's. If you weigh 150  on Earth, you would weigh 336 pounds on Alpheus. So the average mass of human on Alpheus would be just as heavy as an person!

Lying at 0.174 s or 89% the radius of the planet is the maximum distance where a moon can shred to pieces caused by the planet's tidal forces, called its. However, this doesn't apply to all kinds of moons. The roche limit depends on the moon's density, a value of 3 g/cm³ is set as the standard because I believe that the average moon density is around that value. Denser moons would have to orbit closer to its parent planet in order for tidal forces to shred to pieces, and vice versa. The, which is the radius of the within where moon's orbits are stable, is 92 LD or 468. If moons are orbiting beyond that sphere, gravitational influences of the grandparent star would cause moons to have unstable orbits around the planet until it begins to orbit the star. The further away from the parent planet's hill sphere, the sooner it takes for those objects to start orbiting the star. The, where the satellite's orbital period is identical to rotation period of the planet, is 0.417 LD or 2.07. The stationary velocity, the orbital velocity at stationary orbit, is 23.0 km/s or 14.3 mi/s. Since the planet takes 18 hours to rotate, then a moon would also take 18 hours to orbit the planet at stationary orbit.

Interior
Below Alpheus' outer envelope (atmosphere), the weight of all the gases pressing down produce a tremendous pressure. That pressure allow and  to condense in the upper mantle despite the higher temperatures deeper down. In the middle mantle lies liquid where hydrogen can conduct electricity under even greater pressure heated beyond its. In the middle mantle, the temperature is 12,100 K (11,800°C, 21,300°F) and the pressure 710. In the lower mantle, there is narrow layer of solid metallic hydrogen at a pressure of 1.8 and temperature 27,900 K (27,700°C, 49,800°F). At the center lies a core of rock and metal with a mass 13 Earth masses, roughly 4.3% the total mass of the planet. The temperature of the core is estimated to be 35,300 K (35,000°C, 63,000°F) and an estimated pressure 3.4 GPa.

Atmosphere
Alpheus' atmosphere composes about 91% and 9%. However, Alpheus also contains trace amounts of other gases, including, , and , having concentrations of 25, 18, and 7 respectively. The atmosphere also contains 4 ppm, which is about 2½ times more concentrated than methane on Earth. There are also tiny amounts of gases with concentrations of only in or even in, such as  (a molecule composing of two s of hydrogen), , , and.

Alpheus contains banded clouds of ammonia and water and this planet would appear as orange and white stripes from space. The ammonia clouds are in the cooler, upper deck and water clouds in the warmer, lower deck. The planet's temperature is 150 K (−123°C, −189°F). This planet radiates twice the amount of energy than it receives from the parent star. Like Jupiter, there are hundreds of s and s, which can produce violent long-lasting storms and high winds.

Magnetic field
This planet has a strong, about 3.28 , which is three quarters the magnetic field strength of Jupiter.

That powerful 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 beautiful, vivid e 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
Alpheus has 159 and it has 20 dusty. The largest moon has mass 4.6 Lunar masses (0.056 Earth masses) and has diameter 1.556 Lunar diameters (3,359 miles, 5,406 kilometers), which is the same mass as and slightly larger. There are two other moons that are bigger than our, five with diameters between 1000 miles and the diameter of our Moon, and 29 have diameters between 100 and 1000 miles. All the rest (122) are less than 100 miles in diameter, the majority of which (79) are less than 10 miles.

Future studies
The probability that Alpheus will Gliese 651 can be a slim 0.31% chance, but it is speculated that Alpheus will transit. Finding this transiting planet is very challenging because it can only transit the star once every nine years, so the star has to be monitored continuously very likely for years until transit is found. Even one transit would be enough to constrain the size and inclination of this planet. The derivative parameters, including density and surface gravity, can then be calculated using the radius constrained from transit and true mass calculated by inclination. Using the calculated density, astronomers can model the interior of this planet.

If Alpheus does not transit, then this planet can still be studied using different methods, like or. The planet can be studied using astrometry using, (JWST),  (SIM), or even the current  (HST) guidance sensor. The direct imaging can see what the planet may really look like.

Astronomers may eventually use to study the interior, including the extent, features and compositions by layers. Using the mounted on the JWST, the atmosphere can be studied, including temperatures, chemical makeup, and features. Using the same method, the rotation rate can be constrained using s, which in turn can then be calculated using the equatorial circumference.

In orbit around the planet, moons can be detected using the transit across the planet, detecting the wobble of the planet, or even direct imaging. Rings can also be detected using just two methods: transit or direct imaging.

Alpheus can further be studied using the JWST's successor:, due to launch between 2025–35.