Lusus

Lusus (Gliese 304 b, P106) is a which orbits the yellow    Gliese 304 (usually referred as ), similar to our. It is approximately 92 s or 28 s from towards the   in the caelregio Malus.

Lusus is three times more massive than Jupiter, but it is smaller than Jupiter, due to its, making this planet denser than Earth, even though it is a. It orbits at 3 AU and takes over six years to revolve around the star.

is named after the of wine and divine madness.

Discovery and chronology
Lusus was discovered on July 3, 2003 by a team of astronomers led by. The team used the mounted on the  to study this star to look for any evidence of planets. The team looked at radial velocity data of Gliese 304 and found the periodic graph consistent with a 6-year period with a 2.0 M$J$. The semimajor axis of this planet wasn't directly determined, but it was calculated based on its period and the mass of the parent star. The semimajor axis of this planet was determined to be 3.3 AU and had an eccentricity of 0.1.

Lusus is the 99$J$ exoplanet discovered overall, 73$J$ since 2000, and 14$J$ in 2003. Lusus is also the 1$J$ exoplanet discovered in the constellation Puppis and 6$J$ in the caelregio Malus (3$⊕$ in 2003). Lusus is the first and only planet discovered in the Gliese 304 system, hence the designations Gliese 304 b (a is not used because the parent star uses this letter to reduce confusion) and Gliese 304 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$⊕$).

Orbit
Lusus takes 6.1 s to orbit the star at an of 3.37, putting this planet in the A orbit ('A' stands for Alphian, after the exoplanet Alpheus), which ranges from 2.5–5 AU in average distance. Unlike most long-period planets known, Lusus orbits in a circular path with an of 0.034, which is more circular than Jupiter (0.049), but more eccentric than Earth (0.017). The orbital distance varies by 0.23 AU throughout its orbit. The direction of Lusus' orbit is the same as the parent star's rotation, like all planets in our solar system do. The planet orbits at an of 16.5 km/s. The actual of this planet is unknown, but it is speculated to be 42° to  (−1.12° to star's ). The is 277° and the  is 344°, corresponding to the  262° ((277+344)−360).

Parent star observation and irradiance
When viewing Gliese 304 from the orbit of Lusus, that sun would appear 12.5 times fainter than the Sun as seen from Earth, but 2.2 times brighter than the Sun as seen from Jupiter. So the apparent magnitude of Gliese 304 as seen from Lusus is −24.00. The of Gliese 304 as seen from Lusus is 0.134°, which is  the angular diameter of the full moon and sun as seen from Earth, due to fact that Lusus orbits further away from the star than Earth to the Sun and Gliese 304 itself is 19% smaller than our Sun.

Lusus receives an of 72 W/m², which is  the Earth's irradiance but still receives more energy from the star than Jupiter receives from the Sun.

Rotation
Lusus takes only 4 hours and 43 minutes to rotate once on its axis, which is over twice as short as Jupiter, the shortest rotation period of any planet in our solar system. The rotation velocity is 21.4 km/s or 13.3 mi/s, which is faster than Jupiter. Lusus rotates in the same direction as its orbit and rotation of the star, so-called. Its year on Lusus lasts 11370 Lusus days. The tilt of this planet is such that its points to the constellation  (in Avis), while the  points to the constellation  (in Selachimorphus).

Mass and size
Using the speculated inclination, its speculated for Lusus is 2.95  or 5.61 wekagrams. It is classified as super-Jupiter since the planet's mass is between 2 and 13 times that of Jupiter. Despite its mass, Lusus is smaller than Jupiter, about the size of. is the cause why this planet is smaller while more massive than Jupiter. Since it is a gas giant, it has good ability to contract by gravity. Since its formation 3.9 billion years ago, Lusus was three times larger than Jupiter. Lusus is shrinking by approximately 8 cm/yr. Lusus has density 7.135 g/cm³, which is 5.4 times denser than Jupiter and 30% denser than Earth.

Because of its strong gravitational despite its rapid rotation, its  is only 0.01853, considerably more spherical than Jupiter (0.06487) and Saturn (0.09796).

Gravitational influence
Lusus has a gravitational force 11.6 times stronger than Earth's and 4.4 times stronger than Jupiter's. If you weigh 150 on Earth, you would weigh 1744 pounds or   on Lusus, which is about the weight of a very small car parked on Earth!

Since gravity and s of Lusus are so strong, a 3 g/cm³ moon would be torn apart if it orbits within one quarter the Earth–Moon distance at 1.68 planetary radii or 96 s, called its. The orbit where the gravitational influence of the planet is identical to the star, called its, is 128 s (49 gigameters), which is 859 times the radius of the planet or 0.329 AU. The orbit where the satellite's orbital period is identical to the rotation period of the planet, called its, is 0.214 LD (82 Mm), which is 1.43 planetary radii, just within the roche limit. The orbital velocity at stationary orbit, called its stationary velocity, is 51.8 km/s or 32.2 mi/s. Since the planet takes 4 hours to rotate, then a moon would take 4 hours to orbit the planet at stationary orbit.

Interior
Like Jupiter, beneath Lusus' outer envelope, it has a mantle of and  due to its tremendous pressure. Below that layer lies liquid where hydrogen can conduct electricity. At the center lies a core of rock and metal with a mass 18 Earth masses, roughly 1.9% the total mass of the planet. The core is made mainly of, , and s.

Atmosphere
Lusus' atmosphere composes of 90.5% and 9.4%. Lusus has a similar composition to Jupiter. Lusus contains trace amounts of, , , , , , , , , , and. The only ice in the atmosphere is.

The, based on the planet's orbital distance from the star, is 122 K (−151°C or −240°F), which would favor the formation of ammonia clouds. Instead the would cause temperatures to rise. At the "surface", which is the 1-bar layer, the temperature is 329 K (56°C or 132°F), near the hottest temperature ever recorded on Earth, which would favor the formation of sulfuric acid clouds instead. The winds on the planet move at a speed of 800 mph (1300 kph). There may always be a "Great Red Spot" on Lusus similar to the on Jupiter. Like Jupiter, this storm feature often lasts for centuries. There are few more s and s than Jupiter.

Magnetic field
This planet has a powerful that can block radiation coming off from Gliese 304 and causes e at the poles. The magnetic field of Lusus is 4.74 times more powerful than and 66 times more powerful than.

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
Lusus has 101 moons that are larger than 1 km across. A lot of those moons are icy, while some are rocky and cratered. Some moons have subsurface oceans like, some volcanic like , and some have thick atmospheres like. The largest moon (Lusus I, Gliese 304 b1) has a diameter of 1.841 Lunar diameters (3,973 miles, 6,394 kilometers) and has mass 4.39 es, which is larger and more massive than, the largest moon in the , but slightly smaller and less massive than. Lusus I has a thick atmosphere like Titan. The Io-like moon (Lusus II, Gliese 304 b2) has a diameter of 1.239 D$2$ (2,674 mi, 4,304 km) and has mass 1.77 M$2$. The Europa-like moon (Lusus III, Gliese 304 b3) has a diameter of 0.975 D$3$ (3,386 mi, 5,449 km) and has mass 1.16 M$4$. There is a slate moon (Lusus IV, Gliese 304 b4) with sulfur deposits that has a diameter of 0.895 D$3$ (1,933 mi, 3,110 km) and has mass 0.77 M$2$. There are three moons that are larger than 2000 miles across, seven are between 1000–2000 miles, 36 are between 100–1000 miles, and 55 are less than 100 miles across.

The around Lusus are even more tenuous than. The rings are made almost entirely of dust that absorb about 95% of incoming light, making the rings very dark and extremely difficult to find. There are 32 ultra-narrow, ultra-thin and ultra-dark rings. It appears that Lusus has virtually no rings!

Future studies
The method will use to study Lusus might be. It is speculated that Lusus will not its star. The method of direct imaging might be done using space telescopes like or  and can be equipped with  and. Using direct imaging, it can constrain what Lusus actually looks like.

Lusus should be an important target for future studies because it is a long-period Jupiter-like planet. Constraining the inclination using from  is important for calculating its true mass and determine whether it is a planet or a brown dwarf. However it is very likely to be a planet because its minimum mass is 1.97 M$2$ and the inclination must be at least 171.29° or at most 8.71° in order for true mass to be at least 13.00 M$6$, the borderline between planets and brown dwarfs. Constraining the size of this planet is also important. After measuring its size, density and surface gravity can be calculated. Using the density of the planet, astronomers can probe the interior and estimate the mass and size of the core. Astronomers will also study the mantle and its temperature of the core using. The size of Lusus at 0.82 R$3$ is small to being a 2.95 M$8$ planet at the age of 3.9 billion years. It could actually be bigger, about the size of Jupiter. It shall also measure its temperature and chemicals in the atmosphere using spectroscopy. Another important property is constraining its rotation rate, in which the rotation period can be calculated using the circumference of the planet and rotation rate.

The space telescope shall also find moons and even rings around the planet. Finding moons can be done using direct imaging, transit across its host planet, or studying the wobble of the planet. By using all three methods, it will constrain its mass, size (in which its density and gravity can be constrained), surface temperature, atmospheric composition, surface textures, appearance, orbital distance from the planet, orbital period, eccentricity, and rotation period of the moons.