Lusus viewed from the surface of an icy moon. Also a volcanic moon and a slate moon are visible.
Date July 3, 2003
Discoverers Carter et al.
Detection method Radial velocity
Site Anglo-Australian Observatory
Name & designations
Pronunciation /'lus•is/
Adjective Lusian
Planet numbers P106, Gliese 304 P1, Puppis P1,
Malus P6, 2003 P14,
2003 Pup-1, 2003 Mal-3
Star designations PH 83 b, P1 Puppis b, P6 Mali b,
HD 70642 b, HIP 40952 b,
Gliese 304 b, SAO 199126 b
System Gliese 304
Constellation Puppis
Caelregio Malus
Right ascension 08h 21m 28.14s (125.367 23°)
Declination −39° 42' 19.5" (−39.705 41°)
Distance 28.064 pc (91.532 ly)
Orbital characteristics
Semimajor axis 3.366 958 AU (503.683 8 Gm)
Periastron 3.253 715 AU (486.748 8 Gm)
Apastron 3.480 202 AU (520.630 7 Gm)
Eccentricity 0.033 633 7
Orbital circumference 21.150 85 AU (3 164.122 Gm)
Orbital area 35.594 23 AU² (796 581.8 Gm²)
Orbital period 2 230.734 933 d (6.107 419 39 yr)
Avg. velocity 16.473 km/s (3.463 AU/yr)
Max. velocity 16.747 km/s (3.521 AU/yr)
Min. velocity 16.193 km/s (3.404 AU/yr)
Direction of orbit
relative to star's rotation
Inclination 41.741° to line of sight
−1.118° to star's equator
−4.101° to invariable plane
Argument of periastron 277.210°
Longitude of ascending node 344.334°
Longitude of periastron 261.544°
Angular separation 119.970 mas
Observing the parent star
Mean angular star size 0.133 52° (8.011')
Max. angular star size 0.138 17° (8.290')
Min. angular star size 0.129 18° (7.751')
Mean star magnitude −23.997
Max. star magnitude −24.072
Min. star magnitude −23.926
Bulk characteristics
Mean radius 0.818 92 RJ (57.277 Mm)
Equatorial radius 0.806 54 EJ (57.631 Mm)
Polar radius 0.845 36 PJ (56.570 Mm)
Mean circumference 359.883 Mm
Equatorial circumference 362.106 Mm
Polar circumference 355.437 Mm
Surface area 0.671 21 SJ (41 123 Mm²)
Volume 0.549 90 VJ (787 010 Mm³)
Flattening 0.018 53 (1:53.96)
Angular diameter 27.285 μas
Mass 2.957 160 MJ
Reciprocal mass
relative to star
Density 7.135 g/cm³
Gravitational influence
Surface gravity 11.629 g (114.05 m/s²)
Weight on Lusus
(150 lb on Earth)
1 744 lb
Standard gravitational parameter 3.747 × 108 km³/s²
Escape velocity 114.37 km/s
Hill radius 128.066 LD (49.228 4 Gm)
Roche limit
(3 g/cm3 satellite)
0.250 59 LD (96.325 Mm)
Stationary orbit 0.213 96 LD (82.248 Mm)
Stationary velocity 51.836 km/s (11.651 LD/d)
Rotation characteristics
Rotation period 4.709 77 h (0.196 240 d)
Rotation velocity 21.357 km/s (76.44 °/h)
Direction of rotation
relative to orbit
Axial tilt 79.635°
Longitude of vernal equinox 95.686°
North pole right ascension 03h 23m 25s (50.854°)
North pole declination +57° 55' 50" (+57.930°)
North polar constellation Camelopardalis
North polar caelregio Avis
South pole right ascension 15h 23m 25s (230.854°)
South pole declination −57° 55' 50" (−57.930°)
South polar constellation Circinus
South polar caelregio Selachimorphus
Thermal characteristics
Surface temperature 329 K (56°C, 132°F, 592°R)
Mean irradiance 71.5 W/m² (0.0523 I)
Irradiance at periastron 76.6 W/m² (0.0560 I)
Irradiance at apastron 66.9 W/m² (0.0490 I)
Albedo 0.838 (bond), 0.760 (geom.)
Scale height 7.16 km
Surface density 0.516 g/m³
Molar mass 2.23 g/mol
Composition 90.469% hydrogen (H2)
9.377% helium (He)
0.117% ppm water (H2O)
333 ppm ammonia (NH3)
41.0 ppm methane (CH4)
2.74 ppm hydrogen deuteride (HD)
441 ppb phosphine (PH3)
19.5 ppb neon (Ne)
840 ppt hydrogen sulfide (H2S)
775 ppt ethane (C2H6)
501 ppt propane (C3H8)
36.5 ppt argon (Ar)
18.8 ppt benzene (C6H6)
Water (H2O)
Dipole strength 2.03 mT (20.3 G)
Magnetic moment 5.21 × 1021 T•m³
Dipole tilt 8.59°
Satellite system
Number of moons 101
Number of rings 32

Lusus (Gliese 304 b, P106) is a planet which orbits the yellow G-type main sequence star Gliese 304 (usually referred as HD 70642), similar to our Sun. It is approximately 92 light-years or 28 parsecs from Earth towards the constellation Puppis in the caelregio Malus.

Lusus is three times more massive than Jupiter, but it is smaller than Jupiter, due to its gravitational contraction, making this planet denser than Earth, even though it is a gas giant. It orbits at 338 AU and takes over six years to revolve around the star.

Lusus is named after the Roman god of wine and divine madness.

Discovery and chronology Edit

Lusus was discovered on July 3, 2003 by a team of astronomers led by Brad Carter. The team used the spectrometer mounted on the Anglo-Australian Telescope 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 minimum mass 2.0 MJ. 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 99th exoplanet discovered overall, 73rd since 2000, and 14th in 2003. Lusus is also the 1st exoplanet discovered in the constellation Puppis and 6th in the caelregio Malus (3rd 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 brown dwarfs (objects with minimum masses below 13 MJ but with speculative true masses above 13 MJ).

Orbit and rotation Edit

Orbit Edit

Lusus takes 6.1 years to orbit the star at an average distance of 3.37 AU, 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 eccentricity 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 average velocity of 16.5 km/s. The actual inclination of this planet is unknown, but it is speculated to be 42° to line of sight (−1.12° to star's equator). The argument of periastron is 277° and the longitude of ascending node is 344°, corresponding to the longitude of periastron 262° ((277+344)−360).

Parent star observation and irradiance Edit

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 angular diameter of Gliese 304 as seen from Lusus is 0.134°, which is 14 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 irradiance of 72 W/m², which is 119 the Earth's irradiance but still receives more energy from the star than Jupiter receives from the Sun.

Rotation Edit

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 prograde rotation. Its year on Lusus lasts 11370 Lusus days. The tilt of this planet is such that its north pole points to the constellation Camelopardalis (in Avis), while the south pole points to the constellation Circinus (in Selachimorphus).

Structure and composition Edit

Mass and size Edit

Using the speculated inclination, its speculated true mass for Lusus is 2.95 MJ 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 Saturn. Gravitational contraction 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 flattening is only 0.01853, considerably more spherical than Jupiter (0.06487) and Saturn (0.09796).

Gravitational influence Edit

Lusus has a gravitational force 11.6 times stronger than Earth's and 4.4 times stronger than Jupiter's. If you weigh 150 pounds on Earth, you would weigh 1744 pounds or 78 ton on Lusus, which is about the weight of a very small car parked on Earth!

Since gravity and tidal forces 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 megameters, called its roche limit. The orbit where the gravitational influence of the planet is identical to the star, called its hill radius, is 128 lunar distances (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 stationary orbit, 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 457 hours to rotate, then a moon would take 457 hours to orbit the planet at stationary orbit.

Interior Edit

Like Jupiter, beneath Lusus' outer envelope, it has a mantle of liquid hydrogen and helium due to its tremendous pressure. Below that layer lies liquid metallic hydrogen 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 iron, nickel, and silicates.

Atmosphere Edit

Lusus' atmosphere composes of 90.5% hydrogen and 9.4% helium. Lusus has a similar composition to Jupiter. Lusus contains trace amounts of water, ammonia, methane, phosphine, hydrogen deuteride, neon, hydrogen sulfide, ethane, propane, argon, and benzene. The only ice (aerosol) in the atmosphere is water.

The equilibrium temperature, 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 internal heating 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 Great Red Spot on Jupiter. Like Jupiter, this storm feature often lasts for centuries. There are few more jet streams and zonal jets than Jupiter.

Magnetic field Edit

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

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 Europa, some volcanic like Io, and some have thick atmospheres like Titan. 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 Lunar masses, which is larger and more massive than Ganymede, the largest moon in the Solar System, but slightly smaller and less massive than Mars. Lusus I has a thick atmosphere like Titan. The Io-like moon (Lusus II, Gliese 304 b2) has a diameter of 1.239 DL (2,674 mi, 4,304 km) and has mass 1.77 ML. The Europa-like moon (Lusus III, Gliese 304 b3) has a diameter of 0.975 DL (3,386 mi, 5,449 km) and has mass 1.16 ML. There is a slate moon (Lusus IV, Gliese 304 b4) with sulfur deposits that has a diameter of 0.895 DL (1,933 mi, 3,110 km) and has mass 0.77 ML. 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 rings around Lusus are even more tenuous than Jupiter's. 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 Edit

The method will use to study Lusus might be direct imaging. It is speculated that Lusus will not transit its star. The method of direct imaging might be done using space telescopes like James Webb Space Telescope or New Worlds Observer and can be equipped with spectrometer and astroseismometer. 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 astrometry from Gaia 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 MJ 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 MJ, 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 astroseismology. The size of Lusus at 0.82 RJ is small to being a 2.95 MJ 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.