PlanetStar Wiki
An artist's impression about what Dalia may look like
Date April 11, 2011
Discoverers Tinney et al.
Detection method Radial velocity
Site Anglo-Australian Observatory
Name & designations
Pronunciation /'dal•ē•yä/
Adjective Dalian
Planet numbers P517, HD 38283 P1, Mensa P1,
Solarium P34, 2011 P22,
2011 Men-1, 2011 Sol-1
Star designations PH 411 b, P1 Mensae b,
P31 Solaris b, HD 38283 b,
HIP 26380 b, SAO 256213 b
System HD 38283
Constellation Mensa
Caelregio Solarium
Right ascension 05h 37m 02.02s (84.258 41°)
Declination −73° 41' 57.6" (−73.699 34°)
Distance 38.316 pc (124.971 ly)
Orbital characteristics
Semimajor axis 1.023 607 AU (153.129 4 Gm)
Periastron 0.604 387 AU (90.415 0 Gm)
Apastron 1.442 827 AU (215.843 8 Gm)
Eccentricity 0.409 551 6
Orbital circumference 6.225 47 AU (931.318 Gm)
Orbital area 3.002 95 AU² (67 204.6 Gm²)
Orbital period 363.199 198 d (0.994 385 21 yr)
Avg. velocity 29.779 km/s (6.261 AU/yr)
Max. velocity 35.355 km/s (6.284 AU/yr)
Min. velocity 22.882 km/s (6.237 AU/yr)
Direction of orbit
relative to star's rotation
Inclination 12.267° to ecliptic
1.446° to star's equator
−11.260° to invariable plane
Argument of periastron 17.491°
Longitude of ascending node 133.328°
Longitude of periastron 150.819°
Angular separation 26.715 mas
Observing the parent star
Mean angular star size 0.570 84° (34.250')
Max. angular star size 0.966 79° (58.008')
Min. angular star size 0.404 98° (24.299')
Mean star magnitude −27.742
Max. star magnitude −28.886
Min. star magnitude −26.996
Bulk characteristics
Mean radius 1.213 75 RJ (84.892 Mm)
Equatorial radius 1.212 79 EJ (86.659 Mm)
Polar radius 1.215 79 PJ (81.359 Mm)
Mean circumference 533.395 Mm
Equatorial circumference 544.498 Mm
Polar circumference 511.191 Mm
Surface area 1.473 3 SJ (90 483 Mm²)
Volume 1.788 3 VJ (2.559 3 × 106 Mm³)
Flattening 0.062 44 (1:16.01)
Angular diameter 29.620 μas
Mass 1.605 272 MJ
Reciprocal mass
relative to star
Density 1.191 g/cm³
Gravitational influence
Surface gravity 2.876 g (28.20 m/s²)
Weight on Dalia
(150 lb on Earth)
431 lb
Standard gravitational parameter 2.034 × 108 km³/s²
Escape velocity 69.22 km/s
Hill radius 19.033 LD (7.316 1 Gm)
Roche limit
(3 g/cm3 satellite)
0.204 50 LD (78.609 Mm)
Stationary orbit 0.541 76 LD (208.250 Mm)
Stationary velocity 26.303 km/s (5.912 LD/d)
Rotation characteristics
Rotation period 19.569 0 h (0.815 375 d)
Rotation velocity 7.729 km/s (18.40°/h)
Direction of rotation
relative to orbit
Axial tilt 66.347°
Longitude of vernal equinox 148.745°
North pole right ascension 09h 42m 01s (145.505°)
North pole declination −12° 21' 57" (−12.366°)
North polar constellation Hydra
North polar caelregio Felis
South pole right ascension 21h 42m 01s (325.508°)
South pole declination +12° 21' 57" (+12.366°)
South polar constellation Pegasus
South polar caelregio Testudo
Thermal characteristics
Surface temperature 327 K (54°C, 129°F, 588°R)
Mean irradiance 1 742 W/m² (1.274 I)
Irradiance at periastron 4 996 W/m² (3.653 I)
Irradiance at apastron 877 W/m² (0.641 I)
Albedo 0.865 (bond), 0.782 (geom.)
Scale height 71.09 km
Surface density 0.067 g/m³
Molar mass 2.18 g/mol
Composition 92.321% hydrogen (H2)
7.627% helium (He)
77.1 ppm water (H2O)
17.0 ppm ammonia (NH3)
6.33 ppm methane (CH4)
1.27 ppm hydrogen deuteride (HD)
776 ppb argon (Ar)
408 ppb carbon monoxide (CO)
61.8 ppb propane (C3H8)
38.7 ppb neon (Ne)
20.8 ppb krypton (Kr)
12.0 ppb phosphine (PH3)
6.71 ppt Sulfur hexafluoride (SF6)
Water (H2O)
Dipole strength 176 μT (1.76 G)
Magnetic moment 2.83 × 1019 T•m³
Dipole tilt 4.22°
Satellite system
Number of moons 31
Number of rings 8

Dalia (HD 38283 b, P517) is an exoplanet which orbits the yellow F-type main sequence star HD 38283. The star is slightly smaller, cooler and thus dimmer than our Sun. It is approximately 125 light-years or 38 parsecs from Earth towards the constellation Mensa in the caelregio Solarium.

Dalia is a giant gas planet bigger and more massive than Jupiter. It orbits more than five times closer to the star than Jupiter is to our Sun at Earth-like orbit, allowing the existence of habitable moons with bodies of water and complex life. However, Dalia orbits much more eccentrically than Earth's, classifying this planet as an eccentric Jupiter. Dalia would appear as a gray banded globe from space.

Dalia is named after the goddess of fate in Lithuanian mythology.

Discovery and chronology[]

Anglo-Australian Planet Search team (AAPS) began observing HD 38283 in January 1998 using the spectrometer mounted on the Anglo-Australian Telescope (AAT) looking for signs of planets. HD 38283 is a good target star because this star is circumpolar from AAPS, allowing team to continuous watch the star for years. Then on April 11, 2011 the team of astronomers led by Chris Tinney found the signal of a planet around HD 38283. The team determined that the wobble takes 363.2 days to rotate 360°, corresponding that the orbital period of the unseen planet is 363.2 days. Even though Dalia is the 509th exoplanet discovered, it is only the first exoplanet discovered to have orbital period between 360–370 days. Using period and mass of the star, the average planet-star separation is calculated to be 1.02 AU (putting this planet in the middle of its habitable zone. Since radial velocity can't determine the inclination of the orbit, its mass will only be in the lower limit. Based on the wobble deviations and the semi-amplitude of 8.5 m/s, the team determined that the mass of Dalia is at least 0.34 MJ, similar to Saturn, making this the first planet other than super-Jupiter found by AAPS after it found previous 34 planets with minimum masses ranging from 5.1 to 10 MJ. The oval-shaped wobble determined that the planet orbits very eccentrically.

Dalia is the 516th exoplanet discovered overall, 490th exoplanet discovered since 2000, 140th exoplanet discovered since 2010, and 22nd exoplanet discovered in 2011. Dalia is also the 1st exoplanet discovered in the constellation Mensa (1st in 2011) and 34th in the caelregio Solarium (1st in 2011). Dalia is the first and only planet discovered in the HD 38283 system, receiving the designation HD 38283 b (a is not used because the parent star uses this letter to reduce confusion) and HD 38283 P1. Note that the chronology does not include minimum-mass planets that are speculatively brown dwarfs.

Possible existence of a sibling[]

In addition to its 363-day signal of Dalia, the team also found a weak signal at 120 days, indicating that the second planet may exist with an orbital period of 120 days. More observations are needed to confirm the existence of the second planet, whether it is 120 days or not. If the 120-day period second planet is confirmed, Dalia would have a circular orbit instead, thus making the probable habitable moons around Dalia even more conducive to life. If the 120-day period planet is confirmed, then this planet would speculated to have minimum mass 21.9 M.

Orbit and rotation[]


Dalia's orbital period (year) is 363.2 days, very similar to Earth's 365.3 days. However, this planet orbits slightly further from the star than Earth is to the Sun, 1.023 AU vs. 1.000 AU (because the parent star is 8.5% more massive than the Earth's star), putting this planet in the Gaian orbit. And this planet orbits far more eccentrically than Earth's, 0.410 vs. 0.017. Dalia's closest approach to the star is 0.60 AU (closer to its star than Venus is to our Sun) and 1.44 AU (slightly closer than Mars' distance from the Sun). The planet's high eccentricity maybe caused by an undiscovered planet tugging at Dalia's orbit, or even an observational error mentioned above. If the high eccentricity is caused by an undiscovered planet, the perturber would have mass 6.77 MJ and orbits about 2.3 AU beyond the orbit of Dalia near 6:1 resonance. Dalia moves at an average velocity of 6.261 AU/yr (29.78 km/s, 18.50 mi/s), very similar to Earth's 6.283 AU/yr (29.88 km/s, 18.57 mi/s). The planet's fastest velocity at periastron is 6.899 AU/yr (35.35 km/s, 21.97 mi/s) while the slowest velocity at apastron is 5.550 AU/yr (22.88 km/s, 14.22 mi/s). The plane of its orbit is almost face-on with an inclination of 12.3°.

Parent star observation and irradiance[]

Viewed from Dalia, HD 38283 would have an apparent magnitude of −27.74, which would appear 2.5 times brighter than the Sun seen from Earth. Because Dalia orbits very eccentrically, anyone standing on an Earth-like moon would notice that the brightness of its sun would vary by a factor of 5.7 throughout the planet's orbit, compared to the brightness of the Sun seen from Earth only vary by about 7%. During the farthest point of its orbit, its sun would appear faint enough to be seen directly for up to 30 seconds while at the closest approach would be blinded within about half a second. Light from the parent star would take 8:31 to reach Dalia, just 12 seconds longers than light from the Sun to reach Earth. Light would take 5:02 to reach Dalia at periastron and 12:00 to reach Dalia at apastron. Viewed from Dalia, the parent star would appear similar to the size of the Sun seen from Earth, but its angular size would vary dramatically throughout the planet's year from 24 arcminutes at apastron (farthest point in its orbit) to 58 arcminutes at periastron (closest approach).

Despite the same distance from the star as Earth, Dalia receives 27% more energy than Earth receives from the Sun, an average of 1742 W/m², because the parent star is a bit more luminous than our Sun. Since this planet has a very eccentric orbit, at its closest approach, it receives an irradiance of 4996 W/m², while at its farthest approach, it receives just 877 W/m². This planet receives three times more energy from the parent star than heat emission from its interior.


Dalia takes over 19½ hours to rotate once on its axis, shorter than 24 hours for Earth but longer than 17¼ hours for Uranus. A year on Dalia is 445.44 Dalian days compared to 366.26 days on Earth. The planet tilts nearly on its side at 66.3°. The cause of this tilt was early in its history, Dalia was impacted by so much debris during the inward migration that it knocked this planet nearly on its side. The north pole points to the constellation Hydra (subdivision of the caelregio Felis) while the south pole points to Pegasus (in Testudo).

Structure and composition[]

Mass and size[]

Dalia would be a Saturn-like planet if the orbit is edge-on, but at an inclination that is almost face-on, Dalia is actually 61% more massive than Jupiter. Still like Jupiter, Dalia is a mid-Jupiter. Dalia is 21% larger than Jupiter and 1.79 times the volume of Jupiter, making Dalia slightly less dense than Jupiter. Based on its size and planet's distance from Earth, Dalia has an apparent diameter of 29.6 μpc.

Like all other planets, Dalia is not a perfect sphere due to centrifugal effect of its rotation. Its equatorial radius is 5% wider than its polar radius.

Gravitational influence[]

If you weigh 150 pounds on Earth, you would weigh roughly 431 pounds on Dalia, because Dalia's gravitational strength is 2.876 g, which is a little bit stronger than Jupiter's, the strongest gravitational field of any planet in our solar system. So the average mass of a person standing on Dalia would be just as heavy as an ultra-obese person! In response to its stronger gravitational pull than Jupiter, its escape velocity is a little bit higher than Jupiter's. The minimum speed needed to escape Dalia is 69.2 km/s while the minimum speed needed to escape Jupiter is 60.2 km/s.

Based on its periastron distance and mass ratio between planet and star, Dalia's hill sphere radius, the stability zone for satellites, is calculated to be 19 LD. The roche limit, also known as tidal zone, where 3 g/cm³ satellites break up via tidal forces is 0.2 LD or 0.93 planetary radii. Denser moons would be required to orbit closer to the planet in order to break up, and vice versa. The stationary orbit, where the satellite's orbital period is synchronized to the rotation period of the planet, is 0.54 LD or over 225 times the radius of the planet, nearly six times further from the planet than geostationary orbit around Earth. The stationary velocity, the orbital velocity at stationary orbit, is 26.3 km/s or 16.3 mi/s. Since the planet takes 19.57 hours to rotate, then a moon would also take 19.57 hours to orbit the planet at stationary orbit. From the surface of a moon, the parent planet never appears to rotate, so the moon always face the same side of the planet just like our Moon never appears to rotate when viewed from Earth because the Moon is tidally locked to Earth. Whereas from the surface of Dalia, the moon would appear eternally motionless in the sky.


Like all planets, Dalia has a mantle and a core, but no crust since Dalia is a gas giant. Below Dalia's outer envelope (atmosphere), the weight of all the gases pressing down produce a tremendous pressure. That pressure allow hydrogen and helium to condense in the upper mantle despite the higher temperatures deeper down. In the lower mantle lies liquid metallic hydrogen where hydrogen can conduct electricity under even greater pressure heated beyond its critical point. At the center lies a core of rock and metal with a mass 12 Earth masses, roughly 2.4% the total mass of the planet. The temperature of the core is estimated to be 23,700 K (23,400°C, 42,100°F) and an estimated pressure 1.7 GPa.


Dalia's atmosphere composes about 92.3% hydrogen and 7.6% helium. Dalia also contains trace amounts of other gases, including water vapor, ammonia, and methane having concentrations of 77.1, 17.0, and 6.33 parts per million respectively. The atmosphere also contains hydrogen deuteride at 1.27 ppm. The atmosphere also contains tiny amounts of other gases, including carbon monoxide, phosphine, and sulfur hexafluoride.

The mean "surface" temperature (at 1-bar layer) is 327 K (54°C, 129°F), almost identical to the temperature of the Earth. At these temperatures, clouds are made exclusively of water and viewed from space, this planet would appear gray with bands of different shades. Like almost all water giants, this planet has a high reflectivity (albedo). Like Jupiter, there are hundreds of jet streams and zonal jets, which can produce long-lasting storms and high winds.

Since Dalia's distance from the star vary dramatically, rapid changes in temperatures can trigger intense storms and winds over much of the planet. The winds can reach as high as 4000 mph (6500 kph), twice as windier than any planet in our solar system.

Magnetic field[]

This planet has a magnetic field strength of about 1.76 gauss at its equator, which is 25 the magnetic field strength of Jupiter and nearly six times that of Earth's.

That powerful magnetic field is produced by movements of metallic hydrogen in its interior caused by planet's rotation. This mechanism is well known as the dynamo effect. The magnetic field blocks most of stellar and cosmic radiation from reaching the planet, but occasionally it can produce beautiful, vivid aurorae 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[]

Dalia has 31 moons and it has 8 rings made of grains of sand. There is one moon bigger than our Moon, six with diameters between 1000 miles and the diameter of our Moon, and 15 have diameters between 100 and 1000 miles. All the rest (9) are less than 100 miles in diameter.

Some of these moons formed from the planet's accretion disk while others are captured small planets and asteroids during Dalia's type II migration early in its history.

Dalia I[]

Dalia I has mass 12.7 Lunar masses (0.152 Earth masses) and has diameter 1.843 Lunar diameters (3,979 miles, 6,404 kilometers), making this the largest moon of Dalia. Dalia I is 107 the mass of Mars but its diameter is smaller than Mars by 229 miles or 370 kilometers. Dalia I has an atmosphere just thick enough to have bodies of water on its surface, but seasonal variations are chaotic because the planet has a very eccentric orbit. During part of its year, the water is boiling and appearing as steam over the bodies of water and the moon would be dry, while at the other end, the water is frozen. Its atmosphere is seven times thinner than Earth's, still it is about 24 times thicker than Mars'. There are extremophiles on Dalia I, but there are no conventional life because the seasonal variations are too wild.

From the surface of Dalia I, ones may see the parent planet in the sky about half of the times with phase variations just like the Moon as well as other moons. Rings around the parent planet can be visible especially at night, making this scenery even more amazing!


Dalia I takes 38.27 days to orbit the planet, so imagine if there are lifeforms on Dalia I just as intelligent as humans, the timekeeping would be a month lasting 38 or 39 Earth days while the year is the time when the parent planet orbit its star, which is 363.2 Earth days. Dalia I takes 26.23 hours to rotate once on its axis, so on average their calendar days last 26¼ hours instead of 24 hours as it is on Earth. However, the number of hours in a day should be rounded down to 26 hours with 27-hour days (so-called "leap days") occurring every four days. Between leap days, civilizations would see that the sun rises and sets 15 minutes later every day and on succeeding leap days, ones would see the sun rises and sets 45 minutes earlier than a previous day and an hour earlier than would otherwise be. A leap hour would be added after the usual last hour of the day on leap days about halfway between previous sundown and next sunrise. So its month, the time when Dalia I orbits Dalia, is 35 calendar days instead of usual 30- or 31-day months on Earth. So their calendar years last 10 months compared to 12 months for years on Earth. But since the exact year is 10.37 months, an extra day would be added for each month every two years. Each month during the even-numbered years would last 36 days while each month during the odd-numbered years last 35 days. However in odd-numbered years divisible by three, 36-day months are used every six years.

However, alien beings would likely use different units of time and different structure of timekeeping than humans on our home planet.

Future studies[]


Speculatively, Dalia will not transit since the orbit is almost face-on. Finding this potentially transiting planet is very challenging because it can only transit the star once every year, but Kepler can find such planets but this space observatory can only find planets in the constellations Cygnus, Draco, and Lyra but not in Mensa where Dalia was found. So maybe PLATO (to be launched in 2017 or 2018) can find transits of Dalia. 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 Dalia does not transit, as speculated, then this planet can still be studied using different methods, such as astrometry. This method can be used to study this planet using Gaia (launched in December 2013) and James Webb Space Telescope (JWST, to be launched around 2018), or even the current Hubble Space Telescope (HST) guidance sensor.

Direct imaging[]

The direct imaging can see what the planet may really look like. But directly imaging this planet would be highly difficult because it orbits only 1.023 AU (apparently very close to the glare of its star). The angular separation between the planet and the star is 26.7 milliarcseconds. ATLAST (to be launched between 2025–35) would be the easiest telescope to image Dalia and other planets within 1 AU of solar-type stars.

Astroseismology and spectroscopy[]

Astronomers may eventually use astroseismology to study the interior, including the extent, features and compositions by layers. Using the spectrometer 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 Doppler shifts, which in turn rotation period can then be calculated.

Detecting moons and rings[]

In orbit around the planet, moons can be detected using transits 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.