This is what Tuonetar may look like
Date December 14, 2009
Discoverers Vogt et al.
Detection method Radial velocity
Site Keck Observatory,
Anglo-Australian Observatory
Name & designations
Pronunciation /'tū•ne•tär/
Adjective Tuonetarian
Planet numbers P380, 61 Virginis P3,
Virgo P18, Noctua P41,
2009 P74, 2009 Vir-8,
2009 Noc-13
Star designations 61 Virginis d, 134 Noctae d,
BF 4505 d, PH 309 d,
P12 Virginis d, P29 Noctae d,
HD 115617 d, HIP 64924 d,
HR 5019 d, Gliese 506 d,
SAO 157844 d
System 61 Virginis
Constellation Virgo
Caelregio Noctua
Right ascension 13h 18m 24.31s (199.601 31°)
Declination −18° 18' 40.3" (−18.311 20°)
Distance 8.555 pc (27.903 ly)
Orbital characteristics
Semimajor axis 0.476 475 AU (71.279 7 Gm)
Periastron 0.311 962 AU (46.668 8 Gm)
Apastron 0.640 989 AU (95.890 5 Gm)
Eccentricity 0.345 271 8
Orbital circumference 2.926 54 AU (437.804 Gm)
Orbital area 0.669 37 AU² (14 980.2 Gm²)
Orbital period 123.006 388 d (0.336 773 14 yr)
Avg. velocity 41.334 km/s (8.690 AU/yr)
Max. velocity 47.942 km/s (10.079 AU/yr)
Min. velocity 33.446 km/s (7.031 AU/yr)
Direction of orbit
relative to star's rotation
Inclination 75.889° to ecliptic
−2.344° to star's equator
1.887° to invariable plane
Argument of periastron 313.687°
Longitude of ascending node 257.879°
Longitude of periastron 211.565°
Angular separation 55.694 mas
Observing the parent star
Mean angular star size 1.061 82° (63.709')
Max. angular star size 1.621 77° (97.306')
Min. angular star size 0.789 30° (47.358')
Mean star magnitude −28.107
Max. star magnitude −29.027
Min. star magnitude −27.463
Bulk characteristics
Mean radius 0.387 11 RJ (27.076 Mm)
Equatorial radius 0.379 91 EJ (27.146 Mm)
Polar radius 0.402 49 PJ (26.934 Mm)
Mean circumference 170.120 Mm
Equatorial circumference 170.566 Mm
Polar circumference 169.230 Mm
Surface area 0.149 99 SJ (9 212.1 Mm²)
Volume 0.058 092 VJ (83 140 Mm³)
Flattening 0.007 85 (1:127.3)
Angular diameter 42.311 μas
Mass 0.073 923 MJ
Reciprocal mass
relative to star
13 232
Density 1.688 g/cm³
Gravitational influence
Surface gravity 1.301 g (12.76 m/s²)
Weight on Tuonetar
(150 lb on Earth)
195 lb
Standard gravitational parameter 9.365 × 106 km³/s²
Escape velocity 26.30 km/s
Hill radius 3.675 LD (1.412 8 Gm)
Roche limit
(3 g/cm3 satellite)
0.073 27 LD (28.164 Mm)
Stationary orbit 3.021 98 LD (1 161.646 Mm)
Stationary velocity 2.811 km/s (0.632 LD/d)
Rotation characteristics
Rotation period 738.038 3 h (30.751 597 d)
Rotation velocity 231 kph (0.49°/h)
Direction of rotation
relative to orbit
Axial tilt 21.009°
Longitude of vernal equinox 51.493°
North pole right ascension 23h 03m 18s (345.823°)
North pole declination +16° 46' 34" (+16.776°)
North polar constellation Pegasus
North polar caelregio Testudo
South pole right ascension 11h 03m 18s (165.823°)
South pole declination −16° 46' 34" (−16.776°)
South polar constellation Crater
South polar caelregio Felis
Thermal characteristics
Surface temperature 382 K (108°C, 227°F, 687°R)
Mean irradiance 4 515 W/m² (3.302 I)
Irradiance at periastron 10 533 W/m² (7.702 I)
Irradiance at apastron 2 495 W/m² (1.824 I)
Albedo 0.617 (bond), 0.581 (geom.)
Scale height 22.81 km
Surface density 0.250 g/m³
Molar mass 2.30 g/mol
Composition 85.794% hydrogen (H2)
14.197% helium (He)
562 ppm water (H2O)
233 ppm hydrogen sulfide (H2S)
32.5 ppm methane (CH4)
17.2 ppm ammonia (NH3)
3.22 ppm phosphorus pentachloride (PCl5)
1.85 ppm krypton (Kr)
625 ppb octane (C8H18)
133 ppb disulfur dichloride (S2Cl2)
43.4 ppb neon (Ne)
19.8 ppb phosphine (PH3)
16.5 ppb argon (Ar)
12.6 ppb carbon monoxide (CO)
2.01 ppb propane (C3H8)
Dipole strength 0.217 nT (2.17 μG)
Magnetic moment 1.57 × 1013 T•m³
Dipole tilt 0.24°
Satellite system
Number of moons 2
Number of rings 0

Tuonetar (61 Virginis d, P380) is a planet which orbits the yellow G-type main sequence star 61 Virginis, which is similar to our Sun. It is approximately 28 light-years or 9 parsecs from Earth towards the constellation Virgo in the caelregio Noctua.

Tuonetar is the outermost of the three known planets in 61 Virginis system. Tuonetar is the only known ice giant in this system.

Tuonetar is named after the Queen of the Underworld in Finnish mythology.

Discovery and chronology Edit

Tuonetar was discovered on December 14, 2009 by a team of astronomers led by Steven Vogt. The team used the HIRES spectrometer mounted on the Keck Telescope in Hawaii and the Anglo-Austrialian Telescope in New South Wales, Australia and found that the star 61 Virginis has three periodic variations in the spectrum. This implies evidence for a three-planet system around 61 Virginis, including Tuonetar.

Tuonetar is the 373rd exoplanet discovered overall, 347th since 2000, and 75th in 2009. Tuonetar is the 18th exoplanet discovered in the constellation Virgo (8th in 2009) and 41st exoplanet discovered in the caelregio Noctua (13th in 2009). Since Tuonetar is the third planet discovered in the 61 Virginis system, the planet receives the designations 61 Virginis d (a is not used because the parent star uses this letter to reduce confusion) and 61 Virginis P3. 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

Tuonetar orbits the star at an average distance of 0.4765 AU (1 AU is the average distance between the Earth and the Sun) or 71.28 Gm (million km), which is between the orbits of Mercury and Venus in our solar system. Tuonetar has an eccentric orbit. Tuonetar has an eccentricity of 0.3453, which is more eccentric than the dwarf planet Pluto but less eccentric than the dwarf planet Eris. Tuonetar's orbit varies from 0.312 AU (46.67 Gm) to 0.641 AU (95.89 Gm). The planet takes 123 days or ⅓ of an Earth year to make one complete trip around the star at an average velocity of 41.3 km/s, 25.7 mi/s, or 8.69 AU/yr. Tuonetar is in a 13:4 resonance with the middle known planet Tamar and 29:1 resonance with the innermost planet Devana.

Parent star observation and irradiance Edit

Viewed from Tuonetar, the parent star would have a magnitude −28.11 compared to −26.74 for the magnitude of the Sun viewed from Earth. Observed from Tuonetar, 61 Virginis would appear to be 3.5 times brighter than the Sun seen from Earth. Viewed from Tuonetar, the parent star would have an angular diameter of 1.1° on average, which is 2.2 times the angular diameter of the full moon we sometimes see at night.

Tuonetar receives between 1.8 to 7.7 I worth of stellar energy throughout its orbit with a mean of 3.3 I.

Rotation Edit

This medium-size planet takes a long time to rotate once on its axis, which indicate that its rotation rate is low, just thrice the speed of a car on a highway. Tuonetar takes 30.752 days to rotate once on its axis, which is 14 of its orbital period. So the year on Tuonetar lasts exactly 4 days compared to 366 Earth days in an Earth year. Tuonetar tilts 21° to the plane of its orbit, which is slightly less than the Earth's tilt of 23.4°. The planet's north pole points to the constellation Pegasus (in Testudo), while the south pole points to the constellation Crater (in Felis).

Structure and composition Edit

Mass and size Edit

Tuonetar has mass 23.5 Earth masses, classifying it as midplanet in the planetary mass classification scheme. Tuonetar is 62% more massive than Uranus and 227 Jupiter mass. Tuonetar is 4.25 times the diameter of Earth and 25 the diameter of Jupiter. Tuonetar has a density of 1.69 g/cm³, which is ⅓ the Earth's density, implying that this planet is an ice giant like Uranus and Neptune.

Gravitational influence Edit

The gravitational force of Tuonetar is 30% stronger than Earth's. In cgs unit, its logarithmic value is 3.11. So if you weigh 150 pounds on Earth, you would weigh 195 pounds on Tuonetar. The average human weight on Tuonetar would weigh as much as the average weight of a professional baseball player on Earth!

Tuonetar has the hill sphere radius of 52.18 planetary radii or 323 times the distance between Earth and the Moon. The satellite's orbit within the hill sphere is stable while the orbit outside of hill sphere is unstable. The roche limit of Tamar is 28.2 Mm or 1.04 planetary radii. If a 3 g/cm³ satellite orbits within the roche limit, it would tear apart by tidal forces. Denser moons would withstand greater tidal forces and orbiting closer to the planet would be required to tear apart. Tuonetar's stationary orbit, analogous to the Earth's geostationary orbit, is located at a distance of 42.79 planetary radii or 3 lunar distances, which is just beyond the hill sphere. Stationary orbit is an orbit where its orbital period is synchronous with the planet's rotation period. Since the planet takes 49.203 days to rotate once on its axis, then a moon in stationary orbit would also take 49.203 days to orbit the planet, that's 1.6 times longer than the Moon's orbital period around the Earth. So a moon would always present the same face to the planet and the observer on the moon appears that the planet never rotate while observer on the planet's surface appears that the moon always hang in the sky and never move.

Interior Edit

Tuonetar's structure is very similar to the structure of Tamar, except Tuonetar has a much deeper atmosphere. Below Tuonetar's outer envelope (atmosphere), there is the mantle of liquid water, below that is the layer of the exotic form of water called ice VII or "hot ice." Below that is the diamond layer containing an estimated 370 million times more diamond than Earth has and 137 times more diamond than Tamar has. At the planet's center lies a core made out of rock and metal, predominantly of iron and carbon.

Atmosphere Edit

Like all gas giants and ice giants, Tuonetar has a deep atmosphere. The "surface" temperature is 382 K (108°C, 227°F). Tuonetar is seemingly releasing very little heat from its interior because this giant planet is orbiting close to the heat of its star. Heat from its interior raises the temperature by 28 K (28 C°, 51 F°).

Like all giant planets, Tuonetar's atmosphere is mostly hydrogen and helium. This atmosphere also contains trace amounts of water (H2O), hydrogen sulfide (H2S), methane (CH4), ammonia (NH3), phosphorus pentachloride (PCl5), octane (C8H18), disulfur dichloride (S2Cl2), phosphine (PH3), carbon monoxide (CO), propane (C3H8), and few noble gases.

Magnetic field Edit

Tuonetar has an extremely weak magnetic field, about two millionths of a gauss, which is 150,000 times weaker than Earth's. A possible reason for its weakness is because the planet rotates so slowly because the tidal forces of the nearby star slowed the planet's rotation by lot.

Moons and rings Edit

Tuonetar does have two moons. The innermost moon has a diameter 18 that of our moon at 256 miles or 411 kilometers. The moon orbits 0.246 LD from the parent planet. The outermost moon has a diameter more than half that of our moon at 1203 miles or 1937 kilometers. The moon orbits 1.652 LD from Tuonetar.

Tuonetar has no ring system.

Future studies Edit

It is speculated that Tuonetar will not transit since I speculated that the inclination is 55.9°. Studying Tuonetar using the direct imaging would not be a reliable option since the planet orbits way too close to the glare of its star. The planet can best be studied using astrometry using Gaia, James Webb Space Telescope (JWST), or Space Interoferometry Mission (SIM). The astrometry can constrain the inclination and thus calculate the exact mass.

Maybe ATLAST can reliably be used to directly image planets down to 0.01 AU from the sun-like stars. The direct imaging can see what Tuonetar may really look like. The direct imaging can constrain the size of this planet like transit method. The derivative parameters, including density and surface gravity, can then be calculated using the radius and true mass calculated using inclination. Using the calculated density, astronomers can model the interior of this planet.

Astronomers may eventually use astroseismology to study the interior, including the extent, features and compositions by layers. Using the spectrometer mounted on the ATLAST, the atmosphere can be studied.

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.

Related links Edit

  • Devana (61 Virginis b, P378)
  • Tamar (61 Virginis c, P379)