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Lalande 21185 Exoplanet System

The fourth closest star to the Sun: three potential exoplanets //

Lalande 21185, also known as Gliese 411 or HD 95735, is a small star located in the northern constellation Ursa Major. At 8.30 light-years away, it comes in behind Alpha Centauri, Barnard's Star and Wolf 359 as the fourth closest stellar system to the Sun (sixth if brown dwarfs counted). It is also the second closest star after Proxima Centauri to have confirmed exoplanets in orbit. Lalande 21185 is an 8-billion-year-old M2V red dwarf with 0.39 solar masses, 0.37 solar radii and a 3719 K (3446 °C) surface temperature. Despite being close in astronomical terms, it is only around 2% of the Sun's luminosity and cannot be seen with the unaided eye. It is also a variable star, exhibiting regular fluctuations in brightness and the occasional X-ray flare, but is much less active compared to a typical flare star.

Due to its proximity, Lalande 21185 has long been the subject of various astronomical surveys, but it wasn't until 2019 that an exoplanet was robustly detected and confirmed one year later. Designated Lalande 21185 b, this super-Earth is at least 2.69 times the mass of our home planet and orbits its star every 12.94 days. It lies interior to the habitable zone at a mere distance of 0.079 AU and is probably too hot to support liquid water on its surface. However, as with many red dwarf planets, it is likely to be tidally locked, so more favourable conditions along the twilight strip cannot be ruled out.

A second planet, Lalande 21185 c, was proposed in 2021 and confirmed in 2022. It is located at a farther distance of 2.94 AU from its star and takes 2946 days (8.07 years) to complete one orbit. If placed in the solar system, it would sit about halfway between Mars and Jupiter. Lalande 21185 c was initially given a minimum mass of 18.0 times Earth before being revised to 13.6 Earth masses—comparable to Uranus. Since its host is much cooler and dimmer than the Sun, the planet’s distance in the context of the system is more akin to our ice giants, with an estimated equilibrium temperature of a mere 53 K (-220 °C).

The signal of another object between Lalande 21185 b and c was detected in 2021 and initially marked as a false positive. After review, it was determined that a planetary origin could not be ruled out. If this candidate third planet Lalande 21185 d exists, it would be a minimum of 3.89 Earth masses and orbit every 215.7 days at an intermediate distance of 0.51 AU. It sits on a slightly eccentric path just outside Lalande 21185’s habitable zone and probably has the opposite problem to planet b, in that it is too cold to support liquid water on its surface. More observations are needed to confirm or refute the existence of planet d.

This simulation replicates the Lalande 21185 system from actual data wherever possible, though fictional elements may be incorporated due to knowledge gaps:

• The planets of Lalande 21185 do not pass in front of their star from our perspective. This means that our current methods can only constrain their minimum mass but not their true mass or size. Thus, the properties in this simulation are largely speculative and based on modelling by Universe Sandbox itself. If their true mass is much greater than the minimum estimates, it's possible that the planets look very different to what is presented here.
  
• I’ve designed Lalande 21185 b as a tidally-locked temperate airless super-Terra with an average temperature of -19 °C (254 K). However, this value is deceiving because its day and night sides are separated by over 350 degrees. The majority of the planet's water has been stripped by Lalande 21185 during its time as a young, volatile star, leaving behind an ancient, rugged surface. Assuming that Lalande 21185 b possesses a fairly large iron core, Universe Sandbox models its size at 1.28 Earth radii.
  
• I’ve designed Lalande 21185 c as a frigid Neptune with an average temperature of -240 °C (33 K). It possesses a comparable composition to the ice giants in our solar system and is also similar in size to Uranus and Neptune at 4.00 Earth radii. The planet displays a white, blue and violet colour palette, with the atmosphere churned into motion by its internal heat. I have increased the light intensity in this simulation such that the object is visible, but it is likely a very dark and cold world.
  
• The unconfirmed planet Lalande 21185 d is included in this simulation and imagined as a cold arid super-Terra 1.46 times the size of Earth with an average temperature of -156 °C (117 K). It is not tidally locked to its star and rotates slowly on its axis every 5 Earth days. The planet is dotted by sparse clouds and jagged rocky peaks poking out above permanent glaciers. Despite the abundance of ice, it is still “arid” due to the lack of liquid water.

HD 95735, G 147-17, 2MASS J11032023+3558117, TIC 353969903, AC2000 1340883, G 119-52, MCC 594, TYC 2521-2279-1, AG+36 1043, GJ 411, NLTT 26105, UBV 10094, AGKR 9911, GSC 02521-02279, 8pc 392.40, UBV M 16736, AP J11032023+3558117, HIC 54035, PLX 2576, UCAC4 630-046224, ASCC 492123, HIP 54035, PLX 2576.00, USNO-B1.0 1259-00190202, BD+36 2147, IRAS 11005+3615, PM 11006+3617, USNO 837, CCDM J11033+3558A, JP11 2024, PM J11033+3558, uvby98 100095735, Ci 20 604, Karmn J11033+359, PPM 75640, WEB 9772, DO 14335, LAL 21185, RAFGL 411, WISEA J110319.67+355722.4, GAT 62, LFT 756, ROT 1651, XBS J110320.1+355803, GAT 962, LHS 37, SAO 62377, Zkh 149, GC 15183, LSPM J1103+3558, SKY# 21113, [GKL99] 233, GCRV 6818, LTT 12960, StKM 2-691, [RHG95] 1719, GEN# +1.00095735, 2MASS J11032027+3558203, TIC 166646191, Gaia DR3 762815470562110464

• Díaz, R.F., Delfosse, X., Hobson, M.J., Boisse, I., Astudillo-Defru, N., Bonfils, X., Henry, G.W., Arnold, L., Bouchy, F., Bourrier, V. and Brugger, B., 2019. The SOPHIE search for northern extrasolar planets-XIV. A temperate (Teq~ 300 K) super-earth around the nearby star Gliese 411. Astronomy & Astrophysics, 625, p.A17.
  
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