A real-life planet Vulcan and TESS's first find

[ProtoJeb21]

Two new interesting exoplanets were revealed today. First, the Dharma Planet Survey has found a large Super-Earth of about 8.5 Earth masses orbiting an orange dwarf every 42 days. It doesn't seem very special...until you realize that this planet orbits the star 40 Eridani, the system of the fictional planet Vulcan from Star Trek. While 40 Eridani Ab is nowhere near habitable - its equilibrium temperature is over 440 K and it likely has an extremely thick atmosphere - there could be more promising planets in the system that were small enough to initially evade detection.

https://academic.oup.com/mnras/article-abstract/480/2/2411/5056228?redirectedFrom=fulltext

The second discovery is in the system Pi Mensae, also known as HD 39091. Back in 2001 there was a massive (~10x Jupiter mass) gas giant discovered around the star in an eccentric 5.6-year orbit. The TESS spacecraft has currently been surveying the southern sky, including the constellation Mensa, and was able to take a look at this star. It managed to find a 2.1 Earth radius planet in a 6.27-day orbit, far closer than the huge Super-Jupiter of the system. However, the new planet is not even 5 times the mass of Earth with a thick volatile layer (hydrogen, helium, water, and methane) on top of a silicate mantle. For a planet of this size, such a low mass and density is unexpected; I would've guessed a mass of 6-9 Earth masses instead of 4.8. This could be explained if HD 39091 c formed further out and was on its way to become a Neptune-like planet, but the enormous gravitational influence of the migrating HD 39091 b threw it closer to the star before it had a chance to grow any more. The authors of the paper are hoping to analyze the system further due to its brightness and proximity, so more planets may be found in the coming months or years.

https://arxiv.org/abs/1809.05967

[Spaceception]

Ah damn. Gotta edit the HZ thread. I saw articles saying the planet was on the inner edge of the habitable zone. And given some K type-HZ planets can have shorter orbital periods I just figured this was simply the case as well.

 

What're the odds a smaller planet can be found further out?

Edited September 18, 2018 by Spaceception
Should've read a bit closer

[ProtoJeb21]

1 hour ago, Spaceception said:

Ah damn. Gotta edit the HZ thread. I saw articles saying the planet was on the inner edge of the habitable zone. And given some K type-HZ planets can have shorter orbital periods I just figured this was simply the case as well.

 

What're the odds a smaller planet can be found further out?

Unfortunately, not-Vulcan doesn’t reside on the inner edge of the HZ, and it is probably a Mini-Neptune-like planet similar to WASP-47e. For a similar composition of >90% rock/iron and <10% volatiles (water and/or hydrogen), the planet should have a radius of about 1.8-2.0 Earth radii. 

Smaller rocky planets in or out of the habitable zone could exist, although they’ll be difficult to find and will likely be Super-Earths. I calculated that to get anywhere from about 1.3 to 0.8 times the stellar flux of Earth, a planet around 40 Eridani A would have an orbit between 170 and 220 days. Considering how HARPS was able to detect 0.3 m/s radial velocity signals from <2 Earth mass planets around Tau Ceti, a Super-Earth orbiting in 40 Eridani’s habitable zone is within the limits of current technology, although it could take years to find it, if such a planet exists. 

[kerbiloid]

Spoiler

9 hours ago, ProtoJeb21 said:

While 40 Eridani Ab is nowhere near habitable - its equilibrium temperature is over 440 K

Quote

Much of its surface consists of deserts and mountain ranges, and large areas are set aside as wilderness preserves. It is much hotter, it has a stronger surface gravity, and its atmosphereis thinner than that of Earth. As a result of these factors, humans tend to tire out more quickly than native Vulcans.

Well, that fits...

 

[ProtoJeb21]

It turns out not-Vulcan is more likely to be a huge terrestrial planet than I initially thought. I just read that 40 Eridani A is pretty metal-poor, with a metallicity of -0.42 [Fe/H], meaning it has less than half the ratio of heavy elements (oxygen, nitrogen, iron, etc) to hydrogen/helium in it than the Sun. While this may seem like this would limit the size of terrestrial planets, that's surprisingly not the case. Protoplanetary disks around metal-poor stars lack the heavier materials to keep the gas around longer, so when rocky planets begin forming, they will have a harder time accumulating thick hydrogen-helium atmospheres that turn them into Mini-Neptunes. This is the reason for the abundance of small (<3 R_Earth) planets around such stars and explains huge rocky planets like LHS 1140b and K2-56b. Even though similarly large planets can form around high-metallicity stars, such as 55 Cancri e around 55 Cancri, some of them can retain heavier volatile layers of water, nitrogen, and other gases. The best example is WASP-47e, which I believe is more massive than currently believed because there is no way it could still retain such a thick volatile layer while getting 3,900 times more sunlight than Earth. Since not-Vulcan formed in a disk where the hydrogen and helium supply was quickly depleted, it's not hard to imagine that it managed to acquire significant amounts of silicates and metals to become terrestrial, although a thick atmosphere like 55 Cancri e is likely.

[ProtoJeb21]

Here’s a new paper on Pi Mensae c. It turns out that the planet is slightly smaller and denser than reported in the first paper, at about 4.5 Earth masses and just under 1.9 Earth radii with a density around 4 g/cm^3. It is at least 50% silicates by mass, either with a water envelope or a thick gaseous atmosphere above a water mantle. 

https://arxiv.org/pdf/1809.07573.pdf

Edited September 21, 2018 by ProtoJeb21