Nearby Exoplanets Options

[JRehling]

There have been a few topics in recent years pertaining to exoplanets found circling nearby red dwarfs, particularly Proxima Centauri and Trappist-1. There's a new one to report, and I thought I'd give the topic a more general scope rather than specific to this one.

The star in question is Ross 128, and the planet's solar flux is between that of Earth and Venus. There's a good chance that this is potentially the most "habitable" exoplanet yet found, and is happily quite close (13th closest system), so that telescopes will be able to separate the light of the planet from that of the star. This is a circumstance that only a few nearby stars will permit in the foreseeable future, so Ross 128 is likely to figure large in our exoplanet studies over the next century.

https://www.eso.org/public/archives/release...36/eso1736a.pdf

[Decepticon]

A Earth-sized Moon sitting in orbit would be wishful thinking.

[JRehling]

If I had to guess, the large natural moon of a "Neptune" inside the ice line would be about the mass of Titania minus its ice mass, so more like Vesta than Earth, but I'm extrapolating from very low n!

For the hope of a close Earth analogue orbiting Alpha Centauri A, I think we have a better chance that this will be a false detection and a yet-undetected terrestrial planet will coincidentally be in its place. Or, that this planet is real and an earth-sized terrestrial planet will be outside of it, still in the habitable zone.

If this is a Neptune where we think it is, it would stand a good chance of holding this distinction: The exoplanet with the largest apparent diameter, and thus, likely the first we will ever image with detail… whenever appropriately powerful instruments come about.

[HSchirmer]

A Earth-sized Moon sitting in orbit would be wishful thinking.

Eh, not entirely out of the realm of possibilities, but I guess they'd be more likely to be water-worlds?

If I had to guess, the large natural moon of a "Neptune" inside the ice line would be about the mass of Titania minus its ice mass, so more like Vesta than Earth, but I'm extrapolating from very low n!

I will be interesting either way, especially because this may be our first glimpse of an ice giant with the primordial moon system intact. Given that in our solar system "we can't have nice things" and both our ice giants are a bit damaged.....

[JRehling]

This pushes the boundary of "nearby" a bit, but TESS has found a five-planet transiting system orbiting a sunlike star.

https://en.wikipedia.org/wiki/HD_108236

By the nature of TESS, it is a given that any of its discoveries are in short-period orbits, so while the solar system's fifth planet has an orbital period of 12 years, this system's fifth planet has an orbital period of under 30 days.

The estimated densities of the two inner planets are rather similar to Earth while the three outer planets have more Neptune-like densities.

High temperatures should make these planets rather unlike Earth but these are still of great interest because, as transiting planets only 211 LY away, these are still good candidates for follow-on study, because the apparent magnitude of the star (9.24) allows for good signal to noise ratio. For comparison, TRAPPIST-1, though much closer, is a red dwarf, and appears much dimmer. HD 108236's system may permit some breakthroughs in our understanding of the evolution of super Earths. Studies to characterize these planets will certainly book some time on JWST and terrestrial telescopes.

[JRehling]

A new important discovery: Gliese 486 b, a small Super Earth in a transiting orbit only 26 light years away:

https://arxiv.org/pdf/2103.04950.pdf

This is significant for one reason: The proximity and thus the apparent brightness of the star (magnitude ~11.4) make this planet's frequent transits extremely favorable for studying this planet's atmosphere and possibly surface composition. It is, in a nutshell, the single terrestrial planet that offers the best signal-to-noise ratio for such studies, (and it may possibly maintain that distinction forever against any future discoveries.)

The bad news… it's hot, almost certainly much hotter than Venus. This would be worse news if it turns out to be too hot to hold onto an atmosphere, and then the potential for such studies are moot.

It's a sure thing that this world will, along with TRAPPIST-1 and HD 108236, which I mentioned in my last post on this thread, leap to the top of priorities for observation time with JWST and other instruments capable of characterizing exoplanets during transits.

It could well be that within a short time, we'll know more about this planet than we knew about Venus in the 1940s.

[JRehling]

The initial set of TOIs (Tess Objects of Interest) from the main mission has been published.

https://arxiv.org/pdf/2103.12538.pdf

There are 2241 TOIs reported; the data here and its presentation is strongly analogous to Kepler data, even down to the nomenclature, while the astrophysical parameters differ quite a bit from Kepler. In particular, the vast majority of stars were observed for only 27 days each, and most of the rest were observed for only 54 days, which places an upper bound of any possible TOIs' orbital periods for such stars. Accordingly, the set includes a lot of planets that are in the habitable zone of M dwarfs or in the very hot zone of other stars. This is also unlike Kepler in that Kepler looked at one, small favorable part of the sky and certainly left open the possibility of future missions / campaigns conducting the same approach looking at other parts of the sky; in contrast, TESS is looking at virtually all stars of a certain kind, and is performing a kind of definitive and even final search for all such cases – "bright" stars all over the entire sky.

I can't speak with authority as to the characteristics of the data, but with the Kepler equivalent, KOIs, the expectation was that the vast majority actually correspond to planets but that verifying some of them would not be possible with Kepler data alone. I can say for sure that Kepler's instrument had unexpectedly high (and idiosyncratic, non-isotropic) noise and that led to a lot of false positives, and I would optimistically hope for better performance from TESS. I would further project that we can expect approximately 2000 of these to be real planets and that TESS's extended mission(s) will increase the harvest considerably.

All told, we can expect the number of known exoplanets to jump by about 40% as this data is reviewed. Virtually all candidates for "earthlike" planets that transit red dwarfs and are worthy targets for followup science should end up being known at the conclusion of this analysis.

[JRehling]

There's a long-running saga in progress about the violence of red dwarf stars, and rather than try to capture it all, I'll post the latest installment. There is considerable concern that the extremely active nature of red dwarfs – violent flares that increase the stars' brightness by more than 100% in high-energy wavelengths – might blast every otherwise "earthlike" planet orbiting them into an airless state like Luna rather than like Earth.

However, the last big story offers hope: Those flares may congregate at high latitudes, aiming the flares away from the plane of nearby planets.

https://www.washington.edu/news/2021/08/05/superflares-tess/

Over the next two decades, we'll be getting empirical data as we start probing known exoplanets' spectra for signs of atmospheres, and that will be the answer that really matters.