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Nearby Exoplanets
JRehling
post Nov 15 2017, 04:17 PM
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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
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Ron Hobbs
post Nov 15 2017, 09:26 PM
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Excellent! Thank you for the link to the article.

Here is a link to the ESO release, which has cool artistic impressions videos.

https://www.eso.org/public/news/eso1736/

I look forward to hearing more about this system.

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JRehling
post Nov 17 2017, 04:17 PM
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A quick look outward:

Ross 128 is the 8th closest single red dwarf star. So far, we know of two ~earthsized planets in their ~HZs. Not a significant sample, and we don't know how many are yet to be found, but this is certainly suggestive that about a third of such stars have such planets. (The Doppler method can easily miss planets in orbits more or less perpendicular to our line of sight.) The true number may be much higher, because we don't have clear negatives yet on the other 6 of those 8.

Now, between Ross 128 and 20 light years out, there are 36 more single red dwarfs. Between 20 and 30 light years out, there are 75 more. And of course, there are 6 farther than Proxima Centauri and closer than Ross 128, making 117 more red dwarfs within 30 LY. If a third of those have such planets, we have dozens yet to find.

This says seriously good things about the sample we are going to be able to examine with followup studies.

The closest such transiting planet, however, may be quite a bit farther, but distance won't be a serious impediment for detection and characterization of those.

And, of course, there are also many multiple systems and bigger, hotter stars. We already know that the closest single non-red dwarf, Epsilon Eridani, probably has planets.

It's going to be an interesting few decades ahead for studying nearby terrestrial planets.
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JRehling
post Dec 6 2017, 04:40 PM
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A best-of-its-kind discovery: K2-18b (announced in 2015) is a transiting Super Earth 111 light years away. It orbits a red dwarf with a period of 32 days and has an equilibrium temperature very nearly equal to Earth's.

What's good: It transits! Studying the planet's atmosphere, therefore, can be done as the star's light passes through it, or by subtracting the planet's light from the system's as a whole when the star eclipses the planet. There is no need to resolve the planet, so the distance is not highly relevant to follow-up science.

This has also allowed an accurate measurement of mass and therefore density, and this reveals that the planet has a very earthlike density with 2.24x the radius and 8x the mass. This is itself a striking result as part of a new and growing survey of Super Earths, and the mystery of whether they are more like big Earths or little Neptunes. In most cases, we know the radius or the mass but not both.

What's bad: If we're looking for Earth analogues, the larger size may still mean that the atmosphere and climate are radically different from anything we've seen before. On the other hand, with red dwarf systems, there's a fear that flares could strip away an atmosphere and in this case, those two concerns potentially offset one another. Maybe a big planet with this high escape velocity could end up with an intact atmosphere that could even be more earthlike if a bit of it has been blasted away.

Add this to the short list of ones that we'll be watching closely over the next decade +.
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JRehling
post Feb 12 2018, 02:56 PM
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New clues to the density (and composition) of the Trappist-1 exoplanets:

https://www.jpl.nasa.gov/news/news.php?release=2018-022
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JRehling
post Nov 15 2018, 04:38 AM
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An article in Nature announces the discovery of a cold super-earth orbiting Barnard's Star.

https://phys.org/news/2018-11-astronomers-s...rnard-star.html

I wonder if some super-earths might have warm surfaces due to greenhouse effect from a thick atmosphere, but I will leave that up to the experts.

This would mean that both of the closest red dwarfs and at least three of the closest four have planets orbiting them. The evidence is shaping up that nearly all red dwarfs have planetary systems, although the 5th and 6th closest, Luyten 726-8, are a binary pair that come within 2.1 AU of one another, which might make that system in particular an unusual case. This also makes five planetary systems within 11.1 light years. There could also be more with smaller planets, more distant planets, and/or an orbital inclination that hides planets from the radial velocity method.

Intrigue grows for the eventual observations from JWST (2021) and ELT (2024).
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Steve G
post Nov 15 2018, 01:46 PM
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Bernard's star was the first star ever to have a suspected planet. In the sixties Peter van de Kamp claimed that he had detected a perturbation in the proper motion caused by a Jovian-class planet. It was eventually refuted but I'm glad to see the star finally has been confirmed as having a planet. Exciting news to have something so close to home.
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JRehling
post Nov 15 2018, 06:24 PM
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Yeah, Steve, my public library had a book that flat-out stated that Barnard's Star had planets, so it's a sweet resolution to that long-running narrative – and, as is clear, this discovery cannot be the planet(s) that were claimed long ago.

I took a picture of Barnard's Star this summer so I can make a "video" of its proper motion over a span of year(s) – it's a pretty little dot, more orange than red. One distinction held by this system is that it's the closest planetary system (and closest star, period) that can be seen from north of the tropics.

With a nominal orbital distance of 0.4 AU, Barnard's Star b will have about 5 times the angular distance from its star that Proxima b will from Proxima Centauri. That is much less angular separation, however, than cases like Fomalhaut b, which is much farther from its primary than Neptune is from the Sun. Barnard's Star b will likely be one of the easiest exoplanets to resolve and one day reveal non-pointlike images of its surface.
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JRehling
post Jun 19 2019, 04:48 PM
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Various updates:

Two planets, potentially habitable, have been discovered orbiting Teegarden's Star:
https://www.nationalgeographic.com/science/...eegardens-star/

A visual (IR) search for planets orbiting Alpha Centauri A and B is now underway, with observations already far along. It is not likely that a close Earth analogue could be observed now, but other planets whose existence could either boost or deflate the possibility of an Earth analogue could be detected.
https://www.space.com/alien-planets-alpha-c...ough-watch.html

Aggregating reports from different researchers, we now have reports of 7 ~Earth-sized and ~Earth-temperature planets orbiting 6 of the 17 closest red dwarfs, all within 14 light years. This is in line with Kepler estimates of planetary frequency suggesting that "eta Earth" for red dwarfs is on the order of 0.4 to 0.5 such planets per star. (FWIW, Barnard's Star is probably excluded from the possibility because of the detection of other planets orbiting it, with observations that could have detected an earthlike planet if it existed.)

Currently, no planet discovery methods except the transiting method (which requires exceptional luck regarding orbital inclination) is capable of having discovered Earth-sized and Earth-temperature planets orbiting K or G stars, and any such discoveries will likely require visual detection via JWST or 30-meter class ground observatories. This is sensitive to the proximity of such systems, so there are probably four that stand above the rest in future searches: Alpha Centauri A and B, with their unique closeness; Epsilon Eridani, known to possess larger outer planets; and, Tau Ceti, known to possess super-Earths bracketing the habitable zone on both sides with an intriguing gap in between that does not contain any larger planet and which, given known systems, is highly suggestive that some smaller planet could be present right in the habitable zone.

These approximately 11 planets + opportunities largely define the set of any possibly habitable planets which will be characterized with spectroscopy as that capability comes online in the next few years. Of the four M and G stars where we only hope for such a planet to exist, the expected number of such planets is roughly 1, but we will simply have to wait and see if that number is higher or lower. It is not impossible that more candidates will emerge among the remaining nearby red dwarfs, though in many cases, the current status of non-detection equals evidence against the existence of an Earth analogue. Overall, the number of nearby Earth analogue candidates is plausibly between 7 and 15, with the most likely number to be approximately 9.

These nearby non-transiting exoplanets define one major direction for future study. Transiting exoplanets (Earth analogue or otherwise) allow a completely separate technique for followup study, and allow for systems at greater distance to be characterized, but, again, depend on the exceptional luck of a transit, which will only apply to very roughly 2% of cases.

Exciting times!
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ngunn
post Jun 21 2019, 08:17 AM
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The Science Daily article contains these sentences:

But the system is located at a special place in the sky: from Teegarden's star you can see the planets of the solar system passing in front of the Sun.

"An inhabitant of the new planets would therefore have the opportunity to view the Earth using the transit method," says Reiners.


I plotted the RA and Dec in my little star atlas and can see that it certainly lies close to our ecliptic plane. However the planets in our system are very widely spaced compared with Teegarden's, TRAPPIST 1 and the like, also their orbital planes differ significantly, so I'm wondering which ones actually do transit as seen from Teegarden's star? They cannnot all do so for sure. (I've found a crude animation that appears to show them doing so but it uses coplanar orbits and is all out of scale.) Can anyone point me to some good information on this?
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Hungry4info
post Jun 21 2019, 10:46 AM
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The claim in the paper was a bit more nuanced than it has been reported. Right now only Mercury transits, but the others will soon because of the star's high proper motion. The attached figure is from the paper.
Attached thumbnail(s)
Attached Image
 


--------------------
-- Hungry4info (Sirius_Alpha)
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ngunn
post Jun 21 2019, 11:19 AM
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That's perfect, thanks! The figure shows nicely how no external observer anywhere could discover all our planets by the transit method.
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Gladstoner
post Jun 21 2019, 11:39 PM
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Interesting.

Plus, for each Sol system planet, a band can be projected onto our celestial sphere which would contain all stars from which a transit could theoretically be observed. In the case of the earth, it would be 1/2 degree wide along the ecliptic.
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JRehling
post Jun 24 2019, 01:55 AM
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QUOTE (ngunn @ Jun 21 2019, 01:17 AM) *
the planets in our system are very widely spaced compared with Teegarden's, TRAPPIST 1 and the like


A perhaps-transparent explanation: Tidal dynamics vary with the third power of distance, so close-in (portions of) systems are much more likely to be controlled by tides, with planetary orbital inclinations clustering near that of the star, and therefore near one another. This is therefore true of "habitable zones" of red dwarfs and, preferentially, most multi planet systems whose planets were discovered by the transit method.

The future work in characterizing exoplanets with spectroscopy (particularly ones that aren't extremely hot) will to a fair extent break down into a 2x2 matrix: {red dwarf, sunlike [KG dwarfs]} x {nearby enough to allow separation and direct imaging, transiting planets}. By and large, nearby and transiting end up [almost?] totally exclusive because of the low probability of an inclination that allows a transit.

Of those four possibilities, red dwarf + direct imaging will have a very small set of possibilities that could even begin and end with Proxima b for the time being, but hopefully the technology and circumstances will allow a few more. Transiting Earth analogues orbiting sunlike stars will be hard to study, too, for the simple reason that the long orbital period means a long wait between observations, and a few hours once a year means very limited signal-to-noise ratio and a serious constraint for earthbound observatories, which spend half the time in daylight.
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JRehling
post Aug 8 2019, 02:27 PM
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Here's a fresh update on the specific and unique case of Alpha Centauri:

https://www.scientificamerican.com/article/...tauris-planets/

It's interesting to know that relatively massive planets have been ruled out for Alpha Centauri A and B, which leaves us with a large probability, especially for B, that there are either terrestrial planets or none at all. It's also interesting to note that the increasing distance between A and B makes the search easier all the time. Finally, the best and latest visual search will post its results in October.

I'd add that the orbital period of any putative earthlike planet will compare to one year, which means that a visual search, to be thorough, has to include observations spread over at least a year because in any given month, a putative planet might be poorly positioned, either in terms of separation between the planet and its primary star or being positioned for the time being on the side that is unfortunately closer to the other star in the binary pair, and thus experience more stray light interference.

It seems likely that the ELT will resolve the issue definitively, either giving us the ability to perform spectroscopy on terrestrial planets orbiting Alpha Centauri or to establish that any planets there are too small to be very earthlike.
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