About Trappist-1

Star Trappist I, a super cool dwarf about 40ly away, has seven (7) Earthlike planets orbiting it, of which three (3) are in the Goldilocks Zone. (Trappist-1 e, f, and g)

Trappist 1e receives about as much light as Earth.
Trappist 1f receives about as much light as Mars.

Trappist 1e and Trappist 1f are both slightly smaller than Earth.
Trappist 1g is about 13% larger than Earth.

Arrangement and possible history of planets in system suggests they they might well be "super water rich".

There's lot of links out there I'm just summing what I see as the highlights.

A hastily assembled page from the member of the discovery team:
http://www.trappist.one/#

Wiki:
https://en.wikipedia.org/wiki/TRAPPIST-1

 

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I was interested in what it might be like to stand on these planets, so I did some calcs and converted them to diagrams.


......... radius . distance ..... ang star . ang b . ang c . ang d . ang e . ang f
Sol .... 1,400 Mm
Earth ... 12.5 Mm 140,000 Mm .... 0.5d

1 ........ 160 Mm
b ....... 13.6 Mm . 1,540 Mm .... 6.0d
c ....... 12.2 Mm . 2,100 Mm .... 4.3d .. 1.4d
d ........ 9.7 Mm . 2,940 Mm .... 3.1d .. 0.56d .. 0.83d
e ....... 11.5 Mm . 3.930 Mm .... 2.3d .. 0.32d .. 0.43d .. 0.56d
f ....... 12.3 Mm . 5,200 Mm .... 1.8d .. 0.21d .. 0.23d .. 0.25d .. 0.51d
g ....... 14.0 Mm . 6,300 Mm .... 1.5d .. 0.16d .. 0.17d .. 0.17d .. 0.27d .. 0.64d

This a schematic of the heavenly bodies (at closet approach) as seen from each planet , shown to-scale with Sol from Earth.

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For scale, Sol from Earth can be eclipsed by the tip of your pinkie nail at arm's length. That's about .5 degrees.
Trappist-1, from Trappist-1b is so large you would need to hold three fingers up to eclipse it. That's 6 degrees.

No planet spans more than 1.4 degrees as seen from any other planet.

So, there will be no solar eclipses in the Trappist-1 system, however mutual planetary occultations will be quite complex.

 

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......radius . density . surface g's . temp**
Earth 6370 ..... 5.5 ...... 1.00 ..... -18C
b ... 6940 ..... 3.6 ....... .71 ..... 127C
c ... 6750 ..... 6.4 ...... 1.23 ...... 69C
d ... 4900 ..... 4.9 ....... .69 ...... 15C
e ... 5800 ..... 5.0 ....... .83 ..... -22C
f ... 6660 ..... 3.3 ....... .63 ..... -54C
g ... 7180 ..... 5.2 ...... 1.07 ..... -75C

** equilibrium temp (assuming no atmo)

Since it is strongly suspected that all these planets are tidally-locked, one could simply choose a world-ringing zone - either sunward or space-ward of the terminator line - to find a perfectly comfortable climate.

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These bands are arbitrarily made up.

 

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What are the units and table heading abbreviations in your posts #2 and #3? How far apart are the planets, in say, "miles" or "kilometers"?

One thing I've wondered about these planets, is if there are any resonances between the planets -- 2 types:

1) Orbit speeds, like Neptune and Pluto have, so that they are not anywheres near each other?

2) Like Jupiter has with some members of the asteroid belt, keeping them from forming planets and in place in their own orbit before and after Jupiter in its orbit?

Do you have any information on this? I don't know that my post makes any sense though.

 

The units in post 2 are Megametres and angular degrees. (A megametre is 1000km, but using Mm means I can use fewer zeroes. )

The units in post 3 are km and g/cm3.

They are on the order of a million km, which about 4 times the distance form the Earth to the Moon.

Yes. The observations show resonances.

Unknown but unlikely.

 

You know, these planets are so close to each other (orbiting the star), and their orbital period is so fast -- from 1.5 days to 20 days. The orbit of all of them would reside within the orbit of our Mercury.

So, imagine yourself an inhabitant of one of these planets, trying to go to or send a spacecraft to the next planet inward/outward. It sure isn't going to take very long!, if you try the same type of route we take to Mars. But therein lay my problem/question: Would that be more difficult to send a probe to the next planet there than it is to Mars here? It's no gimme sending a probe to Mars; there have been a lot of failures, and that route takes the better part of a year. With orbit periods measured in days, it seems to me there has to be quite a large difference in relative speed between 2 successive planets there ???

 

Let's say that you wanted to travel from c to b on a minimum energy trajectory. You would need a delta v of 5.59 km/sec to get into the proper transfer orbit. Compare this to the 2.7 km/sec you need for the same transfer orbit to Mars. The differences in the delta v needed to match orbital speed on arrival will be of comparable magnitudes.

 

Thanks, I wouldn't have thought that. What about launch and injection windows? I know there are days length launch windows at Earth (going to Mars), but the dependency there might be only the amount of the delta-v our rockets can give to the probe. How much or precise of a launch and injection window would they have, if that can be answered?

 

The size of the launch window depends on how much extra delta-v you allowed for beyond that needed for the optimal trajectory. For example, with the trip from Trappist-1 c to Trappist-1 b, a 1% delta v reserve would allow you to delay your insertion into the transfer orbit by as much as 1.5 hrs. (this includes the needed adjustments to both the insertion into the transfer trajectory and orbital matching at arrival at Trappist-1 b) How much extra fuel this will require depends on the propulsion method (for standard chemical rockets it works out to ~2% additional fuel)

 

Another interesting view is how an outer planet would be seen from an inner one. Here's one example.
It is how Trappist-1 c would appear from Trappist-1b if watched from conjunction to conjunction if you could watch it the whole time (In reality for some small part of the time it would be behind Trappist-1.)

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At its largest apparent size it will be ~2.25 time the apparent size of our Moon, and at its smallest it will be about 40% the size of the Moon.

 

Also, all of the planets are likely to be tidally locked. So, a person's view of the outer planets would be mostly dependent on where they resided on that planet -- they would see only a portion of those above views during the outer planet's orbit, and each planet would be viewable 50% or less of its orbit (the other time it would be blocked by the planet the person resides on and maybe sometimes even the star/sun). It also seems to me the view would be essentially the same every orbit (if the orbits are circular).

That is for viewing the outer planets. For viewing the inner planets, I think that this would happen:

1) If you lived on the dark side of a (tidally locked) planet, you wouldn't be aware that there were inner planets????

2) If you lived on the light side of a (tidally locked) planet, if you could see them in daylight you could see them all of the time except when they were too close or on the other side of the star/sun????

I don't know if that is 100% correct, but it is interesting.

 

Yeah. Quite likely. Explorers, going to see what the brightness was all about would bring back tales of bodies with giant bites out of them*, travelling across the heavens.

*something that would be foreign to them, only seeing their outer bodies in full sun.

[ EDIT ] No. The planets are so close together, they could see their outer planets as ... gibbousous.

Yes, you'd see them in all phases.

 

Ha! That's true, and, they would tell of the beastly bright ball in the sky that was taking those bites out.

 

Excellent thread Dave. Thanks to you and all other posters.
Alex

 

there have to be simple resonances indicated by the orbit radii that increase for six of them by ~ .oo8 AU and 2 of them by half of that or double.

 

Does that mean they are never in opposition (never cross paths as seen by the star/sun), or just that planet "A" makes exactly "X" number of orbits to planet "B" making exactly "Y" number of orbits, "X" and "Y" being integers?

 

Resonance means that the periods of the orbits have a ratio that can be expressed as two integers. For example, if the inner planet completes 3 orbits in the same time as the outer planet completes three orbits, then they are said to have a 3/2 resonance. In the Trappist-1 system b-g are in near resonance, moving outward, the ratios are
b/c-8/5,
c/d-5/3,
d/e-3/2,
e/f-3/2,
f/g-4/3.
I say "near" because the measured orbital period ratios don't work out to be exactly these integer ratios but come close.
The variances are as follows:
8/5 = 1.60000000..... actual ratio 1.60293209
5/3 = 1.66666667...... actual ratio 1.67213273
3/2= 1.50000000..... actual ratio 1.50622282
3/2= 1.50000000..... actual ratio 1.50992973
4/3=1.33333333....... actual ratio 1.34125444

These small variances are likely due to the perturbing effects of the other planets on these resonance pairs.

 

Yeah. That is what I meant by my "Planet "A" and "B", and "X" and "Y"" part.

In my post #4 I was also asking about if there was anything like between Jupiter and the asteroids residing in Jupiter's orbit, and called that a resonance too. I see now it is more properly labeled as asteroids occupying Lagrangian points, or "Trojans", as in this wikipedia page:

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

I don't see any evidence of that -- or even an asteroid belt. Jupiter had something to do with that too (not forming a planet):

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

And I know you know that stuff. Of course there isn't any planet there the size of Jupiter. I just wondered that maybe with the planets that close together there might be some dynamics between them that would lead to novel mechanics, or mechanics that we would never have thought of.

 

Some resonances are destabilizing. This in particular can happen when dealing with small bodies under the influence of a much larger one. An example is the the Kirkwood gaps in the asteroid belt. These are located at distances where an asteroid would be in resonance with Jupiter. In this case, Jupiter's perturbing effect tends to sweep these bands clean of asteroids.

 

Careful here, you seem to be basing this on the Bolometric magnitude rather than the visual one.
The difference is that the Bolometric magnitude includes radiation beyond the visible light range.
Trappist-1, being a red dwarf, radiates heavily in the infrared range, which increases the Bolometric magnitude value.
Trappist-1's visible magnitude is much smaller. It's visual luminosity is ~1/140 of its Bolometric one.

Visible light levels on Trappist 1e would be much lower than that for the Earth.