I discovered something spectacular completely by accident. I was getting ready for the announcement of the discovery of the extra-solar planet Kepler-186 f. You remember, the Earth-sized planet in the habitable zone? It was all over the news (even in French) just a couple months ago.
I made an animation of the Kepler-186 system. The planets went around and around on their orbits. The brightness of the star dipped every time a planet passed between Earth and Kepler-186 (the star).
Here is what the movie looks like (for the month of June):
Each blip in the star’s brightness is a transit. That is when a planet passes between our telescope and the star, blocking a little bit of the star’s light. See that weird blip around June 15th? Let’s watch that part again in slow-motion:
That blip is deeper than the others. It’s got a weird shape too. It’s not from one planet blocking the light from the star, it’s from three! As seen from Earth (or really, from the Kepler space telescope), three planets pass in front of the star at the same time! A triple transit! They are the first-, second- and fourth-closest planets to the star. It would be nice if the habitable zone planet were one one of them. But hey, this is still pretty awesome!
My friends that work on Kepler told me that a triple transit has already been seen (for example, in the Kepler-20 system). What would really be spectacular is if two of planets passed in front of each other while they were blocking their star. The shadow of one planet would fall on the other. A planet-planet eclipse!
This got me really excited. I went ahead and built a model for what will happen on June 15th. Here is what it might look like when the planets pass in front of the star:
Planet c (second planet from the star) starts to transit first. Next comes planet e (fourth) then planet b. Planet b is closest to the star so it orbits fastest. It catches up with planet e and the two planets briefly overlap. This causes a short-lived increase in the brightness of the star. Instead of two separate planets blocking the star, during the planet-planet eclipse there is effectively just one.
What’s so special about planet-planet eclipses anyway? They are powerful tools for studying the orbits of the planets. The shape of a planet-planet eclipse is one of the only ways to determine the inclination (the tilt) between two planets’ orbits. Among all the Kepler data only one clear planet-planet eclipse has been found. These are rare but extremely powerful. They are basically the Bengal tigers of astronomy!
Now, we don’t know the exact path of each planet across the star. The animation I showed is for a lucky geometry. It turns out that there is about a 50% chance of a triple transit happening. If planet c’s path across the star is too high-up or low-down (if its “impact parameter” is too large) then it is already done before planet b’s transit starts.
The planet-planet eclipse is a relatively low probability event. This is because the planets are so small compared with the star! You need a “lucky” setup for them to pass in front of each other. A planet-planet eclipse has just a 5-10% chance of happening. Well, there is never a super high probability of seeing a Bengal tiger except at the zoo. And astronomy takes place in the wild (believe me)! Still, a 5-10% chance of finding something historic seems worth a shot.
Here are three possible configurations of the planets during the June 15th transit:
How could we observe this? The whole thing lasts about 6 hours. The triple transit — when all three planets are in front of the star — can last anywhere from not at all to an hour. The planet-planet eclipse (if it happens) only lasts about 10 minutes.
The signal is small. Each planet only blocks a few ten-thousandths of the star’s light. We need to be able to detect a change in brightness of the star that is that small. And we need to do it fast, since some of these events may only last ten minutes! Plus, Kepler-186 (the star) is not very bright. So this is a very challenging measurement. There is only one telescope capable of making these observations: the Hubble Space Telescope.
I put together a team of experts. People who know how to make this kind of observation happen. Some spectacular people: Avi, Brice-Olivier, Philip, Darin, Elisa, Tom, Franck, Jason, Daniel, Franck, and Emeline.
There are a couple of issues that make the observation with Hubble tricky. First, the telescope has very little on-board memory. We want to carefully measure the brightness of Kepler-186 every minute or so. But there isn’t enough memory to store all the images. And downloading the images to Earth takes about 5 minutes, which would leave big holes in the signal. The solution was not to simply point the telescope at the star but rather to slowly drift past it. That way, the star’s light would be spread out across the camera (after being already passed through a “grism” to disperse it by color). Different parts of the chip would represent different times. The details of this were tricky but a couple of great observers (Avi and Brice) figured them out.
Another problem is that Hubble orbits very close to the Earth. It is in low-Earth orbit. It takes about 96 minutes to go around the Earth once. Hubble can only see our target star when it is not blocked by the Earth. Unfortunately, the star spends almost half of each Hubble orbit out of view, behind the Earth. This leaves a 45-minute hole in the data. A planet-planet eclipse or triple transit is shorter in duration than the length of the observing window. So even if they happen, there is a 50/50 chance that Hubble would miss them. This is a bummer but there is no way around it. The bad thing is that it drops the chance of Hubble seeing a triple transit to about 25%. And the chance of Hubble seeing a planet-planet eclipse to 3-5%.
I was really excited so we kept going. We wrote a proposal to observe Kepler-186 on June 15th. We couldn’t pass through the normal proposal process because this was happening so soon. Normally you have to propose to observe something (a star, galaxy, planet, …) up to a year in advance. I had only discovered the existence of this event in March! So we applied for special, last-minute observing time (“director’s discretionary time”). I sent in the proposal in early May.
Drumroll ……… and a big frowny face. A week later someone at Hubble got back to me. They appreciated the proposal but did not award us any observing time. Bummer!
Why didn’t we get the time? Well, I can understand their point of view. A triple transit is awesome, but we wouldn’t learn any more about the planets than we would from three separate normal transits. In some cases a precisely-timed triple transit could help figure out the planets’ masses (using the “transit timing variations” technique). But, Daniel found that the triple transit wouldn’t help all that much.
Observing a planet-planet eclipse in the system would be spectacular. As I mentioned above, only one has ever been found before. And that was for bigger planets: a super-Earth and a Saturn-sized planet. The possible planet-planet eclipse on June 15th is for two roughly Earth-sized planets. Plus, it is in a very high-value system that includes a potentially habitable planet (and maybe another one).
A planet-planet eclipse would tell us the inclination (tilt) angle between the projected orbits of planets b and e. This would be very interesting to know. A small inclination would tell us that the planets are located in a thin disk. But wait! Don’t we already know the answer? Well, sort of. There is only a small chance of ever finding a system like Kepler-186 with five transiting planets unless the planets’ orbits are confined to a thin disk. So, if Hubble saw a planet-planet eclipse it would almost certainly measure a very small inclination between the orbits of planets b and e. I can see how the Hubble reviewers may have thought that we would not learn anything really new.
There are two strong counter-arguments to this line of thinking. First, we shouldn’t place too much faith in models. I mentioned that we think we already know the answer, that the planets’ orbits must be confined to a thin disk (like a Frisbee). But what if we are wrong? That’s not impossible. And it would actually be much more interesting if our guess was wrong. I think it’s worth testing.
I mean, the transit could look like this:
In this example, the orbits of the two planets that eclipse are inclined by almost 90 degrees with respect to each other! Although I think it’s unlikely, we cannot rule out that this is the true configuration.
My second counter-argument is that planet-planet eclipses are just so so so rare! Among the tens of thousands of transits seen by Kepler, only one planet-planet eclipse has been found. Even a small chance (5%) of finding another seems worth going for. Imagine this: by going through a big nasty dumpster you have a 1 in 20 chance of finding a diamond the size of an apple. Would you do it? The odds are not great, but the potential payoff is spectacular. I would totally do it!
Finally, the next triple transit in the Kepler-186 system won’t happen until the year 2047! I’ll be 70 and probably more interested in controlling things with my mind than in looking for transiting planets. Plus, I’m impatient. I don’t want to wait!
In the end, I don’t blame the people at Hubble who decided not to implement our proposal. They have a very hard job. They get asked for 10-20 times more observing time than they can give. It’s not easy to decide who gets it. I am bummed about it, but I understand their decision.
SUMMARY. This was a spectacularly fun project. I really enjoyed it. I learned all sorts of new things about transits and observing (and making animated gifs). I made some great contacts. I really went for it with the Hubble proposal. I did my best to make it happen. I thought we had a good shot at getting the observing time. But we didn’t get it. I’m disappointed but I feel good that I didn’t hold back.
The main reason I am bummed is because I will never know if a triple transit or planet-planet eclipse happened on June 15. And no one else will either.
UPDATE: After sharing this post, several colleagues told me that they thought that the triple transit might be detectable using a ground-based telescope (as opposed to space-based). My good friend Stephen Kane was able to secure the night of June 15th on a 2-meter telescope at the Indian Astronomical Observatory. This was the right longitude to be able to see the entire triple transit from a single location.
Another drumroll…. and another big frowny face! Patchy weather. Bad seeing. No useful data. Bummer!
That spanking new planet’s already a star.
K-186 f, you know who you are.
You’re making us wonder if we’re all alone.
The planet out there in the habitable zone.
I’ve been on the radio. Been on TV.
Talking ‘bout the planet. Just what can we see?
Just what do we know about this special rock?
Are there little green men? Just how do they talk?
We don’t know nearly as much as we’d like.
And five hundred light years is kind of a hike.
But if there might be life, I’ll clean off my bike.
Or rather my rocket. I’ll set it to zoom.
Or maybe I’ll dust off my old wizard broom.
Wait, stop! This is heading in the wrong direction.
The planet’s too far for a close-up inspection.
If we blast our fastest rocket high up into space
It’ll be 10 million years ‘til we reach that far place.
So we’ll just have to rely on our telescopes,
computers and brains. That’s right, we’re no dopes.
To figure things out we need to stop blundering.
We need to chill out and do some good wondering.
Let’s take a close look at the entire system.
Are there more planets? Might we have missed ‘em?
If there are more planets, they will need space.
Now is there anything that’s out of place?
The four inner planets are crammed like sardines.
No space for another to fit in between.
But then there’s a gap and it’s pretty wide.
You could easily fit another planet inside.
So I sat down and ran some simulations
on my computer. And these calculations
show that a planet really can stick around.
Right in the gap. The gap we just found.
Another planet? Wouldn’t we find it?
Well, not if its orbit is out of alignment.
If it’s just tilted by one small degree
we probably would miss it. It’s that hard to see.
But this extra planet should have the same girth
as the other planets. About like the Earth.
Just like the others it’s a rocky place.
One more rocky planet floating out there in space.
This planet wouldn’t be boiling or freezing.
It should be quite warm. It would be quite pleasing.
It could have oceans and great lakes and rains.
And rivers that wind their way across the plains.
It would be number two in the habitable zone.
Its big brother would not have to be all alone.
Two planets with water, one near and one far.
Two places for life around the same star.
Of course I am being a little bit careless.
It may be, in fact, that these planets are airless.
All we really know is planet f’s size.
And hints there might be this extra guy.
The trickiest part is, how can we find it?
The planet that’s sitting there out of alignment.
I think that it’s there but I have no proof.
And the Kepler satellite last year went poof.
Now Kepler’s back but it’s not as strong.
So it can’t go back to check if I’m wrong.
For now there’s no other ‘scope in the land
or even in space or New York or Japan
that can find this planet. It’s just too faint.
So maybe it’s in there or maybe it ain’t.
I hope that we’ll find it, I’ll never say never.
But someone will have to do something real clever.
I hope it comes soon. Hope it’s not too slow.
I think that it’s there. But for now we don’t know.
If you find a planet you don’t get to name it.
The planet’s not yours. You don’t get to claim it.
But this extra planet has not yet been found
so I don’t think that anyone will make a sound
if I call this planet a special thing.
Planet Marisa, that has a nice ring.
In the spirit of last week’s poetic post, here are two more stanzas for your reading pleasure….
There is a new exoplanet in town.
This planet has only just now been found.
Why should you care? It’s only one more.
Well this is one planet we’d love to explore.
This planet’s orbit is really just right.
It isn’t too cold. You won’t freeze at night.
And it isn’t too hot. You won’t get sun-baked.
In fact you could go take a dip in a lake.
OK, the rhyming stops there…. Let’s get to business.
I want to introduce a newly-discovered planet called Kepler-186 f. I was lucky enough to be part of the team that found this planet (it was led by scientists at NASA and the SETI Institute). What is special is that the planet has the right temperature for water to be liquid on its surface. All life on Earth depends on liquid water, so this is a pretty big deal! Disclaimer: this planet needs to have the right kind of atmosphere and the right composition to actually have liquid water on its surface. We don’t know anything about the planet’s surface or atmosphere.
Kepler-186 f is the most distant planet in the Kepler-186 system. The system was discovered by the remarkable Kepler space telescope. Each of the five planets in the system is slightly bigger than Earth, but none is more than 40% larger. This means that the planets are probably rocky or at least have solid surfaces. Only bigger planets — with sizes larger than about 1.5 to 2 times the size of Earth — are likely to be gaseous “mini-Neptunes”.
Below is an artist’s view of Kepler-186f. You can see the four other planets closer to the star. One is even about to transit in front of the star!
We don’t know anything about the planet apart from its size and orbit. So this is just speculation. Still, let’s let our imagination run wild. To be consistent we need to take into account the fact that the planet’s host star (Kepler-186) is different than the Sun. Kepler-186 f’s sky is not blue because there is not enough blue light reaching the planet. There are also different kinds of clouds than on Earth. As we’ll see below, this planet needs a decent amount of greenhouse heating to maintain liquid water on its surface. This heating could very well come from a relatively dense atmosphere containing carbon dioxide (CO2). If that is the case, then complex climate simulations tell us that Kepler-186 f should have multiple cloud layers. A layer of water clouds should exist at a similar altitude as on Earth. Another layer of CO2 ice clouds should be located much higher up and cover a significant fraction of the planet’s surface. But watch out — the clouds can’t cover up the surface and make the image boring! Everyone wants the planet to have vast oceans but some continents!
This image compares the orbital layout of the Kepler-186 system with the Solar System:
Kepler-186 f’s orbit is located in the outer parts of the habitable zone. It receives one quarter to one third as much energy as the Earth does from the Sun (within the errors bars on the observations). This is less than Mars. The planet needs an atmosphere to heat it up. This heating can come from greenhouse gases in its atmosphere. The most common greenhouse gases in the Solar System are water vapor and CO2. An atmosphere including CO2 (as on Mars and Venus) and a little Nitrogen (as on Earth) can heat the planet’s surface sufficiently for water to be liquid. The exact amount of CO2 and Nitrogen that are needed depend on exactly how much energy the planet receives from the star. It’s comparable to the density of Earth’s atmosphere. Punchline: Kepler-186 f does indeed belong in the habitable zone.
Below is a comparison between the layout of Kepler-186 and three other planetary systems that contain a planet in the habitable zone.
Kepler-186 f is not the hottest planet out there. It is indeed toward the outer parts of the habitable zone. However, it does receive a little more energy from its star than the outermost planet in the GJ 581 system (GJ 581 d). That planet has been studied like crazy. It has been convincingly shown that it represents a potentially habitable planet, as a reasonable atmosphere (with CO2) can heat the planet enough for water to be liquid on its surface.
What makes this new planet special? Well, it’s not the first planet discovered in the habitable zone. And it’s not the first probably-rocky planet discovered in the habitable zone. It is the smallest planet found in the habitable zone so far. But there will certainly be even smaller ones found in the near future. There is no reason to get caught up on labeling this planet the best or more interesting. In my opinion, this is an important planet because it is a stepping stone on the path to finding true Earth analogs around other stars.
Also of interest: simulations of the formation of the Kepler-186 system systematically predict that an additional planet should exist between planets e and f. Another planet could easily “hide” in there and not be detected by Kepler. Such a planet could be in the inner parts of the star’s habitable zone. Pretty exciting (but speculative).
More information: The paper presenting the discovery of Kepler-186 f (led by Elisa Quintana) comes out in Science April 18, 2o14 and can be accessed here. The companion paper presenting a study of the system’s formation, tidal evolution and habitability (led by Emeline Bolmont) was submitted to the Astrophysical Journal and can be accessed here.
Questions, comments, words of wisdom?
Here is a tale ’bout a bright star named Vega.
For years, astronomers have combed through their data
and discovered that Vega is more than a star.
It’s surrounded by dust clouds: one near and one far.
Next to the star is some dust that’s quite hot.
We’ve only just found it. There isn’t a lot.
Farther away, there’s some dust that’s much colder
It comes from collisions between big icy boulders.
The hot dust’s a mystery; it shouldn’t be here.
Every speck of that dust should be gone in a year.
So Vega must have a constant supply
of fresh dust that’s brought in from somewhere on high
and thrown down real close to the star to get fried.
We talked quite a lot, Amy Bonsor and I.
Could a system of planets lie in Vega’s sky?
A system of planets on orbits between
the cold and hot dust and remain sight-unseen?
The planets could orbit right there in the middle,
toss comets from the freezer down onto the griddle.
This means the hot dust must come from the cold.
And make Vega’s warm dust in our very own mold.
Zodiacal dust is found close to the Earth.
But it comes from comets which, at their birth
condensed in the cold, way out far from the Sun.
Then were brought in by planets, kicked in by each one.
For this to work Vega’s planets must shift
They can’t empty out all the comets too quick.
And the outermost planets can’t be too big
or they’ll clean house and shut down the whole gig.
The planets serve as a conveyor belt.
Comets are delivered inward to melt.
But a gas giant planet far out in the chain
would break down the belt and the hot dust will wane.
So that is the story. We think that Vega
has a system of planets. We hope that new data
will find these planets within a few years.
For the gory details please lend me your ear,
Or rather your eyes. The paper’s right here.