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Extrasolar Transits
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Imagining Other Earths
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Kepler and Transits
Transits have been an important event for astronomy for over 400 years. This lecture describes the transit of Venus and how it was used to measure the size of our Solar System. This lecture then discusses how NASA’s Kepler mission observes planetary transits and how its observations have shaped our understanding of the properties of extrasolar planetary systems.
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David SpergelCharles Young Professor of Astronomy on the Class of 1897 Foundation and Chair
Department of Astrophysics
Just as the observations of transits, of Venus around our sun,
broaden our understanding of our solar system 250 years ago.
The study of transits of extrasolar planets around their host stars
is playing an increasingly important role in the classification and
discovery of new planets.
The idea here is very similar.
A planet passes in front of its host star, producing a dimming.
And by detecting this dimming, which can be quite small,
the Kepler mission which will be the focus of the rest of this lecture,
has detected dimmings of one part in a million.
This tiny diminution in the brightness of the host star
can lead to the discovery of the planet, and measurements of its period and size.
The key mission for studying extrasolar planets through
the transit method has been NASA's Kepler mission.
This is a dedicated mission that was designed just to look for
transits basically.
The mission stares at one region of the sky.
This is what it did for the first part of its life.
This one region of the sky was a rich region of our own Milky Way.
So what it was doing was looking off towards one of the spiral arms
of our own MilkyWay galaxy, and it was detecting planets around stars
in this cone, looking out about 3,000 light years.
About 1000 parsecs away from our Sun, towards the constellation Sagittarius.
You can see what a powerful mission this has been by looking at
the number of planet discoveries.
This plot here shows the number of new planets discovered.
First in blue by all other methods, then in red by the Kepler mission.
And here's the most recent Kepler announcement.
And you can see that Kepler has vastly increased the number and
also the type of extrasolar planets known to humanity.
The Kepler mission has found not just large planets,
planets the size of jupiter or larger.
But Kepler has discovered large numbers of planets that are earth-size and
super earth-size.
One of the real surprises but,
about the Kepler mission, was when we looked at our own solar system,
we saw planets the size of Earth, Earth and Venus, even smaller, Mars and Mercury.
Large planets, like Uranus, Neptune, Jupiter, and Saturn.
But in our own solar system, there was a gap between the size of the Earth
up to four size, times the size of the Earth.
We didn't know of any planets whose radii were 1.5 or 2 or
3 times the size of the Earth.
Many people had inferred that this was telling us something about
planet formation.
But what we've learned from Kepler is that there's actually a continuity,
are planetary properties, and many super earth sized planets.
And these super earth sized planets are quite common.
Kepler has also revealed a number of exciting planets.
Planets that are potentially habitable.
One of the ones that was discovered in April of this year, is Kepler 186-f.
Kepler 186-f is part of a multi planet system.
And the final part of this lecture we're talk more about these multi
planet systems.
And it's orbiting together with it's four companions around a red dwarf star.
Recall that a red dwarf star is a star that's dimmer than our Sun, so
you need to be closer to the star in order to be habitable.
But Kepler 186-f which is roughly 10% times bigger than the Earth,
is in about 130 day period around it's host star, so is significantly closer.
Close enough that it's surface temperature lightly lies between
the surface temperature of Earth and Mars.
And this is an example of an Earth-like planet in the sense of
it's characteristic temperature and size.
And perhaps it's a planet that's capable of hosting life.
All we know so far about 186-f is its size and
its distance from the star, and the, and the shape of its orbit.
We don't know yet whether it hosts an atmosphere.
Whether it has water, whether it hosts life.
We will need missions beyond Kepler to be able to make those kinds of
discoveries about extrasolar planets.
This shows the region searched by Kepler, and
shows the range of sizes that Kepler has discovered.
And one of the things as we mentioned already with 186-f that's been very
exciting about Kepler is its had the sensitivity to discover planets small
enough that they are sometimes even smaller than the earth.
When we talk about Kepler discoveries.
The Kepler team divides them into two groups.
First objects are identified,
our transitive events are identified as potentially interesting.
There are so called Kepler objects of interest.
And there's over two thousand of those.
These are likely to be planets.
The exact probably is a subject of some discussion but,
say, roughly 90% of them probably are planets.
But they've not been confirmed by other methods.
Among the Kepler candidates, there's a smaller subset.
And these are numbered, for example, Kepler 20-F is a system that is
a confirmed planet that was promoted from a Kepler object of interest.
A likely suspect that contains planets to a system that definitely hosts planets.
Now, let's turn to what we actually measure with Kepler.
What we measure with Kepler is a transit.
So this plot below shows what kind of data you see from Kepler.
The brightness of the star is measured as a function of time.
And here you can see the star is close to
constant; no detectable variation in its brightness.
Then the planet passes in front of the star.
When it does, it blocks some of the light, and the brightness of the star drops.
This is a relatively large planet close to the star.
As a result, it produces a drop in the star's brightness of 0.6%.
That's large by Kepler standards.
Kepler's actually sensitive to much smaller drops in brightness.
As you can see, the fluctuations due to noise in measurements are quite small.
So what we can measure is the depth of the suppression.
We can measure it's duration.
We can even measure the shape of this curve.
And all this is telling us about properties of the planet.
We can also measure the period.
How long it is between repeats.
And for Kepler to declare a discovery of a planet,
the Kepler team insists on seeing multiple transits so
they know that this is not a fluke produced by some erroneous event.
Or more likely by perhaps,
the passage of a companion rather than a planet in front of it.
And by seeing multiple transits, we can measure the period,
p of the planet around the host star.
We can then make use of Kepler's laws.
Once we know the period of the planet, we know the mass of the host star,
and we can infer that by looking at the properties of the stars.
The stars studied by Kepler are all relatively bright, and
as a result we have a pretty good estimate of their mass.
And we can then use Kepler's laws to infer the distance of the planet from
the host star.
Once I know the distance, and
I can measure the duration of the transit, we can infer
the relative size and from the suppression of the planet to the star.
So that Kepler tells us the planet's size and
the planet distance from the host star.
Two of the basic properties we'd like to know about the planet.
Kepler, and a single planet system does not tell us about the planetary
mass but does give us some of the basic things we need to know,
to understand the planetary properties.
Discovering a Kepler planet is a multi step process.
The Kepler team starts with many time variable events found in the searches.
Identifies several objects that look transit like.
Identifies the Kepler objects of interest.
And then basically it's a sifting process that aims to throw out objects like this.
Situations where you have a bright star,
where the companion's not a planet but a binary star.
That instead of moving across like this, the way a planet might,
has a grazing incidence, producing a little bit of
dimming here rather than blocking the whole thing as a smaller planet might.
So we need to separate this event from a true transit, or
alternatively, separate events like this where we have a triple system.
Where we see the transit of a small star around a secondary star,
and confuse this with the transit of a planet in front of the primary star.
And while the challenge is for the Kepler team.
And they separate these by detailed studies of the light curve, together with
measurements of that use the radial velocity technique to look for companions.
To separate out
these false positives from the real objects in the survey.
Now when Kepler searches to find planets,
like any other method, it has observational biases.
It's more likely to see the planets that are easiest to see.
So Kepler tends to detect big planets, and planets that are close to the star.
So this plot here shows the range of planets detected by Kepler.
This shows all the Kepler objects of interest,
plotted in terms of their size relative to the Earth in radius.
So this is an Earth size planet.
One that's ten times the size of the Earth, and it's orbital period and dates.
This is data up to 2013.
And you can see Kepler is more likely to see planets that are close to the star.
Those are going to be short period.
And more likely to see bigger planets.
This plot makes it look like most of the objects are around here,
in terms of their characteristic size.
But the way we interpret this, is there's actually a distribution of
planetary properties with in fact more small planets than large planets.
And the reason we see the excess here not down here, or
the largest numbers here not down here, is that it's easier to detect planets that
are higher up in this plot and closer to this side.
So what we need to do when we study the Kepler data to get the demographics of
planets is to take the points we see in this observational plane.
Understand our observational selections and
work backwards to infer the properties of planets.
And what Kepler has taught us is that planets are common.
More than one out of every ten stars host planets.
It's taught us that multiple plant systems are common,
that'll be the subject of the final section of the selector.
And it's taught us that planets range in size from at least down to earth mass,
all the way up to ten times the size of jupiter.
And they are over this large range,
we don't find any special features in mass they are,
at least in size, and that we don't, you know, find any characteristic separations.
We find planets with a wide range of periods and a wide range of sizes.
There's tremendous diversity in the properties of extra-solar planets.
Kepler has been an incredibly powerful mission.
And many of us were very excited by its results, were greatly saddened when
one of the driver's wheels that allows the Kepler telescope to point and
stare for a long time at this field and get detailed measurements failed.
When that happened, most of us feared that the Kepler mission was over.
And that while it had four years of success,
it had lived up to its design specifications.
NASA, when they built it, they meant it to last at least this long and
it lasted as long as it was supposed to last.
But it was such a successful mission.
Almost all of us in the astronomy community were really
deeply disappointed when we heard of its problems.
But fortunately a number of very clever people,
associated with the mission, have figured out a way to continue the Kepler mission
in a revived form that's called K2.
And this is the continuation of the mission that's happening today.
And the way K2 is going to operate, and it's operating in fact right now,
is that rather than staring at that one field for it's entire time,
it's going to operate and be stabilized by making use of the solar panel.
And keeping the panel oriented relative to the Sun in a way to maintain its pointing.
And by using its gyros and the solar panels, there'll be enough
information on alignment that it can stare at a single field for roughly 80 days.
So it will stare at a single field.
And in doing so, it will be able to study brighter, more near, bright stars.
Then change it's orientation, stare at the next field.
Rotate again and stare at the next field.
And we'll have a series of campaigns of roughly 80 days, that should
enable it to detect additional planets and continue the very fruitful Kepler mission.
And one of the things to look forward to this year and into next year,
are some newer results coming from this revive Kepler mission, K2.
But fortunately, they're things that go, will go even beyond Kepler, and
the next step in studying transits is a NASA mission called TESS.
And here's an artist's design of TESS, TESS is scheduled to launch in 2017.
And while Kepler stared deeply at one field in the sky,
for the first stage of its life, and found planets around stars whose
brightnesses are typically, say, 13, 14th magnitude,
TESS is going to be staring at nearby stars.
A lot of TESS' targets are stars that you could see with your binoculars as
opposed to the Kepler stars, which are further away and require telescopes.
And while Kepler's done a wonderful job discovering many,
many planets around bright stars.
TESS will be able to discover earths and super earths, and
larger planets around, closer by, brighter stars.
There are a number of advantages of the TESS strategy.
Perhaps most important is that the TESS stars are going to
be easier to follow up on.
The stars are so
bright that we can point telescopes, existing telescopes like Hubble, and
most excitingly the Hubble successor the James Webb Space Telescope, a much larger
telescope scheduled to be launched in 2018 that will study stars in the infrared.
JWST will be able to follow up on these TESS targets.
And if we're fortunate, there'll be a TESS target around a nearby Red Dwarf.
Perhaps a planet somewhat akin to 186F, but
closer, that might host even water and oxygen.
And perhaps, if we're truly fortunate sometime
in 2019, 2020 JWST will be able to detect signs of life
around a habitable planet discovered by the TESS mission.
Kepler's worthy successor.
Let's now turn to a question on the Kepler mission, and
then come back and we'll talk more about multiplanet systems in a moment.
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