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How do we find planets around other stars?

How do we find planets around other stars?

For Greenwich Astronomers, 2019 November 17

David W Hogg

April 13, 2019
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Transcript

  1. How do we find planets
    around other stars?
    David W. Hogg
    New York University
    Max-Planck-Institut für Astronomie, Heidelberg
    Flatiron Institute, New York City

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  2. What is machine learning?
    • Say you want to find all the kittens in all YouTube videos.
    • “Learn” a very flexible function that takes a video clip as input and
    returns kittens! or not-kittens! as output.
    • Use enormous numbers of labeled videos as a training set.
    • What does this have to do with science?
    • In it’s basic form: Not much!

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  3. What are exoplanets?
    • Planets around other stars.
    • The first were discovered in the 1990s. Now thousands are known.
    • A very large fraction of stars have planets.
    • There are billions of planets in our Galaxy.
    • How could we know this?

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  4. Astronomy reminders
    • The Earth orbits the Sun once every 365-ish days.
    • The Sun is 10 million times the mass of the Earth.
    • The Sun orbits the Milky Way.
    • The Milky Way contains tens of billions of stars.
    • The Solar System is 4.6 billion years old.
    • The Universe is 13.8 billion years old.

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  5. Planets
    • In the Solar System, inner planets are made of rock and metal, outer
    are made of gas.
    • Inner planets are heated by the Sun, outer planets get sunlight but
    also have residual heat from formation.
    • There appears to be a continuum between Jupiter-like planets and
    stars.

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  6. Planet discovery: Radial velocity
    • Star–Planet system orbits common center of mass.
    • The star wobbles.
    • Radial velocities can be measured as tiny red- and blue-shifts.
    • Hundreds of discoveries from many telescopes around the world.

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  7. Planet discovery: Transits
    • If we are very lucky (percent-level):
    • The planet passes between us and the star.
    • Planet blots out a tiny fraction of the light.
    • The signal is periodic.
    • An Earth-like planet blots out 100 parts per million.
    • NASA Kepler spacecraft alone found thousands.

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  8. Planet discovery: Direct imaging
    • Young planets are hot, and shine in the infrared.
    • Old planets reflect starlight.
    • Dozens of young planets have been directly imaged.
    • The Sun is billions of times brighter than Earth.
    • And yet, we dream!

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  9. Planet discovery: Other methods
    • Gravitational lensing.
    • Disturbances in stellar accretion disks.
    • Tides and dynamical perturbations.
    • Pulsar or pulsation timing.
    • Astrometry.
    • …

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  10. What are exoplanets?
    • Planets around other stars.

    View Slide

  11. What are exoplanets?
    • Exceedingly tiny signals in boring data.

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  12. What aren’t exoplanets?
    • They sure as h*ck aren’t what you see in artists’ conceptions!

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  13. View Slide

  14. View Slide

  15. View Slide

  16. NASA Kepler
    • Incredibly simple mission:
    • Stare at 150,000 stars, and deliver a brightness measurement for
    every star every 30 minutes.
    • 4.1-year mission, 10 billion stellar measurements.
    • Found thousands of planets.
    • Followed by the K2 Mission which did even more. And TESS, on now!

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  17. View Slide

  18. View Slide

  19. 1 10 100 1000 10000
    orbital period [days]
    1
    10
    planet radius [R ]

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  20. 4000
    2000
    0
    2000
    4000 raw: 264 ppm
    EPIC 201374602; Kp = 11.5 mag
    10 20 30 40 50 60 70 80
    time [BJD - 2456808]
    400
    0
    400 residuals: 31 ppm
    relative brightness [ppm]

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  21. 10 20 30 40 50 60 70 80
    time [BJD - 2456808]
    800
    400
    0
    400
    relative flux [ppm]

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  22. 0.4 0.2 0.0 0.2 0.4
    time since transit [days]
    600
    400
    200
    0
    200
    relative flux [ppm]

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  23. NASA Astrophysics data
    • All data from NASA missions is open and publicly available.
    • This includes NASA Kepler, K2, and TESS.
    • Many discoveries have been made by non-professionals.

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  24. NASA Kepler: Results
    • There are comparable numbers of planets as stars, maybe more!
    • Many stars have very different planetary systems from our own.
    • Planets of around twice Earth’s radius are the most common.
    • Planets with very short periods are common.
    • Jupiter-like outer planets are very common.

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  25. 1 10 100 1000 10000
    orbital period [days]
    1
    10
    planet radius [R ]

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  26. Finding an Earth analog
    • An Earth transit blots out 100 parts per million of the Sun’s light.
    • It does this for 13 hours every 365.25 days.
    • Stellar variability, spacecraft issues, and photon noise all are larger in
    amplitude than the signals we seek.
    • So, is this an impossible task?

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  27. We have a lot of data!
    • Common behaviors across many different stars are extremely
    informative:
    • We can use the enormous numbers of stars we have
    • Learn a very flexible model that predicts what a star can do in the
    future given what it has done in the past…
    • …and what one star can do in response to spacecraft motions, given
    what other stars have done.
    • Remember the kittens?

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  28. Machine learning in science
    • Many problems in natural science have this character:
    • The thing we care about has a very predictable, simple form.
    • The things we don’t care about are stochastic and complicated.
    • The tools of machine learning can be harnessed for these tasks.
    • (Commercial entities usually want performance, not understanding.)

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  29. Machine-learning methods
    • Linear models
    • Gaussian processes
    • Deep learning, especially recurrent networks
    • Generative adversarial networks

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  30. We know what we are looking for
    • We train flexible models for stellar and spacecraft variations.
    • We have a very rigid expectation about what a planet transit is!
    • Periodic, for example.
    • Very simple shape.
    • Duration and period are related.
    • This contrast between the flexible and rigid models is what makes
    planet discovery possible.

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  31. Radial velocities
    • Everything I have said about transits carries over to radial-velocity
    projects.
    • Trying to measure velocities to better than 1 m/s.
    • Stellar surface speeds and atmospheric distortions all much larger
    than this.
    • Elliptical orbit signature is (once again) very rigid.

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  32. Dirty laundry
    • The most interesting planets are small (Earth-like) and have long
    periods (few transits in 4.1 years).
    • These signals are the hardest to find in the data.
    • There is no way to independently validate these discoveries.
    • We’re still pretty confident, but there are limits to what we can know.

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  33. 1 10 100 1000 10000
    orbital period [days]
    1
    10
    planet radius [R ]

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  34. Astrophysical epistemology
    • The objects of study are incredibly remote.
    • No chance of sample return or even radar bounces!
    • What we find depends heavily on chance.
    • Photons (light waves) are our only (ish) messengers.
    • No controlled experiments or counterfactuals.
    • And yet…

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  35. We know a lot
    • (In another talk: black holes, expanding Universe, dark matter, etc.)
    • Planets are common.
    • Our Solar System is not obviously typical.

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  36. What do I want you to take home?
    • Planets are plentiful around other stars.
    • Many of them are very different from the planets around the Sun.
    • Planets are (mainly) found indirectly, as tiny signals.
    • New data-science technologies are critical to these discoveries.

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