How do we find planets around other stars?

For Greenwich Astronomers, 2019 November 17

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
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!
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?
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.
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.
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.
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.
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!
9. Planet discovery: Other methods • Gravitational lensing. • Disturbances in stellar accretion disks. • Tides and dynamical perturbations. • Pulsar or pulsation timing. • Astrometry. • …
10. What are exoplanets? • Planets around other stars.
11. What are exoplanets? • Exceedingly tiny signals in boring data.
12. What aren’t exoplanets? • They sure as h*ck aren’t what you see in artists’ conceptions!
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!
19. 1 10 100 1000 10000 orbital period [days] 1 10 planet radius [R ]
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]
21. 10 20 30 40 50 60 70 80 time [BJD - 2456808] 800 400 0 400 relative flux [ppm]
22. 0.4 0.2 0.0 0.2 0.4 time since transit [days] 600 400 200 0 200 relative flux [ppm]
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.
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.
25. 1 10 100 1000 10000 orbital period [days] 1 10 planet radius [R ]
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?
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?
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.)
29. Machine-learning methods • Linear models • Gaussian processes • Deep learning, especially recurrent networks • Generative adversarial networks
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.
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.
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.
33. 1 10 100 1000 10000 orbital period [days] 1 10 planet radius [R ]
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…
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.
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.