Diversity of Data in the Search for Exoplanets Rachel Akeson NASA Exoplanet Science Institute

1. Diversity of Data in the Search for Exoplanets Rachel Akeson NASA ExoplanetScience Institute California Institute of Technology

2. Astronomy is old Babylonian cuneiform record of observations of Halley’s comet in 164 BC

3. But not always quantitative Halley's Comet in 1456 (lubieniecki) Halley's Comet in 1835 (William Herschel)

4. Good observations can change our place in the universe (or at least our view of it) • Tycho Brahe’s measurements of the position of Mars were so precise (~0.1 degree) that they forced Johannes Kepler to reject a circular orbit for Mars and to develop his laws of planetary motion • Mars’ eccentricity is only 0.09

5. Exoplanets are new 25 years ago there were 9 known planets All in our own Solar System In 1995, two research groups announced detection of a periodic signal in the spectra of a nearby, sun-like star that they attributed to the gravitational influence of a planet around that star

7. The big questions How many stars have planets? How big are those planets and where are they located? What drives the diversity of planetary systems? How many planets are habitable? Are we alone?

8. The NASA Exoplanet Archive Funded by NASA’s Exoplanet Exploration Program and run by the NASA Exoplanet Science Institute

9. Archive Holdings >1700 confirmed exoplanets 1200 references, 35000 data values >3000 planet candidates 2 million data values >21,000,000 light curves (stars searched for planets) Updated weekly with new planets or new data on existing planets Supports ground and space-based missions

10. Strategic data plan Maintain list of exoplanets as vetted by archive scientists Use only data from peer-reviewed literature Include multiple determinations of measured parameters where available Host large datasets not available anywhere else and difficult for smaller groups to maintain Add value by having archive scientists cross-match objects between surveys Partner with other NASA exoplanet efforts to maximize data provided to the science community and preserve that data after missions are complete Lists of exoplanet candidates from the Kepler mission

11. Issue 1: Keeping up with the peer-reviewed literature In 2013, the main astronomical journals had 500 papers with the keyword exoplanets An archive scientist reviewed the abstract for each of these to determine if it contained data which should be included in the archive All data are reformatted and validated before ingestion into the archive

12. Solution 1: Brute Force Astronomical reference times We have 2 archive staff devoted entirely to extracting data from papers However, there are no standard formats for much of the data Time (Zero point and reference frame) Units (Solar mass, Jupiter mass, Earth mass) Sometimes the data isn’t even in a table and has to be extracted from the text by hand Working with other NASA astronomy data archives to document best practices for publishing data

13. Issue 2: Scientists tend not to publish non-detections If you want to know how many stars have planets you need to count both the stars with planets and the stars without Detection rates range from 0.1 to 5%, so there are many more non-detections But researchers get much more “credit” for publishing one planet detection than 99 non-detections

14. Solution 2: Long term: Encourage change of culture to value publishing complete sample over positive detections only Short term: Work with groups with large data sets to publish complete survey results Provide support for grad students, page charges Provide venue in archive for large tabular results

15. Issue 3: Data Diversity Each method of discovering or characterizing a planet measures a different subset of the physical properties of the planet and its orbit around the star And no method gets them all

16. Physical properties of exoplanet systems • Central star • Mass • Radius • Luminosity • Metallicity • Rotation • Distance • Planetary Orbit • Semi-Major axis • Period • Time of periastron • Inclination • Longitude of periastron • Planet • Mass • Radius • Composition • Atmosphere • Rotation

17. 4 main methods of planet discovery Transits Detect decrease in flux from star as planet passes in front Requires alignment of orbit to line-of-sight to Earth

18. 4 methods 2. Radial Velocity (wobble) Detect change in stellar velocity due to gravitational influence of planet

19. 4 methods Fomalhaut 3. Imaging Detect light directly from planet (either scattered from star or intrinsic) Requires blocking light from star HR 8799

20. 4 methods 4. Microlensing Detect increase in stellar brightness due to gravitational perturbation as another star passes in front If the passing star has a planet, the planet can do the same

21. Current exoplanet population by discovery method • Transits • Planetary Orbit • Semi-Major axis • Period • Inclination • Planet • Mass • Radius

22. Current exoplanet population by discovery method • Transits • Planetary Orbit • Semi-Major axis • Period • Inclination • Planet • Mass • Radius • Radial Velocity • Planetary Orbit • Semi-Major axis • Period • Inclination • Planet • Mass * sin inclination • Radius

23. Current exoplanet population by discovery method • Transits • Planetary Orbit • Semi-Major axis • Period • Inclination • Planet • Mass • Radius • Imaging • Planetary Orbit • Semi-Major axis • Period (in some cases) • Inclination • Planet • Mass (from models) • Radius (from models) • Radial Velocity • Planetary Orbit • Semi-Major axis • Period • Inclination • Planet • Mass * sin inclination • Radius

24. Current exoplanet population by discovery method • Microlensing • Planetary Orbit • Semi-Major axis (if known distance) • Period (in some cases) • Inclination • Planet • Mass (from models) • Radius • Transits • Planetary Orbit • Semi-Major axis • Period • Inclination • Planet • Mass • Radius • Imaging • Planetary Orbit • Semi-Major axis • Period (in some cases) • Inclination • Planet • Mass (from models) • Radius (from models) • Radial Velocity • Planetary Orbit • Semi-Major axis • Period • Inclination • Planet • Mass * sin inclination • Radius

25. Result: Sparsely populated table

26. Solution 3 No real solution to the fundamental problem as the planets detected by one method are generally not detectable by another Transits Imaging Current sensitivity limits for the main planet detection methods Radial Velocity Microlensing Transits

27. Current Exoplanet Population The different methods probe different parts of exoplanet phase space Note: to make this plot we “cheat” and assume inclination = 90 for radial velocity planets

28. Exoplanet Archive approach Our goal is to help researchers as much as possible Allow filtering based on presence/absence of data All data linked to original paper/source Provide quick links to subsets of data Provide counts for those doing statistical work

29. Exoplanet population synthesis This is where the archive comes in Mordasini et al (2014)

30. The Gold Standards Some planets have both radial velocity and transit data and these are the best characterized planets Image:Kaltenegger • From the mass and radius, you get the density and can study composition

31. The Brightest Gold Standards And for the brightest transiting exoplanets, we can even detect molecules in the atmospheres Molecules detected: CO CO2 H2O Methane

32. Summary We discovered over 1700 planets around other stars Understanding how these planets formed and the differences between them our own Solar Systems has just begun As with all science, we need more data but we also need to better understand the context and biases of the data we already have

33. The Future More surveys (and exoplanets) coming • NASA: TESS • Transit survey of 500,000 brightstars • 1000’s of nearby exoplanets • ESA: GAIA (2014) • Measuring the position of 1 billion stars within the galaxy • ~2500 massive planets • ESA: PLATO (2022) • Transit survey of 1 million stars • 1000’s planets