Diversity of Data in the Search for Exoplanets

Rachel AkesonNASA Exoplanet Science Institute

California Institute of Technology

Astronomy is old

Babylonian cuneiform record of observations ofHalley’s comet in 164 BC

But not always quantitative

HALLEY'S COMET IN 1456 (LUBIENIECKI)

HALLEY'S COMET IN 1835 (WILLIAM HERSCHEL)

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

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

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?

The NASA Exoplanet ArchiveFunded by NASA’s Exoplanet Exploration Program and run by the NASA Exoplanet Science Institute

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

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

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

Solution 1: Brute Force

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

Astronomical reference times

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

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

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

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

4 main methods of planet discovery

1. Transits

Detect decrease in flux from star as planet passes in front

Requires alignment of orbit to line-of-sight to Earth

4 methods

2. Radial Velocity (wobble)

Detect change in stellar velocity due to gravitational influence of planet

4 methods

3. Imaging

Detect light directly from planet (either scattered from star or intrinsic)

Requires blocking light from star

HR 8799

Fomalhaut

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

Current exoplanet population by discovery method

TransitsPlanetary Orbit•Semi-Major axis•Period•InclinationPlanet•Mass•Radius

Current exoplanet population by discovery method

TransitsPlanetary Orbit•Semi-Major axis•Period•InclinationPlanet•Mass•Radius

Radial VelocityPlanetary Orbit•Semi-Major axis•Period•InclinationPlanet•Mass * sin inclination•Radius

Current exoplanet population by discovery method

TransitsPlanetary Orbit•Semi-Major axis•Period•InclinationPlanet•Mass•Radius

Radial VelocityPlanetary Orbit•Semi-Major axis•Period•InclinationPlanet•Mass * sin inclination•Radius

ImagingPlanetary Orbit•Semi-Major axis•Period (in some cases)•InclinationPlanet•Mass (from models)•Radius (from models)

Current exoplanet population by discovery method

TransitsPlanetary Orbit•Semi-Major axis•Period•InclinationPlanet•Mass•Radius

Radial VelocityPlanetary Orbit•Semi-Major axis•Period•InclinationPlanet•Mass * sin inclination•Radius

ImagingPlanetary Orbit•Semi-Major axis•Period (in some cases)•InclinationPlanet•Mass (from models)•Radius (from models)

MicrolensingPlanetary Orbit•Semi-Major axis (if known distance)•Period (in some cases)•InclinationPlanet•Mass (from models)•Radius

Result: Sparsely populated table

Solution 3

No real solution to the fundamental problem as the planets detected by one method are generally not detectable by another

Transits

Current sensitivity limits for the main planet detection methods

Transits

Radial Velocity

Microlensing

Imaging

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

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

Exoplanet population synthesis

Mordasini et al (2014)

This is where the archive comes in

The Gold Standards

Some planets have both radial velocity and transit data and these are the best characterized planets

• From the mass and radius, you get the density and can study composition

Image:Kaltenegger

The Brightest Gold Standards

And for the brightest transiting exoplanets, we can even detect molecules in the atmospheres

Molecules detected:COCO2

H2OMethane

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

The Future

More surveys (and exoplanets) coming

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

NASA: TESS•Transit survey of 500,000 brightstars•1000’s of nearby exoplanets