Transcript Radial Velocity

Diversity of Data in the
Search for Exoplanets
Rachel Akeson
NASA Exoplanet Science Institute
California Institute of Technology
Astronomy is old
Babylonian cuneiform
record of observations of
Halley’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 Archive
Funded 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
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
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 nondetections
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
Fomalhaut
3. Imaging
Detect light directly from
planet (either scattered
from star or intrinsic)
Requires blocking light from
star
HR 8799
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
Transits
Planetary Orbit
• Semi-Major axis
• Period
• Inclination
Planet
• Mass
• Radius
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
Current exoplanet population by discovery method
Transits
Planetary Orbit
• Semi-Major axis
• Period
• Inclination
Planet
• Mass
• Radius
Imaging
Radial Velocity
Planetary Orbit
• Semi-Major axis
• Period (in some cases)
• Inclination
Planet
• Mass (from models)
• Radius (from models)
Planetary Orbit
• Semi-Major axis
• Period
• Inclination
Planet
• Mass * sin inclination
• Radius
Current exoplanet population by discovery method
Microlensing
Transits
Planetary Orbit
• Semi-Major axis (if
known distance)
• Period (in some cases)
• Inclination
Planet
• Mass (from models)
• Radius
Planetary Orbit
• Semi-Major axis
• Period
• Inclination
Planet
• Mass
• Radius
Imaging
Radial Velocity
Planetary Orbit
• Semi-Major axis
• Period (in some cases)
• Inclination
Planet
• Mass (from models)
• Radius (from models)
Planetary Orbit
• Semi-Major axis
• Period
• Inclination
Planet
• Mass * sin inclination
• 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
Current sensitivity
limits for the main
planet detection
methods
Imaging
Transits
Radial Velocity
Transits
Microlensing
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
This is
where the
archive
comes in
Mordasini et al (2014)
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
The Brightest Gold Standards
And for the brightest transiting exoplanets, we can
even detect molecules in the atmospheres
Molecules detected:
CO
CO2
H2O
Methane
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
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