General Introduction to STEP
Ding Chen
Quy Nhon, Vietnam April 25, 2014
Search for Earth-twins
Msini [Earth Mass]
Habitable Zone
0.5-5 ME
Rocky Planet
Orbital Period [days]
Different Techniques to Search for
Exoplanets
• Kepler mission, occultation
– Jupiter 1%, Earth 10-4
• Radial Vel (doppler shift of stellar
spectrum) (ground based)
– Hot Jupiter ~50m/s, Earth 0.09m/s
• MIcrolensing (sideways wobble)
– Jupiter 500 uas, Earth 0.3 uas
• Direct Detection (IR interferometer or
visible large telescope) Block the light
of the star, so we can see the light of
the planet.
– 0.1 arcsec separation, 1010 contrast
Overview of STEP
• Satellite Specifications / Payloads:
– Orbit: Solar-earth L2 Halo
– Mass: 500 kg
Life time: 5 year
– Payloads: TMA Astrometry Telescope
(Primary Aperture: 1.2m, f=50m, FOV: 0.44°)
 Highlights
 Extremely-high-precision(0.5uas)
astrometry space mission
 Able to detect the habitable planets at
earth criterion
 Get the actual planetary masses and
the full orbital geometry for all
components of the detected planetary
system
STEP’s niche (fresh search)
A. Search for Earth twin:
a) Earth-mass planets in the habitable zone around solar type stars.
b) population census (eta of Earth)
c) 3D orbits and Earth-twin with probability of transits
d) multiple habitable planets & dynamical architecture of systems
B. Dynamical evolution of hot and cold Jupiters:
a) around high-mass main sequence stars (cf with those around PMS)
b) around young solar-type stars (eccentricity & hot Jupiter evolution)
c) around rapid rotators & magnetically active stars (close-in planets)
C. Super-Earths (short-period and several Earths mass or size):
a) true Mp and period distribution (eta of super-Earths)
b) eccentricities & longitudes (liberation vs circulation, transit targets)
c) coplanarity (range)
3D orbit reconstruction
3D orbital reconstruction & accurate distance
8/35
STEP’s niche (follow-ups of RV)
A. Properties of host stars of known planets:
a) distance, radius, mass, age, & internal [Fe/H] (MS & PMS stars)
b) 3D orbits of binary companions (coplanarity with planets)
B. 3D orbits of known isolated planets:
a) sin i and Mp (true mass-period distribution)
b) eccentricities, longitudes of nodes (cold Jupiters)
c) transit probability (phase of conjunction)
d) obliquity (spin measured by stellar oscillation or spots)
C. Multiple siblings of known planets
a) are hot Jupiters loners (trend for distant perturbers or companions)
b) super Earth siblings of hot Jupiters and eccentric gas giants
c) sin i and Mp (kinematic properties) distribution
d) eccentricity and longitudes distribution (secular interaction)
e) coplanarity (dynamical relaxation & Kozai effect)
f) solar system twins (retention of Earths with cold Jupiters)
17/35
STEP’s niche (follow-up of transits)
A. Confirmation:
a) elimination of false positives (background binaries)
b) long-period stellar companions (dynamical origin of JP)
c) stellar parameters: distance, luminosity, mass, radius
B. 3D orbits of known individual planets:
a) Mp and density (origin of structural diversity)
b) eccentricities & longitudes (orientation, precession & Q-value)
c) obliquity versus peri apsi distance (tidal capture model of hot Jup)
d) tidal distortion (phase lag of tidal bulge)
C. Multiple siblings of known planets
a) additional companions (multis & Kozai companions)
b) period, mass & density distribution (formation, atmospheric losses)
c) eccentricity, relative longitudes & coplanarity (dynamical origin)
d) resonant systems: TTV mass determination & nodal precession
STEP’s niche (follow-ups of direct
imaging)
A. Properties of host stars:
a) distance, mass, & age (how young)
b) 3D velocity (for common proper motion long-period candidates)
B. 3D orbits of known planets:
a) orbital parameters (period, semi major axis, eccentricity)
b) luminosity (hot versus cold start, recent impact)
c) dynamical evolution (eccentricity of young and mature systems)
d) Mp determination
C. Multiple siblings of known planets
a) orbits: a, e, and coplanarity (dynamical relaxation, GI versus CA)
b) signs of dynamical ejection (for freely floating planets)
c) shorter-period companions (hot and cold Jupiters)
32/35
Approach
Astrometry
Periodogram of Inner Solar System
4
10
3
10
2
Periodogram Power
10
1
10
0
10
-1
10
-2
10
-3
10
-4
10
0
1
2
3
4
5
6
freq cycles/year
7
8
9
10
What’s the Best Astrometry in Space?
• Ground based astrometry is limited by atmospheric turbulence.
• In Space, HST astrometry (with CCD camera) is perhaps the
most accurate. ~100uas. (l/D ~40mas, critically sampled,
1/200 pixel)
• With STEP we hope to do 0.5uas (in 1~2 hour) with a 1.2m
single-dish telescope. Long focus length(~57m)
– 100X higher accuracy than HST
– Centroid to 1/50,000 pixel
What are Key Issues in Getting 1uas Astrometry?
• Instrument/Calibration
– Calibrate errors in the CCD/SCMOS detectors.
• Without Calibration HST astrometry limited ~1/100 pixel, for STEP
with a 1.2m telescope 1/100 pix ~ 1 milliarcsec not 1 uas.
– Calibrate field distortion biases
• The original concept proposed for ESA used a prime focus focal
plane. This resulted in an extremely long telescope tube (~40m).
STEP uses a much more compact TMA telescope but the additional
optics (and possible errors in the design/fabrication of those optics
gives rise to field distortion. This has to be calibrated to very high
accuracy.
• The most promising approach so far is to use Globular clusters.
There are ~200,000 known stars in 47 Tuc imaged by HST. HST
folks have used 47 Tuc to calibrate field distortion of HST and
achieved accuracy limited by CCD imperfections.
Key Issues Cont
•
•
Telescope, optical accuracy and stability
– Imperfections in the telescope affect precision astrometry in two ways
• Lower strehl (The image is not good as a diffraction limited image)
– This results in lower SNR when fitting the CCD image to a model
PSF. But ultimate as long as the wavefront across the field of view
is < l/10~l/20 the strehl is pretty good and only minor (67% and
90%). 90% is of course better than 67% but the loss of accuracy is
not large
– Stability of the telescope is much more important, if we are even
somewhat successful in calibrating the field distortion from imperfect
optics.
• The use of globular clusters for field distortion calibration means we
can calibrate only every few hours (~10~40). The telescope distortion
has to be stable to a few uas between calibrations.
This contrasted with the stability of the CCD focal plane. The CCD focal plane is
calibrated with laser metrology and that can be done every ~ minute.
Key Technology in STEP:
Calibrating CCD Centroiding Errors!
• Two/three classes of errors
– Pixels are not uniformly spaced
– QE within a pixel is not uniform
– Error in the assumed PSF
• Measuring Pixel positions at the upixel level
• Measuring QE variations within a pixel
• Nyquist sampling and measuring the PSF.
• Multi-color calibration
• The best lab results to date, 1e-5 of l/D => STEP ~1 uas
4/8/2015
14
60
30
5
0.02
50
25
10
0.015
20
40
15
30
15
0.01
20
20
10
25
0.005
10
5
30
5
10
15
20
-12
6
25
30
5
10
15
20
25
30
Least Square Fitting
x 10
5.5
Variance
5
4.5
4
3.5
3
-5
-4
-3
-2
-1
0
1
Displacement (Unit : Pixel)
2
3
4
-6
x 10
Orbit: L2
序号
名称
取值
1
Ax
23.4万千米
2
Ay
74.6万千米
3
Az
30万千米
4
周期
约180天
Rocket: CZ-3C
View distance and Objects
 ~20pc
 200
F,G,K stars
 Expect to detect
1000 exoplanets
 1~30 earthtwins(ηE=10%)
Phase A works
• We need more people to work on Blind Test,
Simulation, Demonstration, Completeness, Dynamics,
Astrometry, Metrology, Distortion Correction,
Algorithm , Data Pipeline, Website…
• Ground based observations on some nearby Stars.
Target selection, strategy,etc.
• RV, Transit research on the target list.
• Complementary/Collaborator missions
• Position like International Senior Experts, 1000 Youth
Talents Plan, 100 Talents Program, Staff, Visitor
Scientist, Post-doc, PhD, etc. are open!
summary
• Astrometry is a powerful method for planet
detection and characterization
• STEP has the potential to discover Earth twins and
to establish their frequency.
• STEP can be utilized to extract critical information
for planets discovered by other methods
• Synergy with CHEOPS, TESS, WFIRST, JWST
Thank you！