Radio Astrometric Detection and Characterization of Extra-Solar Planets:
  A White Paper Submitted to the NSF ExoPlanet Task Force
Radio Astrometric Detection and Characterization of Extra-Solar
Planets:
A White Paper Submitted to the NSF ExoPlanet Task Force
Geoffrey C. Bower1, Alberto Bolatto1, Eric Ford2, Paul Kalas1, Jim Ulvestad3
ABSTRACT
The extraordinary astrometric accuracy of radio interferometry creates an
important and unique opportunity for the discovery and characterization of exo-
planets. Currently, the Very Long Baseline Array can routinely achieve better
than 100 µas accuracy, and can approach 10 µas with careful calibration. We
describe here RIPL, the Radio Interferometric PLanet search, a new program
with the VLBA and the Green Bank 100 m telescope that will survey 29 low-mass,
active stars over 3 years with sub-Jovian planet mass sensitivity at 1 AU. An
upgrade of the VLBA bandwidth will increase astrometric accuracy by an order
of magnitude. Ultimately, the colossal collecting area of the Square Kilometer
Array could push astrometric accuracy to 1 microarcsecond, making detection
and characterizaiton of Earth mass planets possible.
RIPL and other future radio astrometric planet searches occupy a unique
volume in planet discovery and characterization parameter space. The parameter
space of astrometric searches gives greater sensitivity to planets at large radii than
radial velocity searches. For the VLBA and the expanded VLBA, the targets of
radio astrometric surveys are by necessity nearby, low-mass, active stars, which
cannot be studied efficiently through the radial velocity method, coronagraphy,
or optical interferometry. For the SKA, detection sensitivity will extend to solar-
type stars. Planets discovered through radio astrometric methods will be suitable
for characterization through extreme adaptive optics.
The complementarity of radio astrometric techniques with other methods
demonstrates that radio astrometry can play an important role in the roadmap
for exoplanet discovery and characterization.
1Astronomy Department & Radio Astronomy Laboratory, University of California, Berkeley, CA 94720;
gbower,bolatto,pkalas@astro.berkeley.edu
2Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138; ericb-
ford@gmail.com
3National Radio Astronomy Observatory, P.O. Box 0, Socorro NM 87801, U.S.A. ; julvesta@nrao.edu
http://arxiv.org/abs/0704.0238v1
– 2 –
1. Radio Astrometry and Extra-Solar Planets
Radio astrometry has long been the gold standard for definition of celestial reference
frames (Fey et al. 2004, AJ 127, 3587) and has been used to obtain the most accurate
geometric measurements of any astronomical technique. Astrometric results include mea-
surement of the deflection of background sources due to the gravitational fields of the Sun
and Jupiter (Fomalont & Kopeikin 2003, ApJ, 598, 704), the parallax and proper motion of
pulsars at distances greater than 1 kpc (Chatterjee et al. 2005, ApJ, 630, L61), an upper
limit to the proper motion of Sagittarius A* of a few km s−1 (Reid & Brunthaler 2004, ApJ,
616, 872), the rotation of the disk of M33 (Brunthaler et al. 2005, Science, 307, 1440), and
a < 1% distance to the Taurus star-forming cluster (Loinard 2006, BAAS, 209, 1080).
The Very Long Baseline Array (VLBA) images nonthermal radio emission and can
routinely achieve 100 µas astrometric accuracy, but has achieved an accuracy as high as 8
µas under favorable circumstances (Fomalont & Kopeikin 2003). Nonthermal stellar radio
emission has been detected from many stellar types (Güdel 2002, ARA&A, 40, 217), including
brown dwarfs (Berger et al. 2001, Nat, 410, 338), proto-stars (Bower et al. 2003, ApJ, 598,
1140) , massive stars with winds (Dougherty et al. 2005, ApJ, 623, 447), and late-type stars
(Berger et al. 2006, ApJ, 648, 629). Only late-type stars are sufficiently bright, numerous,
nearby, and low mass to provide a large sample of stars suitable for large-scale astrometric
exoplanet searches. Radio astrometric searches can determine whether or not M dwarfs, the
largest stellar constituent of the Galaxy, are surrounded by planetary systems as frequently
as FGK stars and if the planet mass-period relation varies with stellar type. The population
of gas giants at a few AU around low mass stars is an important discriminant between planet
formation models.
Radio astrometric searches have a number of unique qualities:
• Opportunity to discover planets around nearby active M dwarfs at large radii;
• Ability to fully characterize orbits of detected planets, without degeneracies in mass,
inclination, and longitude of ascending node;
• Sensitivity to long-period planets with sub-Jovian masses currently and Earth masses
ultimately;
• Complementary with existing planet searching techniques: most targets cannot be
explored through other methods;
• Ability to follow-up detected planets with imaging and spectroscopy; and,
• Absolute astrometric positions within the radio reference frame for stars and planets.
The quality and uniqueness of radio astrometry for planet searches are the result of two
factors:
– 3 –
Fig. 1.— Sensitivity of different methods in planet mass and semi-major axis space for
radio astrometric surveys and other methods. “Exp. VLBA” refers to the upgraded VLBA
described in § 4. The semi-major axis at the minimum in the astrometric search curves is
determined by the search duration, which is 3 years for RIPL and the Exp. VLBA campaign.
• High precision of radio astrometry: The VLBA can routinely achieve 100 µas
accuracy through relative astrometry. This precision is an order of magnitude better than
obtained from laser-guide star adaptive optics (e.g., Pravdo et al. 2005). Future instruments
will have one to two orders of magnitude more accurate astrometry, comparable to the best
accuracy achievable with the proposed SIM spacecraft.
• Active stars are difficult to study in optical programs: Our target stars are
active M dwarfs, which have radio fluxes on the order of 1 mJy. These radio stars are
difficult to study through optical radial velocity techniques because they are faint and because
the activity in these stars distorts line profiles, reducing the accuracy of radial velocity
measurements.
We give a sketch of the parameter space for RIPL, future radio astrometric searches,
the Space Interferometric Mission, radial velocity searches, and coronagraphic searches in
Figure 1. A comparison of the radial velocity and astrometric amplitudes indicates that
astrometric techniques are favored over radial velocity techniques for long period ( ∼> 1 year)
planets for these faint objects, for an astrometric accuracy of ∼ 100 µas (Ford 2006, PASP,
118, 364).
– 4 –
In Section 2, we describe the sensitivity and methods of radio astrometry. In Section 3,
we describe a new program with the VLBA and the Green Bank 100m telescope to search for
planets around nearby M dwarfs. In Section 4, we demonstrate that a bandwidth upgrade for
the VLBA will increase astrometric accuracy or stellar sample sizes by an order of magnitude.
In Section 5, we discuss the role that the Square Kilometer Array can play with its three
order of magnitude increase in sensitivity over the VLBA.
2. Radio Astrometry Sensitivity and Methods
Astrometric exoplanet searches must be able to detect an astrometric signal that has
an amplitude of
θ = 2
= 1400 µas ∗
∗Mp/MJ ∗
0.2M⊙
, (1)
for a planet of mass Mp orbiting a star of mass M∗ with a semimajor axis a at a distance D
from the Sun (a mass of 0.2 M⊙ corresponds to a M5 dwarf). To robustly detect a planet,
observations must span at least a significant fraction of a period
T = 2.2 yr ∗
0.2M⊙
. (2)
The ultimate accuracy that can be obtained through a radio astrometric technique is
σast = σbeam/SNR, (3)
where σbeam = b/λ is the synthetic beam size for an array with maximum baseline b, λ is the
observing wavelength, and SNR is the signal to noise ratio of the target source detection.
For the VLBA σast ≈ 500 µas/SNR.
The astrometric position is defined relative to nearby (∼ 1◦) compact radio sources.
Typical observations include switching on minute timescales between the calibrator and the
target sources, with less frequent observations of secondary calibrators. The use of mul-
tiple calibrators is intended to determine the differential delay in position on the sky due
to varying path length from tropospheric water vapor. The extent to which this cannot
be calibrated sets the final astrometric accuracy in observations that are not SNR-limited.
The nearer the calibrators and the greater sensitivity at which they can be detected typ-
ically determines this error. The error decreases linearly with decreasing calibrator-target
separation. The increased sensitivity of future arrays will increase the calibrator density
and therefore decrease the typical separation from calibrator to target and the uncalibrated
astrometric error. For sufficiently small target to calibrator separation, the calibrator will be
in the primary beam of the antenna, enabling simultaneous observations of the target and
calibrator that also remove temporal dependence of tropospheric variations.
– 5 –
RIGHT ASCENSION (J2000)
22 01 13.3054 13.3052 13.3050 13.3048 13.3046 13.3044 13.3042 13.3040
28 18 25.142
25.140
25.138
25.136
25.134
25.132
25.130
25.128
25.126
25.124
RIGHT ASCENSION (J2000)
22 01 13.3054 13.3052 13.3050 13.3048 13.3046 13.3044 13.3042 13.3040
28 18 25.142
25.140
25.138
25.136
25.134
25.132
25.130
25.128
25.126
25.124
RIGHT ASCENSION (J2000)
22 01 13.3054 13.3052 13.3050 13.3048 13.3046 13.3044 13.3042 13.3040
28 18 25.142
25.140
25.138
25.136
25.134
25.132
25.130
25.128
25.126
25.124
Fig. 2.— Images of GJ4247 in three separate epochs on 23, 25, and 26 March 2006 (right
to left) from the VLBA Precursor Astrometric Survey. Contour levels are -3, 3, 4, 5, 6, 7, 8
times the rms noise of 95 µJy. The synthesized beam is shown in the lower left hand corner
of each image.
3. RIPL: Radio Interferometric Planet Search
RIPL is a 1400-hour, 3-year VLBA and GBT program to search for planets around 29
nearby, low-mass, active stars. The program will achieve sub-Jovian planet mass sensitivity.
The observing program will be completed in 2009.
The most serious limitation to astrometric accuracy may be from stellar activity that
jitters the apparent stellar position. Most evidence, however, indicates that this radio astro-
metric jitter is small. For instance, White, Lim and Kundu (1994, ApJ, 422, 293) model the
radio emission from dMe stars as originating within ∼ 1 stellar radius of the photosphere.
At a distance of 10 pc for a M5 dwarf a stellar radius is ∼ 100 µas, roughly an order of
magnitude smaller than the astrometric signature of a Jupiter analog. We conducted the
VLBA Precursor Astrometric Survey (VPAS) in Spring 2006 to assess the effect of stellar
jitter on astrometric accuracy (Bower et al. 2007, in prep.).
For each star, three VLBA epochs were spread over fewer than 10 days. Seven stars were
detected in at least one epoch and four were detected in all three epochs (Figure 2). All stars
have proper motions and parallaxes determined by Hipparcos or other optical methods with
a precision of a few mas per year, yielding predicted relative positions accurate to ∼ 100 µas
during the length of the study. For all stars detected with multiple epochs, the motions
match the results of Hipparcos astrometry well with rms in each coordinate ranging from
0.08 to 0.26 µas. Deviations in the positions appear to be limited by our sensitivity; i.e., the
effect of stellar activity on their positions is unimportant.
In fact, the small differences in the fitted proper motion and the Hipparcos proper motion
already eliminate brown dwarfs as companions to these objects (Figure 3). The measured
differences are consistent with noise in the VLBA astrometry (200 µas /3day ∼ 20 mas/yr).
The typical reflex motion due to a long period brown dwarf is ∼ 100 mas/yr, which would be
apparent. The much longer time baseline and better sensitivity of RIPL will reduce proper
motion errors by ∼ 2 orders of magnitude.
– 6 –
r (AU)
GJ4247:  0.144 AU y−2
2 4 6 8 10
r (AU)
GJ896A:  0.070 AU y−2
2 4 6 8 10
Fig. 3.— Region of planetary mass and semi-major axis phase-space rejected by acceleration
upper limits based on combination of 3 epochs of radio astrometric measurements and optical
astrometry, primarily from Hipparcos. Different contours indicate confidence intervals for
excluded regions.
3.1. Synergy with other Planet Searches
RIPL is synergistic with the existing and future planet-search programs, as well as cur-
rent ground-based planet searches (including radial velocities, transits, adaptive optics, and
interferometry). RIPL provides an opportunity to search for planetary systems in a unique
area of parameter space that will not be targeted by other planet searches until the launch
of NASA SIM - Planetquest.
Ground based transit searches are most sensitive for very short periods (P ∼ days), and
the Kepler mission aims to detect planets with orbital periods of slightly more than a year.
Thus, RIPL will make a valuable contribution to our understanding of the frequency of
long-period planets around M stars. Further, unlike transits and radial velocity observations
astrometric measurements directly measure the planet mass, which is important for testing
models of planet formation. While the unknown inclination is less of an issue for studying
large samples of planets, measuring individual inclinations will be particularly valuable for
planets around M dwarfs, since a relatively modest number of M dwarfs are being surveyed
by RIPL (∼ 30 vs ∼ 3000 stars by radial velocities).
Ground-based optical and near-infared interferometers (e.g., PTI, NPOI) require bright
stars and are not appropriate for faint low-mass stars. The RIPL astrometric accuracy is an
order of magnitude better than the astrometric error from Keck Laser Guide Star Adaptive
Optics astrometry (Pravdo et al. 2005, ApJ, 630, 528). Thus, RIPL is the best means for
an astrometric search of M dwarfs until SIM launches (now estimated for no earlier than
2016).
– 7 –
A long-period planet detected by RIPL would enable exciting scientific investigations
such as photometric and spectroscopic observations to determine the planets physical prop-
erties. While space based missions such as TPF-C and TPF-I are expected to be extremely
powerful and aim to directly detect terrestrial mass planets, these missions are not expected
to launch for at least a decade in the future. Knowing which stars have giant planets suitable
for direct imaging would enable direct probes of an extrasolar planet.
4. VLBA Upgrade and Planet Detection
The VLBA is presently being upgraded from a typical data rate of 256 Mbit/s to 4
Gbit/s, with project completion estimated by 2010. This will result in a sensitivity increase
by a factor of 4, or about a factor of 8 increase in areal density of reference sources on the
sky. Thus, the typical distance between a target star and its nearest reference source will
decrease by a factor of ∼ 3. A few years later we expect a data rate of 16 Gbit/s, yielding
a target-calibrator separation more than 10 times smaller than current values. Since in
the limit of infinite SNR the astrometric error depends linearly on the separation from the
reference source, relative astrometric errors of . 10 µas should be fairly routine; in principle,
this would permit detection of a planet with a mass of less than 10% of the mass of Jupiter.
The sensitivity increase afforded by these upgrades will also permit a sizable increase of the
late-type dwarf sample.
5. Square Kilometer Array
The Square Kilometer Array (SKA; Carilli & Rawlings 2004, New AR, 48, 979) is a
proposed future radio telescope that would have a collecting area of a square kilometer,
approximately 200 times the collecting area of the VLBA. The SKA would be built toward
the end of the next decade; it is planned to cover the frequency range from 0.1 to 25 GHz, with
the 5–10 GHz range being most useful for astrometric planet detection. If 25% of the SKA
area at ∼ 8 GHz is constructed on baselines of 1000-5000 km, it will supply revolutionary
astrometric accuracy (Fomalont & Reid 2004, New AR, 48, 1473). With dish antennas of
12m diameter, the combination of sensitivity and wide field of view often will enable many
astrometric reference sources to be found in the same antenna field of view as the target
star, allowing all temporal variations in Earth’s atmosphere to be removed. In such a case,
the relative astrometric accuracy may reach ∼ 1 µas, competitive with SIM and enabling
astrometric detection of Earth-mass planets.
The sensitivity of the SKA will enable astrometric detection of thermal emission from
stars. The Sun, for instance, would be detectable to a distance of 10 pc with the SKA. Thus,
the SKA will be capable of detecting and characterizing planets around Sun-like stars.
	Radio Astrometry and Extra-Solar Planets
	Radio Astrometry Sensitivity and Methods
	RIPL: Radio Interferometric Planet Search
	Synergy with other Planet Searches
	VLBA Upgrade and Planet Detection
	Square Kilometer Array
