# A Volume-Limited Radio Search for Magnetic Activity in 140 Exoplanets with
the Very Large Array

Kevin N. Ortiz Ceballos  Center for Astrophysics ${\rm\mid}$ Harvard &
Smithsonian, 60 Garden St, Cambridge, MA 02138, USA  Yvette Cendes  Center for
Astrophysics ${\rm\mid}$ Harvard & Smithsonian, 60 Garden St, Cambridge, MA
02138, USA  Department of Physics, University of Oregon, Eugene, OR 97403, USA
Edo Berger  Center for Astrophysics ${\rm\mid}$ Harvard & Smithsonian, 60
Garden St, Cambridge, MA 02138, USA  Peter K. G. Williams  Center for
Astrophysics ${\rm\mid}$ Harvard & Smithsonian, 60 Garden St, Cambridge, MA
02138, USA

###### Abstract

We present results from a search for radio emission in 77 stellar systems
hosting 140 exoplanets, predominantly within 17.5 pc using the Very Large
Array (VLA) at $4-8$ GHz. This is the largest and most sensitive search to
date for radio emission in exoplanetary systems in the GHz frequency range. We
obtained new observations of 58 systems, and analyzed archival observations of
an additional 19 systems. Our choice of frequency and volume limit are
motivated by radio detections of ultracool dwarfs (UCDs), including T dwarfs
with masses at the exoplanet threshold of $\sim\\!13\,M_{J}$. Our surveyed
exoplanets span a mass range of $\approx\,10^{-3}-10\,M_{J}$ and semi-major
axes of $\approx\,10^{-2}-10\,$AU. We detect a single target – GJ 3323 (M4)
hosting two exoplanets with minimum masses of 2 and 2.3$\,M_{\oplus}$ – with a
circular polarization fraction of $\approx\,40\%$; the radio luminosity agrees
with its known X-ray luminosity and the Güdel-Benz relation for stellar
activity suggesting a likely stellar origin, but the high circular
polarization fraction may also be indicative of star-planet interaction. For
the remaining sources our $3\sigma$ upper limits are generally
$L_{\nu}\lesssim\,10^{12.5}\,\mathrm{erg}\,\mathrm{s}^{-1}\,\mathrm{Hz}^{-1}$,
comparable to the lowest radio luminosities in UCDs. Our results are
consistent with previous targeted searches of individual systems at GHz
frequencies while greatly expanding the sample size. Our sensitivity is
comparable to predicted fluxes for some systems considered candidates for
detectable star-planet interaction. Observations with future instruments such
as the Square Kilometer Array and Next Generation Very Large Array will be
necessary to further constrain emission mechanisms from exoplanet systems at
GHz frequencies.

Star-planet interactions (2177); Exoplanets (498); Non-thermal radiation
sources (1119); Planetary magnetospheres (997); Magnetospheric radio emissions
(998)

##  1 Introduction

Observational constraints on the magnetic activity of exoplanets are extremely
limited. While the magnetic fields of all magnetized solar system planets have
been measured directly via astronomical observations or in-situ measurements
(Stevenson, 2003), no confirmed direct detection of a magnetic field has been
achieved for an exoplanet. Several techniques exist for indirectly estimating
the magnetic field strength of exoplanets. Observations of star-planet
interactions have been used to constrain exoplanet magnetic fields, for
example by identifying modulations in Ca II chromospheric emission from the
star in phase with the planetary orbit (Shkolnik et al., 2003, 2005; Gurdemir
et al., 2012; Cauley et al., 2019), as well as periodic X-ray emission in
phase with the orbital period (Acharya et al., 2023). Transit observation of
atmospheric bow shocks (Cauley et al., 2019) and evaporating atmospheres (Ben-
Jaffel et al., 2021; Schreyer et al., 2023) have also been used to estimate
planetary magnetic fields. However, these methods are indirect and offer
uncertain estimates at best.

In the solar system, radio observations serve as direct probes of the magnetic
fields of the giant planets (Burke & Franklin, 1955; Zarka et al., 1997). The
solar system planets emit radiation at radio frequencies through the Electron
Cyclotron Maser Instability (ECMI) mechanism, which causes emission up to a
maximum frequency directly proportional to the maximum magnetic field strength
(Zarka, 1998). The nonthermal, incoherent gyrosynchrotron process is also
present in Jupiter’s radio emission, but it is a much weaker signature due to
its inefficiency (Zarka et al., 2015), making ECMI measurements the strongest
diagnostic of planetary magnetic field in the solar system.

Searches for radio emission from exoplanet systems, across MHz to GHz
frequencies, have so far yielded non-detections (e.g., Winglee et al., 1986;
Zarka et al., 1997; Bastian et al., 2000; Lazio et al., 2004; Lazio & Farrell,
2007; Lazio et al., 2009; Lynch et al., 2017; O’Gorman et al., 2018; Route,
2019; Cendes et al., 2021; Route & Wolszczan, 2023) or tentative detections
(e.g., Lecavelier des Etangs et al., 2011, 2013). In general, the detection of
stellar emission at radio frequencies is still challenging. While the very
closest stars are sometimes detectable in their thermal emission (e.g.
$\alpha$ Centauri; Trigilio et al., 2018), these are exceptions due to their
extremely close distances. Rather, stars are often observable in the radio due
to non-thermal emission, such as cyclotron masers and gyrosynchrotron
radiation (Dulk, 1985), a variable type of emission found across a large
portion of the radio spectrum (Hughes et al., 2021). Recently, non-targeted
searches through source location cross-matching on radio sky surveys have
permitted new discoveries of radio-bright main-sequence stars at MHz
(Callingham et al., 2021; Gloudemans et al., 2023) and GHz (Driessen et al.,
2023) frequencies. However, there is yet no evidence that these signals are
definitively tied to exoplanets in these systems. A recent promising detection
of flaring $2-4$ GHz radio emission from YZ Ceti, which hosts a short-period
planet, may be co-periodic with the planet’s orbit, potentially indicating
star-planet interaction (Pineda & Villadsen, 2023).

Searches that have sought to find emission directly from exoplanets (as
opposed to from star-planet interactions) have more recently focused on the
MHz regime. Jupiter’s ECMI emission, caused by its 14 G magnetic field,
reaches a maximum cyclotron frequency of about 40 MHz (Zarka et al., 2012). An
exoplanet with a magnetic field similar to Jupiter, or up to a few times
stronger, would still emit at tens or hundreds of MHz. Two results in this
regime have so far been presented as tentative detections. A potential signal
from the Tau Bootis system (Turner et al., 2021) was detected with LOFAR, but
was seen only once and could not be ruled out as being of stellar origin;
follow-up observations showed no sign of emission (Turner et al., 2024).
Another signal, from the direction of GJ 1151, has also been reported from
LOFAR data (Vedantham et al., 2020), but follow-up radial-velocity
measurements rule out the presence of a Jupiter-mass companion (Pope et al.,
2020). Later observations revealed a long-period ($P=390$ d) exoplanet, likely
too low mass ($M_{p}\sin i=10.62$ M⊕) to be the source of the signal (Blanco-
Pozo et al., 2023). Further LOFAR detections of circular polarization in a
subset of M dwarfs have been likewise attributed to exoplanet interactions
(Callingham et al., 2021), although all but two of these newly detected
sources are not known to host exoplanets.

On the other hand, GHz frequency radio observations of very low mass stars and
brown dwarfs (hereafter, ultracool dwarfs, UCDs) have proved fruitful (e.g.
Berger et al., 2001; Berger, 2002; Hallinan et al., 2007; Route & Wolszczan,
2012; McLean et al., 2012; Route & Wolszczan, 2016; Kao et al., 2016). Over
two dozen brown dwarfs with spectral types L and T, have been detected in the
radio (Berger, 2002; McLean et al., 2012; Williams, 2018; Kao & Sebastian
Pineda, 2022). The detection of emission from the T2.5 dwarf SIMP
J01365663+0933473 ($M=12.7\pm 1.0$ MJ) established that even planetary-mass
objects can emit at GHz frequencies (Kao et al., 2018). Unlike the magnetic
field of a star like our Sun, which is generated by shear in the tachocline
(Parker, 1955), the dynamos of UCDs are thought to be convection-generated,
which is also the case for planets in our solar system (Christensen et al.,
2009). This dynamo process was initially predicted to generate only weak
magnetic fields, but this has now been refuted by the properties of the radio
emission, which require kG-level large-scale fields (Berger, 2002; Williams et
al., 2014; Hallinan et al., 2015). In fact, recent results have shown
spatially resolved emission around the UCD LSR J1835+3259, which potentially
indicates the presence of a planet-like radiation belt (Kao et al., 2023;
Climent et al., 2023), suggesting that the strong magnetic fields in UCDs may
be “planet-like” in nature (Williams, 2018). The detection of GHz frequency
radio emission from UCDs thus implies that exoplanets may also be capable of
generating strong enough magnetic fields to cause detectable radio emission at
these frequencies, where sensitive searches can be carried out. This serves as
the main motivation for this work.

In Cendes et al. (2021), we conducted a pilot search for GHz frequency
emission from a small sample of five systems with eight exoplanets, which had
all been discovered via direct imaging. Directly-imaged exoplanets are an
attractive sample due to their comparable mass scale to T dwarfs, and due to
their resolvable angular separation from their host stars in the VLA
observations. Furthermore, these planets are generally younger and warmer, and
thus expected to have stronger convection and a more active dynamo (Reiners &
Christensen, 2010). Our pilot study did not detect any of these targets, but
established luminosity upper limits of $\lesssim 10^{12.5}$ erg s-1 Hz-1,
comparable to the detected emission from some T dwarfs (Pineda et al., 2017).

The number of nearby directly-imaged exoplanets is currently small, especially
in the context of radio detection rates of UCDs of $\sim 5-10\%$ (Berger,
2002; McLean et al., 2012; Route & Wolszczan, 2016). To achieve statistically
meaningful results that could constrain the presence of radio emission from
exoplanet systems requires a much larger sample of nearby systems. Such a
sample will also naturally span a wide range of masses, thereby exploring
radio emission from Earth-mass to multi-Jupiter mass systems. Here, we present
the results of the first large-scale GHz-frequency survey of nearby exoplanet
systems, predominantly within 17.5 pc using the Very Large Array (VLA),
combining new data with archival observations. In §2 we present the survey and
experimental design. In §3 we present the results of the observations, and in
§4 we discuss their implications; we end with concluding remarks in §5.

Program ID | Dates Observed | Configurations | Targets Observed | Targets Used
---|---|---|---|---
22A-186 | 2022-03-01 to 2022-07-02 | A, BnA$\rightarrow$A | 37 | 35
23A-270 | 2023-03-29 to 2023-05-14 | B | 23 | 23
15B-326 | 2015-11-17 to 2016-01-21 | D, DnC | 21 | 5
18B-048 | 2019-01-14 to 2019-02-16 | C, C$\rightarrow$B | 27111The number of targets observed in C-band for this program. | 14
Table 1: VLA programs used in this study.

##  2 Sample Selection and Observations

We constructed a target sample using the NASA Exoplanet
Archive222https://exoplanetarchive.ipac.caltech.edu/, which included about
5,500 confirmed exoplanets at the time of the sample construction in early
2023. We imposed the following selection criteria: (i) companion mass of $<13$
MJ to ensure exoplanet targets; (ii) distance of $<17.5$ pc to ensure that we
can reach luminosity limits of about $10^{12.5}$ erg s-1 Hz-1, comparable to
the faintest UCDs, in a reasonable amount of observing time; and (iii)
declination of $>-25^{\circ}$ for accessibility and ease of scheduling with
the Very Large Array (VLA). This led to a complete, volume-limited target
sample of 83 targets containing 145 exoplanets. Of these targets, we conducted
new observations of 58 targets 333One additional target, 61 Vir, was also
observed in our program but its location was contaminated by bright emission
from a nearby source; we therefore consider it an unobserved target.. We
further supplemented these observations with analysis of archival data for an
additional 12 targets. In total, we present results for 70 of the 83 targets
in this first target sample. In addition, we also include in our survey 7
targets that are beyond the 17.5 pc cutoff: One target (70 Vir) for which we
obtained new observations, and 6 targets that were included in the archival
datasets we analyzed. A summary of the number of targets observed, and the
number of targets used in the results of this study, is provided in Table 1.

###  2.1 New VLA Observations

We obtained observations with the VLA as part of programs 22A-186 (PI: Cendes)
and 23A-270 (PI: Ortiz Ceballos); details are shown in Table 1. All
observations were performed in the C-band, with continuous spectral coverage
at $4-8$ GHz. We selected C-band due to its optimal sensitivity, and since UCD
radio emission has been predominantly detected at this frequency range (e.g.
Berger et al., 2005, 2009; Williams et al., 2013; Kao et al., 2016, 2018). We
selected observing times proportional to the distance to each target to
achieve a luminosity limit of $\approx 10^{12.5}$ erg s-1 Hz-1 or better
across the sample.

###  2.2 Archival Data

We additionally identified unpublished data in the VLA archive that include
exoplanets within our 17.5 pc cutoff (or close to it). These programs are
listed in Table 1, along with their observational details. For program 18B-048
(PI: Bastian), we only used observations in C-band to match our own data. In
the case of both archival programs, we also excluded targets that we already
observed as part of our new programs, given our greater sensitivity.

###  2.3 Data Analysis

Figure 1:  Luminosity upper limits as a function of system distance. A dashed
line shows the intended sensitivity of the survey at $L_{\nu}\lesssim
10^{12.5}$ erg s-1 Hz-1. Each dataset studied is shown in a different color,
and upper limits on luminosity as a function of distance are presented as
markers with dotted lines pointing downwards. Results from the literature are
also shown for reference; these correspond to the four systems in the
unobserved portion of the sample that have published radio observations in the
$4-8$ GHz range, taken from Bower et al. (2009); Bastian et al. (2018); Pineda
& Hallinan (2018); Cendes et al. (2021). Figure 2:  Luminosity limits as a
function of planet mass. Here, each planet in the sample is plotted, with the
luminosity measurement corresponding to its system. We include the same 4
literature systems as in Figure 1. We also include the measured luminosities
and estimated masses for the available radio-detected T-dwarfs in the
literature: SIMP J013656.5+093347.3, 2MASS J10475385+2124234, 2MASS
J12373919+6526148, 2MASS J12545393-0122474 and WISE J062309.94–045624.6. These
literature measurements are taken from Kao et al. (2016, 2018); Rose et al.
(2023) Figure 3:  Luminosity limits as a function of planet orbital
separation. Literature values correspond to the same four systems from Figures
1 and 2.

For programs 18B-048, 22A-186, and 23A-080, the calibrated measurement sets
were obtained from the National Radio Astronomy Observatory (NRAO) archive,
having been processed by the Common Astronomy Software Application package
(CASA; McMullin et al., 2007) standard 6.4.1 pipeline. In the case of program
15B-326 (PI: Bastian), the calibration files were separately obtained from the
NRAO archive and used to calibrate the raw visibilities with CASA 4.3.1.

Images for each target were made using the standard CLEAN algorithm with the
CASA tclean procedure, with a pixel size of 1/3 of the synthesized beam size
for each observation. We then obtained Gaia DR3 coordinates and proper motions
for each target, which have sub-mas and sub-mas/year precision, respectively,
for all targets in our sample (Gaia Collaboration et al., 2016, 2023). We
generated proper motion-corrected coordinates for the time of observation for
each target.

We used these coordinates to perform point-source photometry on the images at
the location of the targets using the imtool fitsrc feature of pwkit (Williams
et al., 2017). Of the 77 targets, only 1 resulted in a $>5\sigma$ detection of
a point source. The resulting flux densities were scaled to spectral
luminosities using the distances from the Gaia parallaxes. Results are
tabulated in Tables 2, 3, 4 and 5.

##  3 Results

We obtained one detection and 76 non-detections from the 77 systems,
containing 140 exoplanets. The results are shown in Figure 1, where we plot
luminosities as a function of distance. At $\lesssim 8$ pc, our luminosity
upper limits are $\approx 10^{11}-10^{12}$ erg s-1 Hz-1, and they reach our
nominal target limit of $\approx 10^{12.5}$ erg s-1 Hz-1 to $\approx 17.5$ pc;
the limits beyond 17.5 pc (from archival data) are shallower by about 0.5 dex.
Our detection of GJ 3323 is at a level of $\approx 10^{12.5}$ erg s-1 Hz-1,
and we discuss this in more detail in §3.1. These results are consistent with
previous searches for radio emission from exoplanet systems at GHz frequencies
(e.g. Bastian et al., 2000; Route, 2019; Cendes et al., 2021), which have
found no emission from similar targets, although with much smaller sample
sizes.

In Figures 2 and 3 we show the same luminosity limits but now for each
exoplanet with respect to their mass and orbital separation, respectively. Our
survey probes a wide planetary mass range of $\approx 10^{-3}-10$ MJ. We also
compare our results with a few existing measurements of low-mass UCDs for
which quiescent radio emission is detected and a mass estimate is available.
Unlike planets, for which masses can be measured from their orbital motion,
these low-mass stars require comparing observed spectra with atmospheric
evolution models to estimate the object’s mass. Finally, Figure 3 shows that
we probe orbital separations from $10^{-2}$ to $10^{1}$ AU.

In all three Figures we also present a few existing observations from the
literature from comparable 4-8 GHz observations. Pineda & Hallinan (2018) find
a limit of $<1.43\times 10^{12}$ erg s-1 Hz-1 from $4-8$ GHz observations on
TRAPPIST-1. Bower et al. (2009) found a limit of $<6.5\times 10^{12}$ erg s-1
Hz-1 for GJ 625 as part of a survey of stars. Bastian et al. (2018) detected
$\epsilon$ Eridani at $(1.0\pm 0.2)\times 10^{12}$ erg s-1 Hz-1 but conclude
that the detection is likely of stellar origin. We also include the result for
the one target in our pilot study (Cendes et al., 2021) that falls within our
distance cutoff, Ross 458. That study found a limit of $<1.4\times 10^{12}$
erg s-1 Hz-1. Unlike the limits presented in this work, that limit constrains
emission from the planet directly since the planet was resolvable in the
observation. All of these measurements were taken with the VLA.

Given the individual non-detections, we generated stacked images for each
observing program with a sufficient number of targets (i.e., 18B-048, 22A-186,
and 23A-270) by aligning the individual images centered on the known position
of each source; we stack the images in this manner given the different VLA
array configurations (and hence angular resolution) of each program. In the
22A-186 stack we excluded GJ 3323 given its individual detection. The stacked
images are shown in Figure 4, and do not reveal any significant emission at
the source locations. The resulting rms noise levels are 2.1, 1.1 and 1.0
$\mu$Jy for the 18B-048, 22A-186, and 23A-270 stacks, respectively.
Collectively, this indicates that any steady emission from exoplanets at this
frequency range has a typical flux density of $\lesssim 1-2$ $\mu$Jy.

Figure 4:  Stacked images for targets from three of the VLA programs reported
in this paper. Each stack is made using a weighted average of a $31x31$ pixel
region centered on each target star. The center pixel is marked with a red
outline. Images were made with a cell size of 1/3 the synthesized beam size,
but there may be more than one beam width per stack. The resulting RMS values
for the stacks are 2.1 $\mu$Jy (18B-048), 1.1 $\mu$Jy (22A-186), and 1.0
$\mu$Jy (23A-270).

###  3.1 Detection of GJ 3323

Our single detection from the survey is of the GJ 3323 system (5.37 pc), which
consists of an M4 star with two terrestrial planets, GJ 3323 b ($M_{p}\sin
i=2.02M_{\oplus}$, $P=5.36$ d) and GJ 3323 c ($M_{p}\sin i=2.31M_{\oplus}$,
$P=40.54$ d) (Astudillo-Defru et al., 2017). GJ 3323 has been previously
detected with the Chandra X-ray Observatory with a luminosity of $\log
L_{X}=27.28$ erg s-1 ($0.5-8$ keV), and with ROSAT with a luminosity of $\log
L_{X}=27.45$ erg s-1 ($0.1-2.4$ keV; Wright et al. 2018). Furthermore, we
identify the source in the SRG/eROSITA all-sky survey Data Release 1 (Merloni
et al., 2024), with a luminosity of $\log L_{X}=27.32$ erg s-1 ($0.2-2.3$
keV). GJ 3323 has an estimated Rossby number of 0.87 that places it in the
“unsaturated” regime of the rotation-activity relation (Boudreaux et al.,
2022).

In our VLA observation, we detect GJ 3323 with a flux density of $86\pm 10$
$\mu$Jy, corresponding to a luminosity of $\log(L_{\nu})=12.47\pm 0.05$ erg
s-1 Hz-1. We also detect it in Stokes V (circular polarization) with a flux
density of $35\pm 9$ $\mu$Jy, corresponding to a circular polarization
fraction of $\approx 40$%. The VLA detection is shown in Figure 5, with the
total intensity (Stokes I) in the top panel and the circular polarization
(Stokes V) in the bottom panel.

Figure 5:  VLA images of the region centered on the Gaia DR3 proper-motion
corrected position of GJ 3323. The contour levels are $-3,3,5,7,9\,\sigma$,
where $\sigma$ is the RMS of the image as shown in each cutout. GJ 3323 is
detected with a flux density of $86\pm 10$ $\mu$Jy in Stokes I and $35\pm 9$
$\mu$Jy in Stokes V.

Using the radio and X-ray luminosities, we can compare the results for GJ 3323
to the Güdel-Benz Relation (GBR; Guedel & Benz 1993; Benz & Guedel 1994), an
empirical power law relation between the radio and X-ray luminosities of
active stars. Stars of spectral types earlier than M7 typically closely follow
this relation, spanning almost ten orders of magnitude in radio and X-ray
luminosities (Williams, 2018).

We find that GJ 3323 is located close the GBR, indicating that the radio
emission is consistent with having a stellar origin. Our GJ 3323 detection
places it 0.57 dex perpendicular from the GBR best-fit line, while the
perpendicular scatter of the original Güdel-Benz sample is 0.2 dex (Williams
et al., 2014). However, stars of spectral type M0–M6 with radio and X-ray
detections tend to skew to the left of the GBR fit (Williams et al., 2014),
and GJ 3323 is not exceptional in this (see the inset of Figure 6).

It is important to note that M dwarf X-ray and radio emission can show flaring
and variability on a timescale of minutes to hours (e.g. Berger, 2002; Stelzer
et al., 2006; Antonova et al., 2007), such that relying on non-simultaneous
observations for placing targets in the GBR can be risky. However, the
consistent X-ray luminosity from Chandra, ROSAT, and eROSITA suggests that the
X-ray emission is quiescent in nature. For our radio observation, the light
curve did not vary over the 11 minute duration, but the short observation time
makes further characterization difficult. We also checked VLA Sky Survey
(VLASS; Lacy et al., 2020) epochs 1, 2 and 3 for emission from the proper-
motion corrected location of GJ 3323 but did not detect a source (to shallow
$3\sigma$ limits of $\approx 0.40$ mJy at 3 GHz).

Despite the overall consistency with a stellar emission origin, the relatively
high circular polarization fraction could point to a planetary origin, which
we discuss in more detail in §4.

Figure 6:  The Güdel-Benz Relation (GBR) between X-ray and radio luminosities.
The red arrows indicate upper limits on GBR placement obtained from our radio
luminosity upper limits, and X-ray luminosity values from Stelzer et al.
(2013). The red star indicates the placement of GJ 3323 in the GBR from our
detection. Gray circles are from the original result of Benz & Guedel (1994),
and green circles and arrows are detections and upper limits, respectively, of
early M-dwarfs (M0-M6) from Williams et al. (2014). The inset plot shows the
distribution of offsets perpendicular to the GBR fit in units of dex for the
original Benz-Güdel sample (grey) and for the Williams et al. (2014) sample
(green), with GJ 3323 as the red line.

##  4 Discussion

The possibility of radio emission from exoplanet systems has been discussed in
the literature in the context of three possible processes: star-planet
interaction, direct planetary emission, or stellar emission. We discuss each
of these scenarios in turn:

###  4.1 Star-planet interaction

In the solar system, the strength of radio emission from magnetized planets
(the radio power output) is directly proportional to the electromagnetic
Poynting flux incident on the magnetopause of the planet due to the solar
wind, a relation known as the Radiometric Bode’s Law (RBL) (Desch & Kaiser,
1984). Historically, the RBL has been used as a scaling law to predict the
strength of putative radio emission from exoplanets from their estimated
magnetic fields (Lazio et al., 2004; Zarka, 2007). However, the RBL is an
empirical relation determined only from planets orbiting the same star, our
Sun. Given the dependence of this behavior on the solar wind, it is risky to
extrapolate this to other stellar systems, especially to systems with stars
much different than the Sun.

In the case of M dwarfs, it becomes particularly necessary to take into
account that these stars are known to be significantly more active and have a
distinct environments from Sun-like stars. Many of these systems also have
close-in exoplanets, which have been proposed to be ideal targets for
searching for exoplanet-induced radio emission due to increased possibility of
observable star-planet interaction stemming from these short orbital
separations (Cuntz et al., 2000). Planets in close orbits around their stars
are immersed in flowing magnetized plasma from the stellar environment. The
planets themselves thus become obstacles to the plasma flow, and interact with
it, causing waves in this flow. In sub-Alfvénic modes, energy gets transported
back to the star and can also be observed as radio emission (Saur et al.,
2013). No solar system planets have this kind of interaction with the Sun,
owing to their large orbital distances; sub-Alfvénic interaction is
responsible for the observed “Jupiter-Io” effect in which periodic radio
emission and auroral activity is observed in phase with the orbit of Io
(Zarka, 2007), but this is due to magnetospheric currents generated by
Jupiter’s rotation instead of a wind.

Sub-Alfvénic interaction, however, may be the case in the GJ 3323 system, as
GJ 3323 b is estimated to be within the Alfvén surface radius of its host star
(with GJ 3323 c just outside the radius) raising the possibility of star-
planet interaction as a driver of radio emission (Farrish et al., 2019). The
radio emission observed from the Jupiter-Io system is coherent and nonthermal,
caused by the electron-cyclotron maser instability (ECMI) (Zarka, 2007). In
this mechanism, the observed frequency of emission (the cyclotron frequency
$v_{\mathrm{c}}$) is proportional to a magnetic field strength and provides a
“point estimate” of this field strength ($B$) at the point of emission in the
object where the cyclotron maser occurs. This means that the field does not
need to be this strong everywhere, or even on average, just somewhere in the
system. The cyclotron frequency is given by:

$v_{\mathrm{c}}=\frac{eB}{2\pi m_{e}c}\approx
2.8\left(\frac{B}{1\,\mathrm{kG}}\right)\,\,\mathrm{GHz}.$ (1)

ECMI emission exhibits a sharp drop-off in flux at frequencies higher than the
cyclotron frequency, such that the mere detection of ECMI diagnoses the
cyclotron frequency and thus the magnetic field strength. For our frequency
range of $4-8$ GHz, the above equation yields magnetic field strengths of
$1.4-2.8$ kG. It should be noted that the time-dependence of ECMI bursts, as
well as significant beaming effects that occur in ECMI emission, introduce
additional challenges towards detecting this kind of emission with a short
observations like ours (Zarka, 2007).

ECMI emission is typically characterized by a high circular polarization
fraction (Treumann, 2006). The circular polarization fraction of 40% we detect
from GJ 3323 is unfortunately ambiguous, and especially at GHz frequencies,
insufficient to identify the observed emission as caused by ECMI (Villadsen &
Hallinan, 2019). Furthermore, the brief observation presented here cannot
truly check for or rule out star-planet interaction since, as a single
snapshot, it cannot be correlated with the planets’ orbital periods.

Figure 7:  Flux predictions from Turnpenney et al. (2018) for 6 nearby
exoplanets, and our measured upper limits. We establish upper limits more
stringent than their predicted fluxes for three planets. Two of these planets
belong to the same system, GJ 876.

Beyond the detection of GJ 3323, we also investigate our upper limits in
comparison with existing predictions. Turnpenney et al. (2018) examined the
closest M dwarf systems with close-in planets, and modeled this behavior to
predict their radio fluxes. In Figure 7, we show the predicted fluxes for
these systems in comparison to our observed limits. We observed 6 of the 11
exoplanets identified by Turnpenney et al. as the strongest likely emitters.
For three of these planets, our observations establish upper limits that are
between 3 and 10 times fainter than the predicted flux. For the remaining
three, our limits are about a factor of 2 times higher than the predicted
flux.

It is important to note that these predictions involve poorly constrained
assumptions about the planetary and stellar magnetic field strengths of the
systems in question and the stellar wind mass outflow rates. Furthermore, the
ECMI emission that is considered in this model is taken to have a flat
spectral profile up to an unspecified cutoff frequency in the GHz range at
which the brightness declines rapidly. The cutoff frequency is proportional to
the stellar magnetic field strength in the region of emission. While global
magnetic fields for M dwarfs can often reach a few kG (Reiners et al., 2009),
what matters for ECMI emission is the magnetic field strength at the location
of emission. Notably, it can plausibly reach the $2-4$ kG threshold probed by
our $4-8$ GHz observations even in stars with low global field strength
(Pineda & Hallinan, 2018).

Recently, Pineda & Villadsen (2023) published a detection of coherent emission
from the YZ Ceti system at $2-4$ GHz using the VLA. Two bursts of emission
were detected, and they coincided with the same moment of orbital phase of the
only planet in the system, YZ Ceti b, which has a 2 day period orbit. Trigilio
et al. (2023) independently observed emission in the $0.55-0.9$ GHz band,
using the Giant Metrewave Radio Telescope (GMRT), also in-phase with the
planetary orbit. This is tentative evidence that the bursts may be caused by
star-planet interaction. In this case, the actual emission may be coming from
the star itself, similar to the observed Jupiter-Io effect in the solar
system.

We note that of the observed bursts, one lasted 1 hour and the other lasted
only 1 minute. If these signals are in fact the result of star-planet
interaction, their occurrence will depend on the planetary orbital period.
Given their short duration with respect to a full orbit, then the non-
detections presented in this work do not rule out that any of our targets may
exhibit these interactions. The bursts observed from YZ Cet peaked at a
luminosity of $L_{\nu}\sim 10^{13}$ erg s-1 Hz-1, within the sensitivity of
our survey.

###  4.2 Direct emission from exoplanets

Beyond emission from star-planet interactions, it is also important to
consider direct planetary radio emission. In principle, ECMI emission could be
produced and detected directly from an exoplanet. As mentioned previously, if
emission were caused by ECMI, a detection at our observed frequencies would
correspond to a kilo-Gauss magnetic field; this is beyond the estimated
planetary magnetic field strengths of even the largest exoplanets. However,
low-mass UCDs were long predicted to have weak magnetic fields (Durney et al.,
1993; Mohanty et al., 2002) before the detection of their bright radio
emission. In UCDs, ECMI emission is observed in flares that can be detected
even when the object does not exhibit steady quiescent emission (Berger, 2002;
Route & Wolszczan, 2016). Furthermore, since the observed cyclotron frequency
is proportional to a magnetic field "point estimate", the field does not need
to be as strong everywhere, or even on average, just somewhere in the system
at a time of observation.

While convected energy scaling laws suggest that even super-Jupiter exoplanets
would exhibit much lower ECMI cyclotron frequencies than the GHz range, the
observed UCD emission suggests these scaling laws may not be valid for all
planetary-mass objects (Christensen et al., 2009; Kao et al., 2018). On the
other hand, models suggest that young or more massive planets could have field
strengths as strong as $\sim 0.1$ kG (Hori, 2021); this is still not enough
for direct ECMI emission from these planets to be detectable at GHz
frequencies.

In addition to the field strength, a population of non-thermal electrons is
also required in the planetary environment so that ECMI can take place. These
electrons could be provided by the stellar wind, or perhaps by satellites of
the planet as occurs in the Jupiter system (Noyola et al., 2014, 2016).
Finally, the challenges of the beaming and time-dependence of ECMI bursts
mentioned previously also apply, making the prospect of detecting direct
emission even more uncertain.

An alternative direct emission mechanism could be gyrosynchrotron emission,
which is also present in UCDs in the form of stable, quiescent emission. This
type of emission is caused by mildly relativistic electrons moving in a
stellar/planetary magnetic field (Williams, 2018). Stellar activity could
further exacerbate these electrons into producing synchrotron bursts directly
in the planetary environment, but this behavior has not yet been observed (Gao
et al., 2020). Like with ECMI, both a strong magnetic field and non-thermal
electrons are required to be present. While gyrosynchrotron emission can be
present at much higher frequencies for a given magnetic field strength
compared to ECMI (such that in principle GHz observations could probe weaker
magnetic fields than with ECMI observations), as an incoherent mechanism it is
also much less efficient, and is expected to be around five orders of
magnitude weaker (Zarka et al., 2015), beyond what can be probed with the
sensitivity of the VLA.

###  4.3 Stellar Radio Emission

Figure 8:  Spectral type distribution of our observed sample of stellar
systems. Spectral type for each observed star is taken from the NASA Exoplanet
Archive’s Planetary Systems Composite Planet Data Table.

While target selection for our survey was motivated by the known presence of
exoplanets, our results are also relevant to the broader study of stellar
radio emission. Our target stars span the F, G, K and M spectral types, with
the specific breakdown of spectral types shown in Figure 8.

Notably, we did not observe any UCDs (spectral types $\gtrsim$ M7); the only
two UCDs that meet our survey selection criteria for companion mass, system
distance and target declination are TRAPPIST-1 and Teegarden’s Star.
TRAPPIST-1 has a published luminosity upper limit of
$\log_{10}(L_{\nu})=12.15$ erg s-1 Hz-1 from a $4-8$ GHz observation with the
VLA (Pineda & Hallinan, 2018). UCDs can be significantly bright in the radio,
several orders of magnitude brighter than the GBR would predict (Williams et
al., 2014). Meanwhile, earlier type M dwarfs are generally fainter in the
radio with respect to their bolometric luminosity, and less likely to be
detected at all (Berger, 2006).

In our survey, we observed a total of 53 M dwarfs; 40 early M dwarfs (spectral
type M0-M3), and 13 mid M dwarfs (M4-M6). Out of these 53 observations, we
only detected one star, GJ 3323. Given the large number of M dwarfs observed,
our results are relevant to recent searches for radio activity from these
stars (e.g. Callingham et al., 2021). It is difficult to gauge the consistency
of this survey’s results with previous GHz observations of main sequence
stars, given differences in sample selection. Bower et al. (2009) surveyed 172
active M dwarfs with the VLA at 5 GHz, and detected 29; their survey sample
was built from stars known to be active, for the purpose of identifying bright
targets for astrometric study. Our results are more consistent with those of
McLean et al. (2012), who observed 25 early M dwarfs (M4-M6.5) within 20 pc,
detecting only one. However, a systematic study that does not select for
activity (or as in our case, the presence of known exoplanets) is necessary to
make more definitive conclusions on the radio brightness of these stars.

##  5 Conclusions

We have presented VLA radio observations at $4-8$ GHz of 77 nearby exoplanet
systems, reaching a luminosity sensitivity limit of $\approx 10^{12.5}$ erg
s-1 Hz-1. This sensitivity limit is comparable to our previous pilot study
(Cendes et al., 2021) and to detections of radio emission from UCDs (e.g.,
Berger 2002; McLean et al. 2012), and is more sensitive than previous searches
for exoplanet radio emission at GHz frequencies (e.g. Bastian et al., 2000).
We detect a single target, GJ 3323 (M4) with a spectral luminosity of
$\log(L_{\nu})\approx 12.5$ erg s-1 Hz-1. Comparing this result to the known
X-ray luminosity of this source, suggests that the emission is likely of
stellar origin, but the relatively high fraction of circular polarization may
be indicative of star-planet interaction.

Due to the nature of our survey, observing time was optimized towards reaching
a desired sensitivity for a large number of targets. Bursty or intermittent
emission may have well been missed in our short observations, although our
large number of targets mitigates this limitation in the aggregate, any
individual system observed may still be an intermittent emitter. Future long
term monitoring of dedicated targets may detect intermittent emission, and may
be able to characterize it as of planetary origin through correlation with the
planetary orbital period.

Future searches for exoplanet radio emission in the GHz regime may have the
capacity to go beyond what has been done in this work thanks to the advent of
more sensitive radio telescopes in the next decade, such as the Next-
Generation Very Large Array (ngVLA) and the Square Kilometer Array (SKA)
(Selina et al., 2018; Braun et al., 2019). It is estimated that SKA1 will
achieve an order of magnitude improvement in sensitivity over the VLA for
observations of stellar sources, with sensitivity as low as $\sim 2$ $\mu$ Jy
for 1 hour integrations. SKA2 and ngVLA will improve another order of
magnitude, to $\sim 0.2$ $\mu$Jy (Pope et al., 2019). With these capabilities,
it may be possible to either detect or rule out the more optimistic
predictions for the brightness of radio emission due to star-planet
interactions (Turnpenney et al., 2018).

We thank Tim Bastian for useful discussions. The Berger Time-Domain Group at
Harvard is supported by NSF and NASA grants. The National Radio Astronomy
Observatory is a facility of the National Science Foundation operated under
cooperative agreement by Associated Universities, Inc. This research has made
use of the NASA Exoplanet Archive, which is operated by the California
Institute of Technology, under contract with the National Aeronautics and
Space Administration under the Exoplanet Exploration Program. We also
appreciate the support from the NSF Graduate Research Fellowship (GRFP), grant
number DGE1745303, and of the Ford Foundation Predoctoral Fellowship. This
work has made use of data from the European Space Agency (ESA) mission Gaia
(https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and
Analysis Consortium (DPAC,
https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has
been provided by national institutions, in particular the institutions
participating in the Gaia Multilateral Agreement.

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Table 2: 15B-326 Results

Target System | Distance | Planet | Planet mass | Semimajor axis | RMS | Luminosity
---|---|---|---|---|---|---
| (pc) |  | ($M_{J}$) | (AU) | ($\mu$Jy) | (erg s-1 Hz-1)
Gl 15A | 3.562 | b | 0.010 | 0.072 | 13.6 | $<6.20\times 10^{11}$
|  | c | 0.113 | 5.400 |  |
$\tau$ Cet | 3.652 | e | 0.012 | 0.538 | 23.0 | $<1.10\times 10^{12}$
|  | f | 0.012 | 1.334 |  |
|  | g | 0.006 | 0.133 |  |
|  | h | 0.006 | 0.243 |  |
Gl 876 | 4.672 | b | 2.276 | 0.208 | 35.8 | $<2.81\times 10^{12}$
|  | c | 0.714 | 0.130 |  |
|  | d | 0.021 | 0.021 |  |
|  | e | 0.046 | 0.334 |  |
GJ 176 | 9.485 | b | 0.026 | 0.066 | 23.4 | $<7.56\times 10^{12}$
GJ 3293 | 20.21 | b | 0.074 | 0.143 | 11.5 | $<1.69\times 10^{13}$
|  | c | 0.066 | 0.362 |  |
|  | d | 0.024 | 0.194 |  |
|  | e | 0.010 | 0.082 |  |

Note. — Luminosities determined from 3 times measured RMS (3$\sigma$) and
distance.

Table 3: 18B-048 Results Target System | Distance | Planet | Planet mass | Semimajor axis | RMS | Luminosity
---|---|---|---|---|---|---
| (pc) |  | ($M_{J}$) | (AU) | ($\mu$Jy) | (erg s-1 Hz-1)
Gl 687 | 4.55 | b | 0.054 | 0.163 | 7.1 | $<5.28\times 10^{11}$
|  | c | 0.050 | 1.165 |  |
Gl 581 | 6.3 | b | 0.050 | 0.041 | 6.0 | $<8.55\times 10^{11}$
|  | c | 0.017 | 0.072 |  |
|  | e | 0.005 | 0.028 |  |
Gl 667C | 7.243 | b | 0.018 | 0.050 | 11.0 | $<2.07\times 10^{12}$
|  | c | 0.012 | 0.125 |  |
|  | e | 0.008 | 0.213 |  |
|  | f | 0.008 | 0.156 |  |
|  | g | 0.014 | 0.549 |  |
Gl 433 | 9.077 | b | 0.019 | 0.062 | 7.0 | $<2.07\times 10^{12}$
|  | c | 0.102 | 4.819 |  |
|  | d | 0.016 | 0.178 |  |
Gl 436 | 9.775 | b | 0.070 | 0.029 | 6.1 | $<2.09\times 10^{12}$
Pollux444Target coordinates, proper motion and distance taken from the Hipparcos catalogue (van Leeuwen, 2007) due to unavailability in Gaia. | 10.34 | b | 2.300 | 1.640 | 7.3 | $<2.80\times 10^{12}$
HIP 57050 | 11.03 | b | 0.304 | 0.166 | 6.3 | $<2.75\times 10^{12}$
|  | c | 0.214 | 0.912 |  |
14 Her | 17.9 | b | 8.053 | 2.774 | 7.2 | $<8.28\times 10^{12}$
|  | c | 5.025 | 11.924 |  |
HD 154088 | 18.27 | b | 0.021 | 0.134 | 7.9 | $<9.47\times 10^{12}$
HD 154345 | 18.27 | b | 1.190 | 4.210 | 8.2 | $<9.82\times 10^{12}$
HD 87883 | 18.29 | b | 5.409 | 4.055 | 6.5 | $<7.81\times 10^{12}$
Gl 3634 | 20.39 | b | 0.026 | 0.029 | 9.5 | $<1.42\times 10^{13}$
7 CMa | 20.47 | b | 1.850 | 1.758 | 7.8 | $<1.17\times 10^{13}$
|  | c | 0.870 | 2.153 |  |
Gl 328 | 20.52 | b | 2.510 | 4.110 | 9.6 | $<1.45\times 10^{13}$
|  | c | 0.067 | 0.657 |  |

Table 4: 22A-186 Results Target System | Distance | Planet | Planet mass | Semimajor axis | RMS | Luminosity
---|---|---|---|---|---|---
| (pc) |  | ($M_{J}$) | (AU) | ($\mu$Jy) | (erg s-1 Hz-1)
Ross 128 | 3.375 | b | 0.004 | 0.050 | 7.9 | $<3.23\times 10^{11}$
GJ 273 | 3.786 | b | 0.009 | 0.091 | 8.2 | $<4.22\times 10^{11}$
|  | c | 0.004 | 0.036 |  |
Wolf 1061 | 4.308 | b | 0.006 | 0.038 | 10.0 | $<6.66\times 10^{11}$
|  | c | 0.011 | 0.089 |  |
|  | d | 0.024 | 0.470 |  |
GJ 9066 | 4.47 | c | 0.210 | 0.880 | 13.5 | $<9.68\times 10^{11}$
GJ 3323 | 5.375 | b | 0.006 | 0.033 | 7.2 | $3.31\times 10^{11}$
|  | c | 0.007 | 0.126 |  |
GJ 251 | 5.585 | b | 0.013 | 0.082 | 7.5 | $<8.40\times 10^{11}$
HD 180617 | 5.915 | b | 0.038 | 0.343 | 8.0 | $<1.00\times 10^{12}$
HD 219134 | 6.542 | b | 0.015 | 0.039 | 13.1 | $<2.01\times 10^{12}$
|  | c | 0.014 | 0.065 |  |
|  | d | 0.051 | 0.237 |  |
|  | f | 0.023 | 0.146 |  |
|  | g | 0.034 | 0.375 |  |
|  | h | 0.340 | 3.110 |  |
LTT 1445A555Target coordinates, proper motion and distance taken from the Hipparcos catalogue (van Leeuwen, 2007) due to unavailability in Gaia. | 6.864 | b | 0.009 | 0.022 | 9.4 | $<1.59\times 10^{12}$
|  | c | 0.005 | 0.027 |  |
GJ 393 | 7.038 | b | 0.005 | 0.054 | 9.4 | $<1.67\times 10^{12}$
GJ 1151 | 8.043 | c | 0.033 | 0.571 | 6.5 | $<1.51\times 10^{12}$
GJ 486 | 8.079 | b | 0.009 | 0.017 | 7.1 | $<1.66\times 10^{12}$
Gl 686 | 8.16 | b | 0.021 | 0.091 | 7.2 | $<1.72\times 10^{12}$
GJ 849 | 8.815 | b | 0.893 | 2.320 | 12.8 | $<3.57\times 10^{12}$
|  | c | 0.990 | 4.950 |  |
GJ 357 | 9.436 | b | 0.006 | 0.036 | 6.9 | $<2.21\times 10^{12}$
|  | c | 0.011 | 0.061 |  |
|  | d | 0.019 | 0.204 |  |
GJ 3512 | 9.497 | b | 0.460 | 0.337 | 8.0 | $<2.59\times 10^{12}$
|  | c | 0.200 | 1.292 |  |
Gl 49 | 9.86 | b | 0.018 | 0.090 | 10.0 | $<3.49\times 10^{12}$
GJ 1265 | 10.24 | b | 0.023 | 0.026 | 5.4 | $<2.03\times 10^{12}$
GJ 649 | 10.39 | b | 0.258 | 1.112 | 6.1 | $<2.36\times 10^{12}$
HIP 48714 | 10.52 | b | 0.072 | 0.112 | 4.1 | $<1.63\times 10^{12}$
GJ 740 | 11.11 | b | 0.009 | 0.029 | 3.9 | $<1.73\times 10^{12}$
HD 3651 | 11.11 | b | 0.228 | 0.295 | 3.9 | $<1.73\times 10^{12}$
GJ 414A | 11.88 | b | 0.024 | 0.232 | 5.9 | $<2.99\times 10^{12}$
|  | c | 0.169 | 1.400 |  |
GJ 180 | 11.95 | b | 0.020 | 0.092 | 6.8 | $<3.48\times 10^{12}$
GJ 96 | 11.95 | b | 0.062 | 0.291 | 4.4 | $<2.26\times 10^{12}$
|  | c | 0.020 | 0.129 |  |
|  | d | 0.024 | 0.309 |  |
GJ 179 | 12.41 | b | 0.950 | 2.410 | 4.9 | $<2.71\times 10^{12}$
HD 69830 | 12.58 | b | 0.032 | 0.078 | 4.8 | $<2.73\times 10^{12}$
|  | c | 0.037 | 0.186 |  |
|  | d | 0.057 | 0.630 |  |
55 Cancri | 12.59 | b | 0.831 | 0.113 | 5.5 | $<3.13\times 10^{12}$
|  | c | 0.171 | 0.237 |  |
|  | d | 3.878 | 5.957 |  |
|  | e | 0.025 | 0.015 |  |
|  | f | 0.141 | 0.771 |  |
HD 190007 | 12.72 | b | 0.049 | 0.092 | 5.8 | $<3.37\times 10^{12}$
GJ 3779 | 13.75 | b | 0.025 | 0.026 | 4.2 | $<2.85\times 10^{12}$
$\gamma$ Cep | 13.79 | b | 9.400 | 2.050 | 3.5 | $<2.39\times 10^{12}$
47 UMa | 13.89 | b | 2.530 | 2.100 | 5.0 | $<3.46\times 10^{12}$
|  | c | 0.540 | 3.600 |  |
|  | d | 1.640 | 11.600 |  |
$\tau$ Boo | 15.61 | b | 5.950 | 0.049 | 3.3 | $<2.89\times 10^{12}$
GJ 504 | 17.59 | b | 4.000 | 43.500 | 4.0 | $<4.44\times 10^{12}$
70 Vir | 18.1 | b | 7.490 | 0.481 | 3.4 | $<4.00\times 10^{12}$

Table 5: 23A-080 Results Target System | Distance | Planet | Planet mass | Semimajor axis | RMS | Luminosity
---|---|---|---|---|---|---
| (pc) |  | ($M_{J}$) | (AU) | ($\mu$Jy) | (erg s-1 Hz-1)
GJ 411 | 2.546 | b | 0.008 | 0.079 | 6.2 | $<1.44\times 10^{11}$
|  | c | 0.043 | 2.940 |  |
GJ 514 | 7.628 | b | 0.016 | 0.422 | 6.4 | $<1.34\times 10^{12}$
HD 260655 | 9.998 | b | 0.007 | 0.029 | 5.1 | $<1.83\times 10^{12}$
|  | c | 0.010 | 0.047 |  |
Ross 508 | 11.22 | b | 0.013 | 0.054 | 5.6 | $<2.53\times 10^{12}$
$\upsilon$ And | 13.48 | c | 13.980 | 0.828 | 5.0 | $<3.26\times 10^{12}$
|  | d | 10.250 | 2.513 |  |
|  | b | 0.688 | 0.059 |  |
GJ 480 | 14.26 | b | 0.042 | 0.068 | 2.9 | $<2.12\times 10^{12}$
GJ 685 | 14.31 | b | 0.028 | 0.134 | 3.8 | $<2.79\times 10^{12}$
HIP 79431 | 14.58 | b | 2.100 | 0.360 | 6.2 | $<4.73\times 10^{12}$
GJ 1214 | 14.64 | b | 0.026 | 0.015 | 13.0 | $<1.00\times 10^{13}$
LHS 1140 | 14.96 | b | 0.020 | 0.096 | 3.8 | $<3.05\times 10^{12}$
Gl 378 | 14.96 | b | 0.041 | 0.039 | 2.9 | $<2.33\times 10^{12}$
|  | c | 0.006 | 0.027 |  |
GJ 317 | 15.18 | c | 1.644 | 5.230 | 4.1 | $<3.39\times 10^{12}$
|  | b | 2.500 | 1.151 |  |
HD 238090 | 15.25 | b | 0.022 | 0.093 | 2.3 | $<1.92\times 10^{12}$
TYC 2187-512-1 | 15.48 | b | 0.330 | 1.220 | 3.6 | $<3.10\times 10^{12}$
51 Peg | 15.53 | b | 0.460 | 0.053 | 2.9 | $<2.51\times 10^{12}$
GJ 720A | 15.57 | b | 0.043 | 0.119 | 2.7 | $<2.35\times 10^{12}$
GJ 3929 | 15.83 | b | 0.006 | 0.025 | 2.9 | $<2.61\times 10^{12}$
|  | c | 0.018 | 0.081 |  |
G 264-12 | 15.99 | b | 0.008 | 0.023 | 3.5 | $<3.21\times 10^{12}$
|  | c | 0.012 | 0.052 |  |
HD 190360 | 16.0 | b | 1.800 | 3.900 | 2.5 | $<2.30\times 10^{12}$
|  | c | 0.060 | 0.130 |  |
HD 128311 | 16.32 | b | 1.769 | 1.084 | 4.6 | $<4.40\times 10^{12}$
|  | c | 3.789 | 1.740 |  |
GJ 3942 | 16.95 | b | 0.022 | 0.061 | 2.4 | $<2.48\times 10^{12}$
HD 7924 | 17.0 | d | 0.020 | 0.155 | 2.9 | $<3.01\times 10^{12}$
|  | c | 0.025 | 0.113 |  |
|  | b | 0.020 | 0.060 |  |
$\rho$ CrB | 17.51 | b | 1.093 | 0.224 | 3.0 | $<3.30\times 10^{12}$
|  | c | 0.089 | 0.421 |  |
|  | d | 0.068 | 0.827 |  |
|  | e | 0.012 | 0.106 |  |