Nature | www.nature.com | 1
Article
A planet within the debris disk around the
pre-main-sequence star AU Microscopii
Peter Plavchan
1
✉
, Thomas Barclay
2,13
, Jonathan Gagné
3
, Peter Gao
4
, Bryson Cale
1
,
William Matzko
1
, Diana Dragomir
5,6
, Sam Quinn
7
, Dax Feliz
8
, Keivan Stassun
8
,
Ian J. M. Crossield
5,9
, David A. Berardo
5
, David W. Latham
7
, Ben Tieu
1
, Guillem Anglada-Escudé
10
,
George Ricker
5
, Roland Vanderspek
5
, Sara Seager
5
, Joshua N. Winn
11
, Jon M. Jenkins
12
,
Stephen Rinehart
13
, Akshata Krishnamurthy
5
, Scott Dynes
5
, John Doty
13
, Fred Adams
14
,
Dennis A. Afanasev
13
, Chas Beichman
15,16
, Mike Bottom
17
, Brendan P. Bowler
18
,
Carolyn Brinkworth
19
, Carolyn J. Brown
20
, Andrew Cancino
21
, David R. Ciardi
16
,
Mark Clampin
13
, Jake T. Clark
20
, Karen Collins
7
, Cassy Davison
22
, Daniel Foreman-Mackey
23
,
Elise Furlan
15
, Eric J. Gaidos
24
, Claire Geneser
25
, Frank Giddens
21
, Emily Gilbert
26
, Ryan Hall
22
,
Coel Hellier
27
, Todd Henry
28
, Jonathan Horner
20
, Andrew W. Howard
29
, Chelsea Huang
5
,
Joseph Huber
21
, Stephen R. Kane
30
, Matthew Kenworthy
31
, John Kielkopf
32
, David Kipping
33
,
Chris Klenke
21
, Ethan Kruse
13
, Natasha Latouf
1
, Patrick Lowrance
34
, Bertrand Mennesson
15
,
Matthew Mengel
20
, Sean M. Mills
29
, Tim Morton
35
, Norio Narita
36,37,38,39,40
, Elisabeth Newton
41
,
America Nishimoto
21
, Jack Okumura
20
, Enric Palle
40
, Joshua Pepper
42
, Elisa V. Quintana
13
,
Aki Roberge
13
, Veronica Roccatagliata
43,44,45
, Joshua E. Schlieder
13
, Angelle Tanner
25
,
Johanna Teske
46
, C. G. Tinney
47
, Andrew Vanderburg
18
, Kaspar von Braun
48
, Bernie Walp
49
,
Jason Wang
4,29
, Sharon Xuesong Wang
46
, Denise Weigand
21
, Russel White
22
,
Robert A. Wittenmyer
20
, Duncan J. Wright
20
, Allison Youngblood
13
, Hui Zhang
50
&
Perri Zilberman
51
AU Microscopii (AU Mic) is the second closest pre-main-sequence star, at a distance of
9.79parsecs and with an age of 22million years
1
. AU Mic possesses a relatively rare
2
and spatially resolved
3
edge-on debris disk extending from about 35 to 210
astronomical units from the star
4
, and with clumps exhibiting non-Keplerian
motion
5–7
. Detection of newly formed planets around such a star is challenged by the
presence of spots, plage, ares and other manifestations of magnetic ‘activity’ on the
star
8,9
. Here we report observations of a planet transiting AU Mic. The transiting
planet, AU Mic b, has an orbital period of 8.46days, an orbital distance of
0.07astronomical units, a radius of 0.4Jupiter radii, and a mass of less than 0.18
Jupiter masses at 3σ condence. Our observations of a planet co-existing with a debris
disk oer the opportunity to test the predictions of current models of planet
formation and evolution.
https://doi.org/10.1038/s41586-020-2400-z
Received: 16 February 2019
Accepted: 17 March 2020
Published online: xx xx xxxx
Check for updates
A list of affiliations appears at the end of the paper.
bb
b
1,325 1,330 1,335 1,340 1,345 1,350
1,355
TESS BJD – 2,457,000.5 (days)
0.98
0.99
1
1.01
1.02
1.03
1.04
1.05
Median-normalized brightness ux
Fig. 1 | TESS light curve for AU Mic. Black dots, normalized flux as a function of
time, obtained from the MAST archive. Transit ephemerides of AU Mic b are
indicated as ‘b’ in red. The double-humped sinusoidal-like pattern is due to the
rotational modulation of starspots, with the 4.863-day rotation period readily
apparent. The large, brief vertical streaks of data points deviating upwards
from this slower modulation are due to flares. Data with non-zero quality flags
indicating the presence of spacecraft-related artefacts, such as momentum
dumps (see Fig.2 legend), are removed. The gap at about 1,339days
corresponds to a gap in the data downlink with Earth during the spacecraft’s
perigee. A third transit of AU Mic b was missed during this data downlink data
gap, and thus the orbital period of AU Mic b is one-half of the period inferred
from the two TESS transit events seen. AU Mic exhibited flaring activity with
energies ranging from 10
31.6
to 10
33.7
erg in the TESS bandpass over the 27-day
light curve (±~60%), with a mean flare amplitude of 0.01 relative flux units. 1σ
measurement uncertainties are smaller than the symbols shown (<1parts per
thousand, p.p.t.).
2 | Nature | www.nature.com
Article
NASA’s Transiting Exoplanet Survey Satellite (TESS) mission
10
was
launched on 18 April 2018, and monitored the brightness of AU Mic
during the first 27days of its survey of most of the sky (Fig.1). Two tran-
sits of AU Mic b appear in the TESS photometric light curve. Follow-up
observations with the Spitzer Space Telescope
11
confirm the transits of
AU Mic b. Our analyses show that this transiting planet has an orbital
period of 8.46days, an orbital distance of 0.07astronomical units ()
and a radius of 0.4Jupiter radii. An additional, shallower candidate
transit is observed in the TESS light curve, which suggests the possible
existence of additional planets (Fig.2). Joint radial-velocity (RV) and
high-resolution adaptive optics imaging rules out
12
other planets in
this system more massive than Jupiter interior to about 20. The 3σ
upper limit to the velocity reflex motion semi-amplitude for AU Mic
b is K<28ms
−1
(seeMethods), corresponding to an upper limit for
the mass of AU Mic b of <0.18Jupiter masses (M
Jupiter
) or <3.4 Neptune
masses (M
Neptune
; see Fig.3, Tables 1 and 2).
The proximity, brightness, age and edge-on geometry of the AU
Mic system will permit us to study AU Mic b at an early stage of its
dynamical, thermal and atmospheric evolution, as well as any con
-
nection between the planet and the residual debris disk. The host star
is a red dwarf, one of the most abundant stellar types in our Galaxy.
Their diminutive size, mass and luminosity make middle-aged, com-
paratively inactive M dwarfs favoured targets to search for Earth-size
planets in circumstellar habitable zones. Thus AU Mic is an opportunity
to study a possible antecedent to these important systems. Moreover,
AU Mic, unlike most M dwarfs of a similar age, possesses a debris disk
2
,
and hence may offer insight into connections between planets and dust
disks. This system confirms
13
that gaseous planet formation and any
primordial disk migration takes place in less than 20Myr. The accretion
and migration of this (or additional) planets could have left behind the
Kuiper-belt-like ‘birth ring’ of parent body debris that is hypothesized
6
at about 35, while clearing the interior disk of gas and dust. Further-
more, it is possible that any remnant primordial debris in the inner
disk near the current locations of the planet could be in the process
of being ejected by this planet. Measurement of the spin-orbit obliq-
uity of AU Mic b via the Rossiter–McLaughlin effect (a peak-to-peak
amplitude of 40ms
−1
is expected) or Doppler tomography would be
–0.3 –0.2 –0.1 0.0 0.1 0.2
0.3
Time since transit (d)
–8
–6
–4
–2
0
2
Period = 8.46321 ± 0.00004 d
AU Mic b
Transit 1
Transit 2
Model
Transit 3
–0.3 –0.2 –0.1 0.0 0.1 0.2
0.3
Time since transit (d)
–4
–3
–2
–1
0
1
2
Data
Transit model
a
b
Detrended ux (p.p.t.) Detrended ux (p.p.t.)
Fig. 2 | Light curves of the transits of AU Mic b, and a separate, candidate
transit event. a, Data points show light curves from TESS in visible light (green
and red filled circles for transits 1 and 2, respectively) and from Spitzer IRAC
11
at
4.5µm wavelength (purple filled circles for transit 3). The data for transits of AU
Mic b are shown with an arbitrary vertical shift applied for clarity; flux units are
p.p.t. The transit model (orange curve) includes a photometric model that
accounts for the stellar activity modelled with a Gaussian Process (GP), which is
subtracted from the data before plotting. The frequent flares from the stellar
surface are removed with an iterative sigma-clipping (seeMethods). In
particular, flares are observed during the egress of both the TESS transits of
AU Mic b, and also just after the ingress of the second transit of AU Mic b. The
presence of these flares in the light curve particularly affect our precision in
measuring the transit duration and thus the mass/density of the host star AU
Mic, and consequently the impact parameter and eccentricity of the orbit of
AU Mic b. Model uncertainties shown as shaded regions are 1σ confidence
intervals. The uncertainty in the out-of-transit baseline is about 0.5p.p.t. but is
not shown for clarity. b, The AU Mic candidate single transit signal, identified
by visual inspection of the TESS light curve. The change in noise before and
after the candidate transit signal is due to a ‘dump’ of angular momentum from
the spacecraft reaction wheels which decreased the pointing jitter and
improved the photometric precision; data points during the dump are not
shown.
10
−1
10
0
10
1
10
2
10
3
M/M
ݪ
10
0
10
1
R/R
ݪ
AU Mic b
Terrestrial
worlds
Neptunian
worlds
Jovian
worlds
K2-33 b
Qatar-4 b
Kepler-63 b
KELT-9 b
Kepler-51 b
Kepler-51 c
Kepler-51 d
Qatar-3 b
WASP-52 b
DS Tuc A b
Fig. 3 | Mass–radius diagram showing AU Mic b in the context of ‘mature’
exoplanets and known young exoplanets. Mass M and radius R are
normalized to the values for Earth, respectively M
⊕
and R
⊕
. AU Mic b is shown in
blue; we compare it to the nominal best-fit mass–radius relationship from
known exoplanets orbiting older main-sequence stars
19
, shown as a red
segmented line (dispersion not shown), and known exoplanets from the NASA
Exoplanet Archive with measured masses or mass upper limits, radii, and
estimated stellar host ages ≤400Myr, as follows: DS Tuc A b (mass is estimated
from ref.
19
and not measured), Kepler-51 bcd, Kepler 63 b, K2-33 b, Qatar-3 b,
Qatar-4 b, KELT-9 and WASP-52 b. By combining the radius measurement from
TESS, and the mass upper limit from radial velocities (RVs), we can ascertain an
upper limit to the density of AU Mic b to critically inform models for planet
formation. Our current upper limit for the mass of AU Mic b cannot rule out a
density consistent with Neptune-like planets orbiting older main-sequence
stars, but a more precise constraint or measurement in the future may show it
to be inflated. Uncertainties shown are 1σ for detections, and 3σ for mass upper
limits.
Nature | www.nature.com | 3
an important test of migration models since we expect any obliquity in
this young system to be unaffected by stellar tides and thus primordial.
AU Mic is a member of the β Pictoris Moving Group; the group’s arche-
type β Pic is a much more massive (about 3.5×), luminous (about 100×)
and hotter (approximately 2×) A-type star, also possessing a debris
disk. β Pic has a more massive Jovian planet β Pic b observed by direct
imaging at a semi-major axis of about 9, with a mass of approximately
(11±2)M
Jupiter
determined with astrometry
14
. AU Mic and β Pic are of the
same stellar age, but are very different exoplanet host stars. While AU
Mic b possibly formed at a distance similar to β Pic b and then migrated
inwards to its present location, β Pic b has not substantially migrated
inward. These two coeval systems provide an excellent differential
comparison for planet formation.
Finally, the combined effect of stellar winds and interior planets
have been invoked to explain the high-speed ejection of dust clumps
from the system
6,7
. The observed clumps are dynamically decoupled
from AU Mic b; the ratio of the semi-major axes (0.06 versus >35)
is >100, but the clumps could have originated much closer to the star.
Dust produced in the debris ring at about 35 will spiral inwards pri-
marily as a result of stellar wind drag, which, for AU Mic and a mass
loss rate about 1,000 times that of the solar wind
6
, is estimated to be
3,700 times stronger than Poynting–Robertson drag
2
. To compare
the timescales between collisions of dusty debris and the stellar wind
drag force
15
, we assume a birth ring fractional width of 10% (3.5),
and given AU Mic’s infrared flux excess, find that the stellar wind drag
and dust collision timescales are roughly equal. Thus, some fraction
of the dust grains generated in the birth ring at about 35 may spiral
inward to the host star under the action of stellar wind drag, instead
of being ground down further by dust collisions until blown out of
the system by radiation pressure. For 1-µm-sized solid grains of dusty
debris, the in-spiral time would be approximately 7,500years, much
shorter than the age of the star. Such dust may have been observed by
ALMA
16
at <3, interior to the birth ring at 35. Dust reaching the
orbit of an interior planet could be dynamically ejected, depending on
the Safronov number: we estimate that of AU Mic b to be 0.07 and thus
inefficient at ejecting dust.
There is no other known system that possesses all of these crucial
pieces—an M-dwarf star that is young, nearby, still surrounded by a
debris disk within which are moving clumps, and orbited by a planet
with a direct radius measurement. As such, AU Mic provides a unique
laboratory to study and model planet and planetary atmosphere
formation and evolution processes in detail.
Online content
Any methods, additional references, Nature Research reporting sum-
maries, source data, extended data, supplementary information,
Table 1 | Star parameters
Parameter 68% credible interval Remarks
AU Mic (star)
Distance from the Sun 9.79±0.04pc Gaia mission parallax
Radius (0.75±0.03)R Directly measured with
interferometry
17
Mass (0.50±0.03)M Estimated from
spectral type and age
a
T
eff
3,700±100K Spectral energy
distribution modelling
15
Luminosity 0.09L Spectral energy
distribution modelling
15
Age 22±3Myr Ref.
1
Rotation period 4.863±0.010days RV analysis, TESS light
curve, SuperWASP
light curve
18
Projected rotational velocity 8.7±0.2kms
−1
Ref.
12
Linear limb-darkening
coeficient (TESS)
0.21
−0.15
+0.20
TESS light curve
Quadratic limb-darkening
coeficient (TESS)
0.0
−0.14
+0.18
TESS light curve
Linear limb-darkening
coeficient (Spitzer)
0.17
−0.12
0.22
Spitzer light curve
Quadratic limb-darkening
coeficient (Spitzer)
0.15
−0.21
+0.27
Spitzer light curve
Visible stellar activity
amplitude
145 ms
−1
−14
+17
RV analysis
Near-infrared stellar activity
amplitude
80 ms
−1
−12
+16
RV analysis; K band at
2.3µm
Spot decay half-life 110±30days RV analysis
GP hyper-parameter 4 0.37±0.02 RV analysis
Apparent magnitude TESS=6.76mag TESS light curve
a
Also consistent with independently itting the two transit events in TESS light curve for AU
Mic b.
Table 2 | Planetary parameters
Parameter 68% credible interval Remarks
AU Mic b
Period 8.46321±0.00004days TESS and Spitzer
transit light curve
analysis
Semi-major axis
0.066 AU
−0.006
+0.007
TESS and Spitzer
transit light curve
analysis
Velocity
semi-amplitude, K
<28ms
−1
RV analysis
Mass <3.4M
Neptune
<0.18M
Jupiter
RV analysis
Radius (1.08±0.05)R
Neptune
(0.375±0.018)R
Jupiter
TESS and Spitzer
transit light curve
Density <4.4gcm
−3
RV / TESS analysis
Time(s) of
conjunction
2,458, 330.39153
−0.00068
+0.00070
BJD
a
TESS and Spitzer
transit light curves
Transit duration, τ
14
3.50h
−0.59
+0.63
TESS and Spitzer
transit light curves
R
p
/R
*
0.0514±0.0013 TESS and Spitzer
transit light curve
Impact parameter, b
0.16
−0.11
+0.14
TESS and Spitzer
transit light curve
a/R
*
19.1
−1.6
+1.8
TESS and Spitzer
transit light curve
Eccentricity
0.10
−0.09
+0.17
TESS and Spitzer
transit light curve
b
.
Candidate transit event
Period 30±6days TESS light curve
transit duration
Radius (0.60±0.17)R
Neptune
=
(0.21±0.06)R
Jupiter
TESS transit light
curve
Time(s) of
conjunction
2,458,342.22±0.03days TESS transit light
curve
R
p
/R
*
0.028±0.006 TESS transit light
curve
Impact parameter, b 0.5±0.3 TESS transit light
curve
a/R
*
40±8 TESS transit light
curve
Eccentricity 0.2±0.2 TESS transit light
curve
a
Barycentric Julian Day.
b
Circular orbit assumed for RV analysis.
4 | Nature | www.nature.com
Article
acknowledgements, peer review information; details of author con-
tributions and competing interests; and statements of data and code
availability are available at https://doi.org/10.1038/s41586-020-2400-z.
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© The Author(s), under exclusive licence to Springer Nature Limited 2020
1
Department of Physics and Astronomy, George Mason University, Fairfax, VA, USA.
2
Center
for Space Sciences and Technology, University of Maryland Baltimore County (UMBC),
Baltimore, MD, USA.
3
Institute for Research on Exoplanets, Département de Physique,
Université de Montréal, Montréal, Quebec, Canada.
4
Department of Astronomy, University
of California, Berkeley, CA, USA.
5
Massachusetts Institute of Technology, Cambridge, MA,
USA.
6
Department of Physics and Astronomy, University of New Mexico, Albuquerque, NM,
USA.
7
Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA.
8
Department of
Physics and Astronomy, Vanderbilt University, Nashville, TN, USA.
9
Department of Physics
and Astronomy, University of Kansas, Lawrence, KS, USA.
10
School of Physics and
Astronomy, Queen Mary, University of London, London, UK.
11
Department of Astrophysical
Sciences, Princeton University, Princeton, NJ, USA.
12
SETI Institute, Mountain View, CA, USA.
13
Exoplanets and Stellar Astrophysics Laboratory, NASA Goddard Space Flight Center,
Greenbelt, MD, USA.
14
Department of Astronomy, University of Michigan, Ann Arbor, MI,
USA.
15
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA.
16
NASA Exoplanet Science Institute, California Institute of Technology, Pasadena, CA, USA.
17
Institute for Astronomy, University of Hawaii at Manoa, Honolulu, HI, USA.
18
Department of
Astronomy, University of Texas at Austin, Austin, TX, USA.
19
University Corporation for
Atmospheric Research, Boulder, CO, USA.
20
University of Southern Queensland, Centre for
Astrophysics, Toowoomba, Queensland, Australia.
21
Department of Physics, Astronomy and
Materials Science, Missouri State University, Springield, MO, USA.
22
Department of Physics
and Astronomy, Georgia State University, Atlanta, GA, USA.
23
Center for Computational
Astrophysics, Flatiron Institute, New York, NY, USA.
24
Department of Earth Sciences,
University of Hawaii at Manoa, Honolulu, HI, USA.
25
Department of Physics and Astronomy,
Mississippi State University, Starkville, MS, USA.
26
Department of Astronomy and
Astrophysics, University of Chicago, Chicago, IL, USA.
27
Keele University, Keele,
Staffordshire, UK.
28
RECONS Institute, Chambersburg, PA, USA.
29
Department of Astronomy,
California Institute of Technology, Pasadena, CA, USA.
30
Department of Earth and Planetary
Sciences, University of California, Riverside, CA, USA.
31
Leiden Observatory, Leiden
University, Leiden, The Netherlands.
32
Department of Physics and Astronomy, University of
Louisville, Louisville, KY, USA.
33
Department of Astronomy, Columbia University, New York,
NY, USA.
34
IPAC, California Institute of Technology, Pasadena, CA, USA.
35
Astronomy
Department, University of Florida, Gainesville, FL, USA.
36
Department of Astronomy, The
University of Tokyo, Tokyo, Japan.
37
JST, PRESTO, Tokyo, Japan.
38
Astrobiology Center, NINS,
Tokyo, Japan.
39
National Astronomical Observatory of Japan, NINS, Tokyo, Japan.
40
Instituto
de Astroisica de Canarias (IAC), La Laguna, Tenerife, Spain.
41
Department of Physics and
Astronomy, Dartmouth College, Hanover, NH, USA.
42
Department of Physics, Lehigh
University, Bethlehem, PA, USA.
43
Dipartimento di Fisica “Enrico Fermi”, Universita’ di Pisa,
Pisa, Italy.
44
INAF-Osservatorio Astroisico di Arcetri, Firenze, Italy.
45
INFN, Sezione di Pisa,
Pisa, Italy.
46
Observatories of the Carnegie Institution for Science, Pasadena, CA, USA.
47
Exoplanetary Science at UNSW, School of Physics, UNSW Sydney, New South Wales,
Australia.
48
Lowell Observatory, Flagstaff, AZ, USA.
49
NASA Infrared Telescope Facility, Hilo,
HI, USA.
50
School of Astronomy and Space Science, Key Laboratory of Ministry of
Education, Nanjing University, Nanjing, China.
51
SUNY Stony Brook, Stony Brook, NY, USA.
✉
e-mail: pplavcha@gmu.edu
Methods
TESS light-curve analysis
AU Mic has long been known as a young star exhibiting flares and bright-
ness variations driven by large starspots on the stellar surface rotat-
ing in and out of view
20
. Previous attempts to find transiting planets
were not successful owing to this variability and the redness of the star
combined with secondary atmospheric extinction effects
21,22
, in spite
of reasoning that the orbits of any planets could be aligned with AU
Mic’s edge-on debris disk, and therefore could be more likely to transit
than for a random inclination.
TESS observed AU Mic (TIC441420236) in its first sector (2018 July
25–August 22). The TESS light curve from the 2-min cadence stamp
was processed by the Science Processing Operations Center pipeline,
a descendant of the Kepler mission pipeline based at the NASA Ames
Research Center
23,24
. After visually identifying the transits in the light
curve, we independently validate the existence of the transits from
the 30-min full-frame image (FFI) data. We also extract light curves
with different photometric apertures, and confirm that the transit
signal is robust and consistent. No centroid motion is observed dur-
ing transits, suggesting that it is associated with AU Mic rather than
being an instrumental systematic or contamination from scattered
background light or a distant star. To validate the transit with ancillary
data, we inspect archival sky survey images such as POSS and find no
background stars within the TESS pixels that are present at the location
of AU Mic with a sufficient brightness ratio so as to mimic the observed
transit signals with a background eclipsing binary. Nor do we or others
identify any background eclipsing binaries in high-contrast adaptive
optics imaging
3
or our high-resolution spectroscopy (see below). The
nearest Gaia DR2 source that is capable of producing a false positive if an
eclipsing binary (with G-band contrast=5.7mag, ignoring TESS-G-band
colour terms) is 76arcsec or 3 TESS pixels from AU Mic. Finally, the
interferometric stellar radius determination
17
rules out bound stellar
companions.
We perform multiple independent analyses of the TESS light curve
to identify and model the transits present, including the TESS mission
pipeline planet detection algorithms, ExoFAST v1.0 and v2.0
25,26
, and
asterodensity profiling
27
, which yield consistent results. While Exo
-
FAST does support the simultaneous modelling of light curves and
RVs, it does not include components for modelling the stellar activ-
ity prevalent for AU Mic in the RVs. Thus, we carry out independent
analyses of the light curves and RVs. For the TESS light curve, ExoFAST
and astrodensity profiling do not simultaneously model the exoplanet
transits and detrending of the photometric variability produced by
the rotational modulation of the starspots. Thus to prepare the TESS
light curve for these analysis tools, we first fit four sinusoids to the
light curve with periods equal to the rotation period, and one-half,
one-third and one-quarter thereof. We then apply a 401 data-point
running median filter to remove the remaining photometric modula-
tion due to starspots. The flares present in the transit events were not
removed for these analyses, primarily affecting the determination of
the transit duration of AU Mic b.
Spitzer light-curve analysis
Owing to the data collection gap in the TESS light curve, Spitzer Direc-
tor’s Discretionary Time (DDT; Program ID no. 14214, 17.3h time alloca-
tion) observations were proposed, awarded and collected in 2019 to
rule in or rule out one-half of the orbit period for AU Mic b as seen in the
TESS light curve. Three transits were observed with IRAC at 4.5µm, one
of which is presented herein, the others will be presented in a future
paper. We first clean up the raw images by sigma-clipping outliers and
subtracting off a background estimate from an annulus around the
centre of light. We then sum the flux in a circular aperture centred
around the centre of light of each frame, and do this for several differ-
ent aperture radii. We then follow the procedure from ref.
11
and do a
pixel level decorrelation (PLD; using 3×3pixels) on each radius, and
pick the one that gives the smallest scatter. We adopt a 2.4pixel radius
aperture, binned by a factor of 106.
Joint TESS and Spitzer photometric analysis
We carry out a custom analysis that simultaneously accounts for the
rotational modulation of starspots, the flares and the transit events
for both the TESS and Spitzer light curves to evaluate the impact
our detrending of the spot rotational modulation and flares has on our
analysis of the transit events: this is the analysis we adopt in the main
text (Extended Data Fig.1). We use the TESS pre-search data condi-
tioned light curve created by the TESS pipeline
24,28,29
for this analysis.
To remove flares, we create a smoothed version of the light curve by
applying a third-order Savitzky–Golay filter with a window of 301 data
points, subtracting the smooth light curve, and clipping out data points
more deviant than 1.5× the r.m.s. We performed 10 iterations of this
clipping, removing the majority of stellar flares. We then used the exo-
planet package (https://github.com/dfm/exoplanet) to simultaneously
model the stellar variability and transits. Exoplanet uses several other
software packages: Starry for the transit model (https://github.com/
rodluger/starry) and celerite (https://github.com/dfm/celerite) for the
GP, which we use to model stellar variability. Our GP model consists of
two terms; a term to capture long-term trends, and a term to capture the
periodic modulation of the star’s light curve that is caused by spots on
the stellar surface. The latter is a mixture of two stochastically-driven,
damped harmonic oscillator terms that can be used to model stellar
rotation. It has two modes in Fourier space: one at the rotation period
of the star and one at half the rotation period. The transit model is
parameterized by two stellar limb-darkening parameters, the log of
the orbital period, the log of the stellar density, the time of first transit,
the log of the planet-to-star radius ratio, the impact parameter of the
transit, orbital eccentricity of the planet, and the periastron angle.
We next run a Markov Chain Monte Carlo (MCMC) to fit for the 9
PLD coefficients (the c
i
s), a slope + quadratic ramp to represent the
rotational modulation of the stellar activity still visible for AU Mic in
the Spitzer light curve at 4.5µm, as well as a transit model including
two limb-darkening coefficients for a quadratic limb-darkening law
(Extended Data Fig.2). We leave the photometric uncertainty as a free
parameter, which we fit for during the MCMC. Prior to the MCMC, we
cut out the dip that occurs during the transit, potentially due to a large
spot crossing, from Barycentric Modified Julian Date (BMJD)=58,524.5
to 58,524.53, to make sure we weren’t biasing the transit depth. The
systematics-corrected light curve is used in our light-curve modelling
in the main text.
Ground-based light-curve analysis
Ref.
21
conducted a dedicated ground-based search for planets transit-
ing AU Mic. One candidate partial transit event ingress was observed
(Barycentric Julian Date BJD=2,453,590.885), with a depth (flux dim-
ming of the star) of ~3%. By itself, this could be attributed to a number of
phenomena associated with the star’s youth, debris disk, or systematic
errors. The photometric precision of this light curve is not sufficient to
identify additional transits of AU Mic b or the candidate transit signal
from the TESS light curve.
The SuperWASP team monitored AU Mic for seven seasons as part of
a larger all-sky survey
22
(Extended Data Fig.3). We visually inspect the
SuperWASP light curve for evidence of any photometry consistent with
an ingress or egress from a transiting planet. On several nights, given
the ephemeris of AU Mic b, there are photometry data visually similar
to an ingress (for example, Julian day (JD) ~2,453,978.40) or an egress
(for example, JD ~2,454,232.56). However, the amplitude of the bright-
ness change is comparable to the amplitude of the red (low-frequency)
noise in the SuperWASP light curve, and thus these features are
probably not real. We do not model or confirm these candidate events,
given the stellar activity and relative photometric precision.
