1,546 research outputs found
Transit least-squares survey -- II. Discovery and validation of 17 new sub- to super-Earth-sized planets in multi-planet systems from K2
The extended Kepler mission (K2) has revealed more than 500 transiting
planets in roughly 500,000 stellar light curves. All of these were found either
with the box least-squares algorithm or by visual inspection. Here we use our
new transit least-squares (TLS) algorithm to search for additional planets
around all K2 stars that are currently known to host at least one planet. We
discover and statistically validate 17 new planets with radii ranging from
about 0.7 Earth radii to roughly 2.2 Earth radii and a median radius of 1.18
Earth radii. EPIC201497682.03, with a radius of 0.692 (-0.048, +0.059) Earth
radii, is the second smallest planet ever discovered with K2. The transit
signatures of these 17 planets are typically 200 ppm deep (ranging from 100 ppm
to 2000 ppm), and their orbital periods extend from about 0.7 d to 34 d with a
median value of about 4 d. Fourteen of these 17 systems only had one known
planet before, and they now join the growing number of multi-planet systems.
Most stars in our sample have subsolar masses and radii. The small planetary
radii in our sample are a direct result of the higher signal detection
efficiency that TLS has compared to box-fitting algorithms in the
shallow-transit regime. Our findings help in populating the period-radius
diagram with small planets. Our discovery rate of about 3.7 % within the group
of previously known K2 systems suggests that TLS can find over 100 additional
Earth-sized planets in the data of the Kepler primary mission.Comment: published in A&A, 12 pages, 6 colored Figures, 1 Table; minor textual
corrections; Fig. 5 corrected for the distance scalin
Transit least-squares survey - I. Discovery and validation of an Earth-sized planet in the four-planet system K2-32 near the 1:2:5:7 resonance
We apply, for the first time, the Transit Least Squares (TLS) algorithm to
search for new transiting exoplanets. TLS is a successor to the Box Least
Squares (BLS) algorithm, which has served as a standard tool for the detection
of periodic transits. In this proof-of-concept paper, we demonstrate how TLS
finds small planets that have previously been missed. We showcase TLS'
capabilities using the K2 EVEREST-detrended light curve of the star K2-32
(EPIC205071984) that was known to have three transiting planets. TLS detects
these known Neptune-sized planets K2-32b, d, and c in an iterative search and
finds an additional transit signal with a high signal detection efficiency
(SDE_TLS) of 26.1 at a period of 4.34882 (-0.00075, +0.00069) d. We show that
this signal remains detectable (SDE_TLS = 13.2) with TLS in the K2SFF light
curve of K2-32, which includes a less optimal detrending of the systematic
trends. The signal is below common detection thresholds, however, if searched
with BLS in the K2SFF light curve (SDE_BLS = 8.9) as in previous searches.
Markov Chain Monte Carlo sampling shows that the radius of this candidate is
1.01 (-0.09, +0.10) Earth radii. We analyze its phase-folded transit light
curve using the vespa software and calculate a false positive probability FPP =
3.1e-3, formally validating K2-32e as a planet. Taking into account the
multiplicity boost of the system, FPP < 3.1e-4. K2-32 now hosts at least four
planets that are very close to a 1:2:5:7 mean motion resonance chain. The
offset of the orbital periods of K2-32e and b from a 1:2 mean motion resonance
is in very good agreement with the sample of transiting multi-planet systems
from Kepler, lending further credence to the planetary nature of K2-32e. We
expect that TLS can find many more transits of Earth-sized and smaller planets
in the Kepler data that have hitherto remained undetected with BLS and similar
algorithms.Comment: published in A&A, Vol. 625, id. A31 , 8 pages, 6 colored figure
Revisiting the exomoon candidate signal around Kepler-1625b
Transit photometry of the exoplanet candidate Kepler-1625b has recently been
interpreted to show hints of a moon. We aim to clarify whether the exomoon-like
signal is really caused by a large object in orbit around Kepler-1625b. We
explore several detrending procedures, i.e. polynomials and the Cosine
Filtering with Autocorrelation Minimization (CoFiAM). We then supply a light
curve simulator with the co-planar orbital dynamics of the system and fit the
resulting planet-moon transit light curves to the Kepler data. We employ the
Bayesian Information Criterion (BIC) to assess whether a single planet or a
planet-moon system is a more likely interpretation of the light curve
variations. We carry out a blind hare-and-hounds exercise using many noise
realizations by injecting simulated transits into different out-of-transit
parts of the original Kepler-1625 data: 100 sequences with 3 synthetic transits
of a Kepler-1625b-like planet and 100 sequences with 3 synthetic transits of
this planet with a Neptune-sized moon. The statistical significance and
characteristics of the exomoon-like signal strongly depend on the detrending
method, and the data chosen for detrending, and on the treatment of gaps in the
light curve. Our injection-retrieval experiment shows evidence for moons in
about 10% of those light curves that do not contain an injected moon.
Strikingly, many of these false-positive moons resemble the exomoon candidate.
We recover up to about half of the injected moons, depending on the detrending
method, with radii and orbital distances broadly corresponding to the injected
values. A BIC of -4.9 for the CoFiAM-based detrending indicates an
exomoon around Kepler-1625b. This solution, however, is only one out of many
and we find very different solutions depending on the details of the detrending
method. It is worrying that the detrending is key to the interpretation of the
data.Comment: 16 pages, 12 figures. Accepted for publication by A&
Exomoon indicators in high-precision transit light curves
While the solar system contains about 20 times more moons than planets, no
moon has been confirmed around any of the thousands of extrasolar planets known
so far. Tools for an uncomplicated identification of the most promising exomoon
candidates could be beneficial to streamline follow-up studies.} Here we study
three exomoon indicators that emerge if well-established planet-only models are
fitted to a planet-moon transit light curve: transit timing variations (TTVs),
transit duration variations (TDVs), and apparent planetary transit radius
variations (TRVs). We re-evaluate under realistic conditions the previously
proposed exomoon signatures in the TTV and TDV series. We simulate light curves
of a transiting exoplanet with a single moon. These model light curves are then
fitted with a planet-only transit model, pretending there were no moon, and we
explore the resulting TTV, TDV, and TRV series for evidence of the moon. The
previously described ellipse in the TTV-TDV diagram of an exoplanet with a moon
emerges only for high-density moons. Low-density moons distort the sinusoidal
shapes of the TTV and the TDV series due to their photometric contribution to
the combined planet-moon transit. Sufficiently large moons can produce periodic
apparent TRVs of their host planets that could be observable. We find that
Kepler and PLATO have similar performances in detecting the exomoon-induced TRV
effect around simulated bright () stars. These stars, however, are rare
in the Kepler sample but will be abundant in the PLATO sample. Moreover,
PLATO's higher cadence yields a stronger TTV signal. The periodogram of the
sequence of transit radius measurements can indicate the presence of a moon.
The TTV and TDV series of exoplanets with moons can be more complex than
previously assumed. We propose that TRVs could be a more promising means to
identify exomoons in large exoplanet surveys.Comment: 13 pages, 9 figures, 1 tabl
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