Transit photometry is a promising method for discovering extrasolar planets as small as Earth from spacebased photometers, and several near-term photometric missions are on the drawing board. In particular, NASA’s recently selected Kepler mission is devoted primarily to detecting extrasolar planets. The success of these e!o rts depends in part on the ability to detect transit signatures against the inherent photometric variability of the target stars. While other noise sources such as shot noise and CCD noise are under the control of the instrument designers, this one is not. The photometric variability of solar-like stars presents a fundamental lower limit to the minimum detectable planet radius for a given star and number of observed transits. In this paper we examine the capability of such missions using bolometric data for the only star for which su"cient photometric precision exists to address this question: the Sun. The results indicate that solar-like variability does not prevent the detection of Earth-sized planets even for stars rotating significantly faster than the Sun. Four transits are detectable for mv ! 12 stars with rotation periods as short as " 21 days, while six transitsallowdetectionforstellarrotation periods asshortas" 16days.Indeed, thelimitsposed bysolar-like variability allow for the detection of planets significantly smaller than Earth orbiting Sun-like stars. Planets as small as 0.6 Earth radii exhibitingat least sixtransits can be detected orbiting bright (mv ! 10) solar analogs. Subject headings: methods: data analysis — planetary systems — techniques: photometric

https://iopscience.iop.org/article/10.1086/341136/pdf

The Impact of Solar-like Variability on the Detectability of Transiting Terrestrial Planets

The Kepler Mission is a space observatory launched in 2009 by NASA to monitor 170,000 stars over a period of four years to determine the frequency of Earth-size and larger planets in and near the habitable zone of Sun-like stars, the size and orbital distributions of these planets, and the types of stars they orbit. Kepler is the tenth in the series of NASA Discovery Program missions that are competitively-selected, PI-directed, medium-cost missions. The Mission concept and various instrument prototypes were developed at the Ames Research Center over a period of 18 years starting in 1983. The development of techniques to do the 10 ppm photometry required for Mission success took years of experimentation, several workshops, and the exploration of many 'blind alleys' before the construction of the flight instrument. Beginning in 1992 at the start of the NASA Discovery Program, the Kepler Mission concept was proposed five times before its acceptance for mission development in 2001. During that period, the concept evolved from a photometer in an L2 orbit that monitored 6000 stars in a 50 sq deg field-of-view (FOV) to one that was in a heliocentric orbit that simultaneously monitored 170,000 stars with a 105 sq deg FOV. Analysis of the data to date has detected over 4600 planetary candidates which include several hundred Earth-size planetary candidates, over a thousand confirmed planets, and Earth-size planets in the habitable zone (HZ). These discoveries provide the information required for estimates of the frequency of planets in our galaxy. The Mission results show that most stars have planets, many of these planets are similar in size to the Earth, and that systems with several planets are common. Although planets in the HZ are common, many are substantially larger than Earth.

https://iopscience.iop.org/article/10.1088/0034-4885/79/3/036901/pdf

KEPLER Mission: development and overview.

A COPY of a paper published in the Messenger of Mathematics in March 1898, entitled “On the Oscillations of a Heterogeneous Compressible Liquid Sphere and the Genesis of the Moon; and on the Figure of the Moon”, has been sent to the Editor of NATURE by Mr. W. F. Sedgwick, who graduated from Trinity College, Cambridge, in the year 1894. Mr. Sedgwick has also submitted the manuscript of an unsuccessful essay sent in by him for the Smith's Prize at the end of the year 1895 “On the Vibrations of a Heterogeneous Liquid Sphere, with Applications to the Solar System; and on the Elastic Solid Theory of the Earth”. He states that the applications to the solar system on pp. 170–171 of his published paper were based on considerations set out in much greater detail in his essay. His communications throw additional light on the history of ideas concerning the origin of the solar system. His paper has been generally overlooked in this connexion—not unnaturally, considering the somewhat distant relation between its subject and its setting—but there is no doubt of its relevance, and Sir James Jeans, in a letter to the Editor, remarks: “I regret that Mr. Sedgwick's work had entirely escaped my notice until my attention was recently directed to it. His theory of the origin of the solar system appears to have nothing in common with my own, except that both postulate tidal actions—of very different kinds. But I very gladly acknowledge that Mr. Sedgwick's hypothesis of a tidal origin for the solar system was earlier than my own.”

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The Origin of the Solar System

Having been operational at Kitt Peak for more than a year, the prototype (HAT-1) of the Hungarian Automated Telescope (HAT) has been used for an all-sky variability search of the northern hemisphere. This small autonomous observatory is recording the brightness of stars in the range of IC ≈ 6 – 13 mag with a telephoto lens and its 9° × 9° field of view, yielding a data rate of ~106 photometric measurements per night. We give brief hardware and software descriptions of the system, controlled by a single PC running RealTime Linux OS. We provide an overview of site-specific details and quantify the astrometric and photometric capabilities of HAT. As a demonstration of system performance, we give a sample of 60 short-period variables in a single selected field, all bright, with I < 13 mag, where only 14 were known before. Depending on the observing strategy, a search for extrasolar planet transits is also a feasible observing program. We conclude with a short discussion on future directions. Further information can be found at the HAT Web page.3

https://www.jstor.org/stable/10.1086/342382

System Description and First Light Curves of the Hungarian Automated Telescope, an Autonomous Observatory for Variability Search

The planets of the Solar System divide neatly between those with atmospheres and those without when arranged by insolation ($I$) and escape velocity ($v_{\mathrm{esc}}$). The dividing line goes as $I \propto v_{\mathrm{esc}}^4$. Exoplanets with reported masses and radii are shown to crowd against the extrapolation of the Solar System trend, making a metaphorical cosmic shoreline that unites all the planets. The $I \propto v_{\mathrm{esc}}^4$ relation may implicate thermal escape. We therefore address the general behavior of hydrodynamic thermal escape models ranging from Pluto to highly-irradiated Extrasolar Giant Planets (EGPs). Energy-limited escape is harder to test because copious XUV radiation is mostly a feature of young stars, and hence requires extrapolating to historic XUV fluences ($I_{\mathrm{xuv}}$) using proxies and power laws. An energy-limited shoreline should scale as $I_{\mathrm{xuv}} \propto v_{\mathrm{esc}}^3\sqrt{\rho}$, which differs distinctly from the apparent $I_{\mathrm{xuv}} \propto v_{\mathrm{esc}}^4$ relation. Energy-limited escape does provide good quantitative agreement to the highly irradiated EGPs. Diffusion-limited escape implies that no planet can lose more than 1% of its mass as H$_2$. Impact erosion, to the extent that impact velocities $v_{\mathrm{imp}}$ can be estimated for exoplanets, fits to a $v_{\mathrm{imp}} \approx 4\,-\,5\, v_{\mathrm{esc}}$ shoreline. The proportionality constant is consistent with what the collision of comet Shoemaker-Levy 9 showed us we should expect of modest impacts in deep atmospheres. With respect to the shoreline, Proxima Centauri b is on the metaphorical beach. Known hazards include its rapid energetic accretion, high impact velocities, its early life on the wrong side of the runaway greenhouse, and Proxima Centauri's XUV radiation. In its favor is a vast phase space of unknown unknowns.

https://iopscience.iop.org/article/10.3847/1538-4357/aa7846/pdf

The Cosmic Shoreline: The Evidence that Escape Determines which Planets Have Atmospheres, and what this May Mean for Proxima Centauri B

Results of tests to demonstrate the very high differential-photometric stability of CCD light sensors are presented. The measurements reported here demonstrate that in a controlled laboratory environment, a front-illuminated CCD can provide differential-photometric measurements with reproducible precision approaching one part in 10(exp 5). Practical limitations to the precision of differential-photometric measurements with CCDs and implications for spaceborne applications are discussed.

/pdf/test-of-ccd-precision-limits-for-differential-photometry-1uizuq0fvu.pdf

Test of CCD Precision Limits for Differential Photometry

FRESIP (FRequency of Earth-Sized Inner Planets) is a mission designed to detect and characterize Earth-sizes planets around solar-like stars The sizes of the planets are determined from the decrease in light from a star that occurs during planetary transits, while the orbital period is determined from the repeatability of the transits Measurements of these parameters can be compared to theories that predict the spacing of planets, their distribution of size with orbital distance, and the variation of these quantities with stellar type and multiplicity Because thousands of stars must be continually monitored to detect the transits, much information on the stars can be obtained on their rotation rates and activity cycles Observations of p-mode oscillations also provide information on their age and composition These goals are accomplished by continuously and simultaneously monitoring 500 solar-like stars for evidence of brightness changes caused by Earth-sized or larger planetary transits To obtain the high precision needed to find planets as small as the Earth and Venus around solar-like stars, a wide field of view Schmidt telescope with an array of CCD detectors at its focal plane must be located outside of the Earth's at mosphere SMM (Solar Maximum Mission) observations of the low-level variability of the Sun (∼1:100,000) on the time scales of a transit (4 to 16 hours), and our laboratory measurements of the photometric precision of charge-coupled devices (1:100,000) show that the detection of planets as small as the Earth is practical The probability for detecting transits is quite favorable for planets in inner orbits If other planetary systems are similar to our own, then approximately 1% of those systems will show transits resulting in the discovery of 50 planetary systems in or near the habitable zone of solar-like stars

Fresip: A mission to determine the character and frequency of extra-solar planets around solar-like stars

The current nebular theory postulates that planets are. a consequence of the formation of stars from viscous accretion disks. Condensation from the accretion disk favors the formation of small rocky planets in the hot inner region, and the formation of gas giants in the cool outer region. Consequently, terrestrial-type planet in inner orbits should be commonplace. From geometrical considerations , Borucki and Summers have shown that 1% of planetary systems resembling our solar system should show transits for Earth-sized (or larger) planets. Thus a photometric satellite that uses a wide field of view telescope and a large detector array to simultaneously monitor 5000 target stars should detect 50 planetary systems. To differentiate regularly recurring transits from statistical fluctuations of the stellar flux, one must observe over several orbital periods so that the false positive rate can be reduced to one event or less. A one-meter aperture telescope placed in a halo orbit about either the L1 or L2 Lagrange points and viewing perpendicular to both the orbital and ecliptic planes can view continuously for the required period because neither the Sun, Earth, or Moon would enter the field of view. Model calculations show that the observations should provide statistically significant estimates of the distributions of planetary size, orbital radius, coplanarity, and the frequency of planetary systems that have Earth-sized planets in inner orbits. Because approximately one half of the star systems observed will be binary systems, the frequency of planetary systems orbiting either one or both of the stars can also be determined.

A. F. Granados

Papers

Test of CCD Precision Limits for Differential Photometry

Fresip: A mission to determine the character and frequency of extra-solar planets around solar-like stars

FRESIP: A Discovery Mission Concept To Find Earth-Sized Planets Around Solar Like Stars