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Laboratory for Atmospheric and Space Physics

What is a Transit?

A transit occurs when one object passes in front of another relative to our line-of-sight, similar to an eclipse. he apparent size of the transiting object only casts a very small shadow on Earth-bound and Earth-orbiting telescopes. It is this small shadow that CUTE is designed to observe. A prime example of a transit is that of Venus across the disk of the Sun in 2012 (below, via the Solar Dynamics Observatory).

While eclipses can be seen without the aid of a telescope and with proper eye protection, transits are usually observed with telescopes or satellites. The International Space Station (ISS) is often seen transiting the Sun as it orbits the Earth.

The ISS seen transiting in front of the Sun. Mercury is also seen as the small spot in the lower center of the Sun’s disk. Credit: Thierry Legault.

 

Sketches of the light curves obtained through observations in the (a) optical and (b) near-ultraviolet. CUTE is interested in the change in flux due to an exoplanet’s atmosphere. [Vidotto et al. 2011b]

What can we learn from transits?

Primary exoplanetary transits are measured through the comparison of star light unblocked by its exoplanet to the starlight while the exoplanet is in transit. Transits offer a wealth of information stemming from two primary observations: First, how long does the transit last? Second, how much light was blocked by the transiting planet? If multiple transits are observed, we can also determine the orbital period of the planet.

Comparing the duration of the transit to the orbital period allows us determine the ratio of the star’s radius to the distance between the star and the planet (semi-major axis). Those calculations are often complicated by things like orbital inclination (does the planet orbit at 90 degrees from our perspective?) and radial velocity variations in the host star (how much does the host star move due to the planet’s orbiting?). These parameters have been defined for certain exoplanets through observations carried out by telescopes like Kepler and the Transiting Planets and Planetesimals Small Telescope (TRAPPIST).

CUTE is focused on the second question: How much light is blocked by the transiting planet? How does the planet’s atmosphere alter the starlight during the transit? The shape of the light curve (how the light changes as a function of time) looks similar to that in the light curve sketch to the right. CUTE’s target list is filled with hot Jupiters, gas planets similar to Jupiter in mass but with orbital periods of a few days, i.e., very close to their parent stars. The atmospheres of these hot Jupiters may transit just before the body of the planet does, and we can learn about the planet’s atmosphere from the starlight that shines through it by comparing the flux of the star light with no transiting planet to that of the starlight with the transiting planet.

CUTE’s Dedicated Study

Previous exoplanet transit observations and their interpretation have been complicated by three primary factors: 1) the paucity of observed systems prevent a statistical treatment of short-period exoplanet atmospheres; 2) independent analyses of the few available data sets have produced discrepant results in most cases; 3) many exospheric signatures detected so far appear time-dependent.

A simulated transit light curve, filled out over three exoplanet transits. Multiple transit observations are necessary to fill out a single light curve because as CUTE’s view of the exoplanet’s star is periodically blocked by Earth for a portion of a single transit.

CUTE’s advantage lies in its ability to measure several transits of a single exoplanet in the same spectral window, for an ensemble of targets. Larger space observatories, like the Hubble Space Telescope, can produce fantastic spectra of transiting exoplanets, but are constrained by the large number of diverse observational programs carried out in a given year; obtaining five – six complete transits for the approximately 12 primary CUTE targets would require ~1000 Hubble orbits.

CUTE will provide critical time-domain information by observing ~5 – 10 transits of the same system. This data will allow for the statistical treatment of exoplanet atmospheres, and relay fundamental insights into the atmospheric processes and mass-loss rates of these hot Jupiters, and may even determine if these planets are magnetized.