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Exoplanet XO-1b transit observations
 The XO robotic telescope found slight brightness variations of the star GSC 02041-01657 in Corona Borealis on the beginning of year 2006. Consequent photometric observations, performed by amateur astronomers, confirmed very slight 0.03m brightness drops repeating every 3.94 days. The 193rd exoplanet was discovered—the star was named XO-1 and the newly found planet XO-1b. It is only 10th exoplanet found by detection of planet transits. How difficult is it to actually observe the exoplanet transit?

The XO-1 star is very similar to our Sun. It has almost the same diameter and mass and its spectral type is G1V. Its brightness is 11m and distance approx. 200 (±20) pc. Star coordinates are R.A. = 16h 02m 12s and Dec. = +28° 10’ 11”.

XO-1 field with comparison stars in red color

XO-1 field with comparison stars in red color

The XO-1b planet is very similar to our Jupiter, it is only a bit larger (approx. 1.3 times) and lighter (its mass is 0.9 MJ). But it orbits much closer to its sun—approx. 0.04 AU. The orbital period is only 3.94 days.

The planet transit lasts for about 3.5 hours. But XO-1b transits observable from the Czech Republic appeared on the end of June, when the night is very short. The middle of the transit must occur close to midnight if we wat to observe the whole event. Either way we must start measurement in twilight and finish it on dawn.

The first observable XO-1b transit occurred on June 17th, 2006. The weather was too bad this day so we waited till June 20th and 24th.

The sky was clear on June 20th and the night was exceptionally suitable for astronomical observations. Although the astronomical night did not occur (the Sun passed too close below horizon at midnight at our latitude), clear atmosphere scattered only little sunlight and light pollution of the nearby city Zlin.

The first detected XO-1b transit

The first detected XO-1b transit

The expected brightness drop only 0.03m is so small that we did not know if we can detect it at all. The resulting light curve shows that with quality CCD camera we can observe such events.

We already knew what to expect on June 24th. The weather was much worse and the second half of the night was really bad with high clouds and haze in the atmosphere. Still the XO-1b transit is clearly visible.

The second detected XO-1b transit

The second detected XO-1b transit

Although we started observations on twilight, we missed the actual planet entry and the initial brightness drop.

It is necessary to fulfill two conditions to successfully observe extremely low brightness drops caused by exoplanet transits—it is necessary to observe through a filter (preferably a red one) and to guide all exposures to keep observed star and reference star images on the same pixels of the CCD chip. We did not utilize auto guider so the star images wandered around the CCD chip during observations. Still we achieved very good results. The precision should improve significantly when we start to use auto guiding.

The event was observed at the Zlin city observatory using 27 cm f/8 Newtonian reflector and G2CCD-1600 camera.

All images on this page by Pavel Cagas, Vaclav Pribik and Petr Cagas.

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