Transiting Exoplanet Survey Satellite

The Transiting Exoplanet Survey Satellite is shown before launch. (Photo by Orbital ATK) 

A newly published paper lists 2,241 exoplanet candidates identified with data from the Transiting Exoplanet Survey Satellite (TESS). 

An exoplanet is any planet outside our solar system. The first exoplanet discovery was made only about 30 years ago. Since then, we have found nearly 5,000 extra-solar planets. 

All of the exoplanets we’ve discovered so far are in our galaxy, the Milky Way. Observations indicate that most stars have planets in orbit around them. Of the Sun-like stars that we have looked at with our planet hunting satellites, about 20% have an Earth-sized planet in the “habitable zone.”

The habitable zone is the region around a star where water can exist in liquid form. The habitable zone of the Sun stretches from Venus, the second planet from the Sun, to Mars, the fourth planet from the Sun. 

We detect exoplanets using a variety of methods. The method used by TESS is called the transit method. In this method, we point our telescopes at a nearby star and observe it for long periods of time.

If we see a dip in the amount of light from that star, that generally means that something has passed in front of the star. If the dimming we observe occurs over and over at regular intervals, we conclude that an object is orbiting that star. For this method of detection to work, the plane of the orbit of an exoplanet must be aligned with our line-of-sight from Earth. You can imagine that planets orbit other stars in every possible plane along a 360-degree sphere. As a result, only a small percentage of those planetary orbits will be detectable. 

We use other methods to confirm the detections made using the transit method. The most successful and reliable of these is the radial velocity method, where we measure the back-and-forth motion of stars caused by the gravity of planets in orbit tugging on those stars. 

Using these two methods together, we can determine the size of an exoplanet, the mass of an exoplanet, the density (and therefore composition) of an exoplanet, how much radiation an exoplanet receives, and the distance that an exoplanet orbits from its host star. 

TESS has discovered these 2,000-plus worlds in less than two years of observing. As our technology advances and our methods become more refined, we will detect many tens of thousands more exoplanets in the years to come. 

The Morning Sky

Saturn and Jupiter wait for you in the eastern sky before sunrise. Saturn rises first with Jupiter trailing behind. Jupiter, the fifth planet from the Sun, is much brighter than the sixth planet, Saturn. Try to spot the very thin crescent Moon low in the east on Friday morning.  

The Evening Sky

The Moon reappears on the other side of the Sun on Tuesday night, again presenting a very thin sliver. Look low in the west as the Sun sets to find it. If you miss it on Tuesday, try again on Wednesday, when it will be a bit higher in the west as darkness falls.

Above the Moon is the Pleiades cluster. Take a look at this wonderful object through binoculars before it disappears for the summer. Mars sits in between the horn tips of Taurus further up the western sky. 

Join me at Josephine Sculpture Park on Saturday, April 17, for a telescopic Night Sky Tour. Please see the JSP website for details.

Have a question about astronomy or space science? Send an email to and it might be featured in a future column. 

Dan Price is a NASA/JPL Solar System Ambassador and informal educator. He leads the Night Sky Tours at Josephine Sculpture Park. Have a question about astronomy or space science? Send an email to and it might be featured in a future column.

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