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A size comparison of (L to R) the Earth, Wolf 503b and Neptune. The blue color for Wolf 503b is imaginary; nothing is still known about the atmosphere or the surface of the planet. Credits: NASA Goddard / Robert Simmon (Earth), NASA / JPL (Neptune).
Wolf 503b, an exoplanet twice the size of Earth, was discovered by an international team of Canadian, American and German researchers using data from NASA's Kepler Space Telescope. This discovery is described in a new study led by Merrin Peterson, a student at the Exoplanets Research Institute (iREx) who began her Masters degree at the University of Montreal (UdeM) in May.
Wolf 503b is about 145 light-years from Earth in the constellation Virgo; it revolves around its star every six days and is therefore very close to it, about ten times closer than Mercury to the sun.
"The discovery and confirmation of this new exoplanet was very fast, thanks to the collaboration of my advisor Björn Benneke and myself," Peterson said. "In May, when the latest version of Kepler K2 data arrived, we quickly launched a program that allowed us to find as many interesting exoplanets as possible. Wolf 503b was one of them.
The program used by the team identifies distinct, periodic dips that appear in the light curve of a star as a planet passes in front of it. In order to better characterize the system of which Wolf 503b is a part, astronomers first obtained a spectrum of the host star of the NASA infrared telescope installation. This confirmed that the star is an old "orange dwarf", slightly less luminous than the Sun but about twice as old, and allowed an accurate determination of the radius of the star and its companion.
To confirm that the companion was indeed a planet and to avoid making a false positive identification, the team obtained adaptive optics measurements at the Palomar Observatory and also examined data from the 39; archives. With these, they were able to confirm that there were no binary stars in the background and that the star had no other mate more massive that can be interpreted as a planet in transit.
Wolf 503b size compared to the planets found by the Kepler telescope. Wolf 503b falls just to the right of the "Fulton Gap", a still poorly understood lack of planets between 1.5 and 2 times the radius of the Earth. Credit: B.J. Fulton, based on data from NASA Ames, Caltech and University of Hawaii.
Wolf 503b is interesting, first, because of its size. Thanks to the Kepler telescope, we know that most of the Milky Way planets that orbit near their stars are about as big as Wolf 503b, somewhere between the Earth's size and Neptune (4 times larger than Earth's ). . Since there is nothing like them in our solar system, astronomers wonder if these planets are small and terrible rocky versions of Neptune.
A recent discovery also shows that there are many fewer planets whose size is between 1.5 and 2 times the size of the Earth than those that are smaller or larger than that. This fall, called the Fulton Trench, may be what distinguishes the two types of planets from each other, say the researchers in their study of discovery, published in 2017.
"Wolf 503b is one of the only planets with a near-beam radius that has a bright enough star to be studied in more detail, which will limit its true nature," says Björn Benneke, professor at UdeM and member of iREx. and CRAQ. "This provides a key opportunity to better understand the origin of this ray as well as the nature of intriguing populations of 'super-lands' and 'sub-neptunes' as a whole."
The second reason for the interest of the Wolf 503b system is that the star is relatively close to the Earth and therefore very bright. One of the possible follow-up studies for bright stars is the measurement of their radial velocity to determine the mass of planets orbiting around them.
A more massive planet will have a greater gravitational influence on its star, and the variation in the line of sight speed of the star over time will be greater. The mass, as well as the radius determined by Kepler's observations, give the apparent density of the planet, which in turn tells us something about its composition. For example, at its radius, if the composition of the planet is similar to that of the Earth, it should be about 14 times greater than its mass. If, like Neptune, the atmosphere is rich in gas or volatile substances, it would be about half as important.
Because of its brightness, Wolf 503 will also be a prime target for the upcoming James Webb Space Telescope. Using a technique called transit spectroscopy, it will be possible to study the chemical content of the planet's atmosphere and to detect the presence of molecules such as hydrogen and water. This is crucial to check if it is similar to that of the Earth, Neptune or completely different from the atmospheres of the planets of our solar system.
Similar observations can not be made on most of the planets found by Kepler, as their host stars are usually much weaker. As a result, the apparent densities and atmospheric compositions of most exoplanets are still unknown.
"By studying the nature of Wolf 503b, we will learn more about the structure of the planets near the range of radius and more generally about the diversity of exoplanets present in our galaxy," said Peterson. "I look forward to learning more about it."
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