These will be the first targets of the James Webb Telescope

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During its first months of operation, the James Webb Telescope will focus on two planetary systems housing giant worlds that are still very young. The main objective of these missions will be to determine how these planets are formed.

A few days after its launch next October, the James Webb Telescope will be positioned around the Lagrange point L2, at 1.5 million kilometers of the Earth on the side opposite the Sun. Protected by a sunshade twenty-two meters long and eleven meters wide composed of five very thin layers of extremely reflective materials, it will probe the early Universe, but will also focus on exoplanets.

In recent months, teams of astronomers from around the world have submitted their study proposals. And inevitably, we had to make choices. Some projects have already been validated.

Big and young planets

Most of this work will focus on objects that have no equal in our Solar System, but which have the advantage of evolving at distances far from their star. In addition to not “bathing” in the light of their host, these worlds could also “shine” in the light. infrared light, provided they are hot enough.

Sensitive to these wavelengths and equipped with a cornograph responsible for blocking starlight, the James Webb Telescope will attempt to image these planets directly.

Two of the first targets of the observatory will be the planetary systems 51 Eridani and HR 8799. Found at 96 light years Earth, the first is home to a giant planet measuring about twice the mass of Jupiter and evolving at around 17 billion km of a star similar to the Sun. Found at 133 light years, the second system is home to four planets. These are at least ten times more massive than Jupiter, each evolving to more than 22 billion kilometers of a star slightly more massive than the Sun.

These outer exoplanets are relatively young, ranging from tens of millions to just a few hundred million years old. As a reminder, our Solar System was formed over 4.5 billion years ago. The images of these exoplanets will therefore be essentially images of still very hot “baby planets”.

The star HR 8799 imaged by Hubble in 1998. A coronograph blocks part of its light. A software made it possible to subtract more, enough to reveal three of the four planets of the system. Credits: NASA, ESA and R. Soummer (STScI)

How are the planets formed?

One of the main objectives will be to determine how these worlds were formed. Were they created by an accumulation of material in the disc surrounding its young star enriched with heavy elements, just like Jupiter in our system? Or, did they form from the collapse of a cloud of hydrogen, like a star, before shrinking under the relentless pull of gravity?

To answer these questions, researchers will use JWT instruments to probe the atmospheres of these planets more deeply. For example, NIRCam will be able to detect and measure atmospheric fingerprints of elements such as methane. It will also look at cloud characteristics and surface temperatures.

In the meantime, engineers and technicians are preparing for a final deployment of the telescope’s main mirror. If all goes as planned, it will then be shipped to Guyana where it will be launched on October 31st aboard a Ariane 5 rocket.





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