Science

Hubble observes the formation of a giant planet

In recent years, models of planetary formation have grown thanks to the increasing sensitivity and resolution of observation instruments. However, when it comes to exoplanets, distance always makes observations difficult. But recently, a team of astrophysicists were able to use Hubble to directly image, with astonishing precision, the growth of a giant gas exoplanet in the disk of its host star. Important results that will improve the accuracy of current models.

We just don’t know much about the growth of giant planets. This planetary system gives us the first opportunity to see matter falling on a planet. Our results open a new field for this field of research Says Brendan Bowler of the University of Texas.

Although more than 4000 exoplanets have been cataloged to date, only about 15 have been directly imaged by telescopes. And the planets are so far away and small, they’re just dots in the best photos. The team’s new technique using Hubble to directly image this planet opens a new avenue for further research on exoplanets, especially during a planet’s formative years.

This huge exoplanet, designated PDS 70b, revolves around the orange dwarf star PDS 70, which is already known to have two actively forming planets inside a huge disk of dust and gas circling the star. The system is located 370 light years from Earth in the constellation Centauri.

An estimate of the planet’s growth rate

” VShe system is so exciting because we can witness the formation of a planet. This is the youngest authentic planet that Hubble has ever directly imaged Says Yifan Zhou, also from the University of Texas. Millions of years old, the planet is still gathering material and accumulating mass.

Hubble’s ultraviolet (UV) sensitivity offers a unique look at the radiation from extremely hot gases falling on the planet. Hubble’s observations have allowed astrophysicists to estimate how fast the planet is gaining mass. UV observations, which add to the body of research on this planet, allowed the team to directly measure the planet’s growth rate for the first time.

The distant world has already accumulated up to five times the mass of Jupiter over a period of about five million years. The current measured accretion rate has declined to the point where, if the rate remained stable for another million years, the planet would only increase by about 1 / 100th of Jupiter’s mass.

Zhou and Bowler point out that these observations are a single snapshot in time – more data is needed to determine whether the rate at which the planet is adding mass is increasing or decreasing. ” Our measurements suggest that the planet is at the end of its forming process “.

Growth from the circumstellar disc

The young PDS 70 system is filled with a disk of primordial gas and dust that provides fuel to fuel the growth of planets throughout the system. The planet PDS 70b is surrounded by its own disk of gas and dust that siphons material from the much larger circumstellar disk.

The researchers hypothesize that the magnetic field lines extend from its protoplanetary disk to the exoplanet’s atmosphere and carry matter across the planet’s surface.

The European Southern Observatory’s VLT captured the first clear image of a forming planet, PDS 70b, around a dwarf star in 2018. The planet stands out as a bright spot to the right of the center of the image, which is blackened by the coronograph used to block light from the central star. © ESO, VLT, André B. Mülle

If this material follows the columns of the disc on the planet, it would cause local hot spots. These hot spots could be at least 10 times hotter than the temperature of the planet. These hot plates were found to glow violently in UV light Zhou explains.

These observations provide insight into the formation of giant gas planets around our sun 4.6 billion years ago. Jupiter was certainly formed in the same way, from a similar disk. Its main moons would also have formed from the remains of this disc.

Overcome the obstacle of stellar luminosity

A challenge for the team was to overcome the brightness of the host star. PDS 70b orbits roughly the same distance as Uranus from the Sun, but its star is more than 3,000 times brighter than the planet at UV wavelengths. While Zhou was processing the images, he very carefully removed the glare from the star to leave only the light emitted by the planet. In doing so, he improved the limit of how far a planet can be from its star in Hubble observations by a factor of five.

pds70b exoplanete hubble coronograph
Hubble’s observations highlight the planet PDS 70b. A coronograph on the Hubble camera blocks glare from the central star so that the planet is directly observed. © Joseph DePasquale

Yifan’s observation strategy and post-processing technique will open new windows to study similar systems, if not the same system, over and over with Hubble. With future observations, we could potentially find out when the majority of gas and dust will fall on their planets, and if they do so at a constant rate. », Concludes Bowler.

Sources: The Astronomical Journal

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