the first of many distant worlds

HIP 65426 b is a gas giant exoplanet with a mass of 6-12 Jupiters! It takes it 630.7 years to complete a single orbit around its host star (HIP 65426) and is 92AU away from it. A few days ago, the James Webb Space Telescope directly imaged the exoplanet using the infrared portion of the electromagnetic spectrum. Webb is our most powerful telescope yet and we can see its powers in full display here! 

The planet is quite young relative to other planets - our Earth is 4.5 billion years old, while this planet is about 15 to 20 million years old. But how do we know the age of planets which are so far away from us?! Well, we begin by measuring the age of the host star by looking at its spectral type. We then just assume that the star's planets are the same age as the star itself - that's it! Is this a reasonable assumption to make, though? Funnily enough, my 3rd year research project at university is heavily linked to this question, so I'll do my best to answer it! The answer is yes; planets form in these discs known as 'protoplanetary discs' which surround a star for the first few million years following its birth. Since all planets in the system must form in this protoplanetary disc (which is active for only a blip of the star's existence), it's fine to say that exoplanet age = star age. Although, now that I'm writing this, I realised that planets can also be tidally captured. How do we differentiate these tidally captured planets from planets which formed in the protoplanetary disc of the original host star? I honestly don't really know - I'm currently drafting an email to my supervisor to ask her because I'm very curious myself. If I had to guess, though, I would say that the probability of a planet being tidally captured is quite low to begin with. Furthermore, if the composition of the planet differs heavily from the other planets in the system, then it probably did not form within the star's protoplanetary disc. 

Astronomers originally discovered HIP 65426 b in 2017 using the SPHERE instrument on the European Southern Observatory's Very Large Telescope in Chile and took images of it using short infrared wavelengths of light. Webb's view, at longer infrared wavelengths, gives us much more detail than ground-based telescopes due to the intrinsic infrared glow of the Earth's atmosphere.

Usually, detecting these exoplanets are quite hard due to resolution reasons. Stars are so much brighter than planets - in this case, HIP 65426 b is 10000 times dimmer than its host star in the near-infrared and a few thousand times fainter in the mid-infrared. However, HIP 65426 b is quite far from its host star, allowing us to differentiate between the two. Webb’s Near-Infrared Camera (NIRCam) and Mid-Infrared Instrument (MIRI) are both equipped with coronagraphs, which are sets of tiny masks that block out starlight, enabling Webb to take direct images of certain exoplanets like this one. NASA’s Nancy Grace Roman Space Telescope, projected to launch later this decade, will demonstrate an even more advanced coronagraph.

While this isn't the first image of an exoplanet taken from space (Hubble has captured images of exoplanets previously), HIP 65426 b points the way forward for Webb's exoplanet exploration.

published: 02/09/22 by kaan evcimen