The first direct image of a baby planet being born! (maybe!)(but probably!)

Bad Astronomy
The entire universe in blog form
Oct. 20 2011 7:00 AM

The first direct image of a baby planet being born! (maybe!)(but probably!)

Astronomers may have, for the first time, directly imaged a planet still in the process of formation, gathering material from a debris disk surrounding its parent star.

First: Holy Haleakala!

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  


Second: note the use of the word "may". It looks to me like it's real, though.

Third: Oh, you want to see the picture? Well, let me do the honors:

The alleged planet, called LkCa 15b, is the blue spot in the image. The red shows material which is most likely accumulating onto the planet itself, building up its mass. The central star isn't seen in this image because its light has been blocked out so the fainter material near it can be seen. The star's position is marked by the star icon.

The image is in the infrared, taken using the monster Keck telescope in Hawaii. What's shown in red is light at a wavelength of 3.7 microns (roughly five times what the human eye can see) and blue is from 2.1 microns, about three times what we can see. Warm material around the star is best seen at these wavelengths. If this is a planet, it's at a temperature of about 500 - 1000 K (440° - 1340° F), and has a mass roughly six times that of Jupiter, or about 2000 times the Earth's mass.

So is it a planet? I read the journal paper (PDF) the astronomers wrote, and they make a pretty good case. It's not a background object that's aligned by chance with the star; they observed the system over the course of a year and the object is moving along with the star in the sky, so they're clearly connected. The authors also very carefully eliminate other possible explanations (a low-mass star instead of a planet, a large clump of dust, reflected light from the star), and come to the conclusion that this data is best explained as a young planet in the act of forming. It looks pretty good to me... certainly enough so that I added it to my gallery of directly imaged exoplanets! To make things simple, from here on out in this post I'll assume it's real.

This is a pretty interesting system. The star, called LkCa 15, is only about 2 million years old, and about as massive as the Sun. It's located pretty close to us as these things go, about 450 light years away in the constellation of Taurus. It's known to have a pretty big disk of dust circling it -- called a protoplanetary disk -- stretching well over 20 billion km across. By comparison, Neptune's orbit around the Sun is about 9 billion km across! So it's a big disk.

Also? It has a hole in it. Here's an image of the disk taken in the far infrared comparing the disk to the planet image:

[Click to protoplanetate.]

The disk's hole is about 8 billion km across. Disks like this are seen around other stars, and it's generally thought that the hole is caused by a planet orbiting inside that region sweeping up material. In this case, that looks to be true! If the planet is in a circular orbit, it's about 2.5 billion kilometers from its star, a little closer to its star than Uranus is from the Sun (it's not known if the orbit is circular or elliptical; that'll take a few years of observations as the planet physically moves around the star and the orbit can be calculated). The planet is much hotter than you might expect, but that's because it's so young: material is falling onto it, heating it up. That's why it's glowing in the infrared.

If all this is hard to picture, here's a nice artist's illustration of the situation:

That should help. The star is in the center of the disk, with the planet orbiting in the cleared-out region. Material is still falling onto the planet, so it's still physically in the process of forming.

Nothing like this has been seen before in a planet so young! That's scientifically quite important. Our models of how planets form are complex, and we need detailed observations to see if the models are correct or not. Since planet formation is a process, we need observations of it at different stages, including very early on. That's crucial, since it represents the transition period between the time before planets start to form in the disk, and the time when the planets are all finished and tidied up. We've seen both of those before, so this observation is a first.

All in all, very, very cool. Only a handful of planets have been directly imaged (see the gallery below), so even adding another single case is important; adding one at this stage in its birth process is fantastic.

We'll be seeing more and more of these as time goes on. The astronomers, Kraus and Ireland, are planning more observations of LkCa 15... and it's just one example of a star that's forming in that region of space. They've targeted several others as well, so who knows what else they'll find poking around in there?

Image credit: Kraus and Ireland. Art credit: Karen L. Teramura, UH IfA.

[Below is a gallery of exoplanets that have been directly imaged using telescopes on ground and in space. Click the thumbnail picture to get a bigger picture and more information, and scroll through the gallery using the left and right arrows.]



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