TW Hydrae: The Planet That Shouldn’t Exist

12. 22. 13 by Jaime Trosper

In a physical world that contains phenomena such as pulsars, neutron stars, and black holes, one would think that we have a pretty good grasp on the basics–how our universe evolves and functions; however, you might be surprised to know that some of the basics are actually difficult to discern. One of those problematic areas is planetary formation. That’s right.. .we really have a somewhat-amateur understanding of how  planets form, despite actually living (and evolving) on one.

Our current models say that our solar system, and other planetary systems circling other stars, form from a protoplanetary accretion disk encircling a young star. This elliptical disk of material is typically comprised of gas, ice, rock, and grain. These build up slowly before ultimately coalescing into a planet, shaped by gravity. This process is thought to take tens of millions of years to occur from start to finish, and of course the amount of material concentrated around the protoplanetary disk dictates the number of planets a star has (and their size), which means that the planets should all form during a similar time-frame and there should also come a period when planetary formation is no longer possible.

However, we may have to rethink this theory.

Recently, Hubble took a closer look at a youngish orange-dwarf star (similar in size and mass to our parent star) that is located a mere 176 light-years from Earth in the constellation of TW Hydrae. Hubble spotted a large gap in the protoplanetary disk encircling the star. The gap, which is attributed to a large, yet-to-be-detected planet forming (similar to a snow plow), stretches more than 1.9 billion miles wide (the disk itself is about 41 billion miles wide, located about 7.5 billion miles (66 billion kilometers) away from the star), which is perplexing given the young age of the star. (Less than 10 million year sold).

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The star, known simply as TW Hydrae, has been a popular star for astronomers a while now, particularly since the star its positioned in such a way that its poles align directly toward Earth, giving us an opportunity to study the star’s protoplanetary disk (Said disk is known to host a substantial amount of water too), which should not be in the process of forming planets just yet (though I must say that this is not the first claim of a detected exoplanet. The last of which, occurred back in 2007 and was ultimately discredited after it was shown to be a starspot.).

As I mentioned earlier, planetary bodies are thought to form over the course of tens of millions of years, allowing for the needed time for enough material to collect bit by bit from the protoplanetary disk. This is obviously problematic, as TW Hyrade is only an estimated 8 to 10 million years old. Furthermore, the distance is also disconcerting. The planet (estimated to be between 6 and 28 times more massive than Earth) is orbiting its parent star from a great distance of 7.5 billion miles (on average, Pluto is a mere 3,667,000,000 miles from the sun).

According to John Debes of (from The Space Telescope Science Institute): “A planet 7.5 billion miles from its star should take more than 200 times longer to form than Jupiter did at its distance from the sun because of its much slower orbital speed and the deficiency of material in the disk. Jupiter is 500 million miles from the sun and it formed in about 10 million years.” Therefore.. this planet should not exist.. at least it shouldn’t based on our current models.

Also hindering the stars’ ability to produce a planet so quickly is the noticeable lack of dust grains located in the outer region of the protoplanetary disk. (Starting out at about 5.5 billion miles from the star) No grains of material, no planet. So assuming this discovery pans out, it could be yet another strike against conventional models.

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