Tag: HK Tauri

ALMA telescope catches live planet-forming action for the first time

The ALMA telescope in Chile has, for the first time, observed a star system that might be in the early stages of planet formation. The picture has astronomers drooling over it because the study of the origins of planets has until now been limited to simulated computer models and observations of planets made after they formed.

ALMA image of the protoplanetary disc around HL Tauri.
ALMA image of the protoplanetary disc around HL Tauri. Image: ALMA (ESO/NAOJ/NRAO)

According to a statement put out by the European Southern Observatory (ESO), the observation was one of the first made with the ALMA, which opened in September 2014 for a ‘Long Baseline Campaign’ (ESO is the institution through which European countries fund the telescope). ALMA uses a technique called very-long baseline interferometry to achieve high resolutions that lets it observe objects hundreds of light-years away in fine detail. It makes these observations in the millimeter/sub-millimeter range of wavelengths; hence its name: Atacama Large Millimeter/sub-millimeter Array.

The image shows a disc of gas, dust and other debris orbiting the star HL Tauri, located about 450 light-years from Earth. A system like this is originally a large cloud of gas and dust. At some point, the cloud collapses under its own gravitation and starts to form a star, further accruing matter from the cloud and growing in size. The remaining matter in the cloud then settles into a disc formation over millions of years around the young star.

In the disc, the gas and dust continue to clump, this time into rocky lumps like planets and asteroids. This is why the disc is called a proto-planetary disc. As a planet forms and its gravitational pull gets stronger, it starts to clear a space in the disc of matter by either sucking it for itself or knocking it out. The gaps that are formed as a result are good indicators of planet formation.

According to the ESO statement, “HL Tauri’s disc appears much more developed than would be expected from the age of the system [less than 100,000 years]. Thus, the ALMA image also suggests that the planet-formation process may be faster than previously thought.”

An annotated image showing the protoplanetary disc surrounding the young star HL Tauri.
An annotated image showing the protoplanetary disc surrounding the young star HL Tauri. Image: ALMA (ESO/NAOJ/NRAO)

In the Solar System, similar gaps exist called Kirkwood gaps. They represent matter cleared by Jupiter, whose prodigious gravitational pull has been pushing and pulling the orbits of asteroids around the Sun into certain locations. In fact, Jupiter’s movement within the Solar System – first moving away, then toward, and then away once more from the Sun – has been used to explain why the material composition of some asteroids between Mars and Jupiter is similar to those of Kuiper Belt objects situated beyond the present orbit of Neptune. Jupiter’s migration mixed them up.

Similarly, the gaps forming around HL Tauri, though they may represent planetesimals, may not result in planets in the exact same orbits as they could move around under the influence of subsequent gravitational disruptions. They could acquire unexpectedly eccentric orbits if their star system comes too close to another, as was found in the nearby binary star system HK Tauri in July 2014. Or, the gaps are probably being emptied by the gravitational effects of an object in another gap.

However, astronomers think the presence of multiple gaps is likely evidence of planet formation more than anything else.

At the same time, the resolution in the image is 7 AU (little more than Jupiter’s distance from the Sun), which means the gaps are very large and represent stronger gravitational effects.

Astronomers will use this and other details as they continue their investigation into the HL Tauri system and how planets – at least planets in this system – form. The Long Baseline Campaign, which corresponds to the long-baseline configuration of the ALMA telescope that enabled this observation, will continue into December.

Why do tilted/eccentric orbits form?

For all its mysteries, the Solar System is uniquely ideal in many ways. For one, while it has rocky inner planets and giant, gassy outer ones, astronomers have found that elsewhere, massive exoplanets often orbit close to their stars, as if they formed at a greater distance and then moved in. For another, the orbits of the planets around the Sun are nearly circular and on the same plane. This is unlike what the Kepler space telescope has found in other star systems, where exoplanets often have eccentric orbits and orbits that are tilted with respect to the star’s equator. A July 31 paper in Nature adds one more system to this difference, but this time with some information about why eccentric/tilted orbits could form. Eric Jensen (Swarthmore College) and Rachel Akeson (NASA) discuss the binary star system HK Tauri, whose stars are one and four million years old with 386 AU between them. More importantly, one star, HK Tau B, has a disk of gases and dust around itself that, when viewed from Earth, appears to be edge-on, masking the starlight and making for easier imaging in visible light. Such a disk is called a protoplanetary disk because it contains the materials from which planets form.

The other star, HK Tau A, is also inclined as seen from Earth but not edge-on, so the bright starlight bleaches observations made in visible light. So Jensen and Akeson observed it in millimeter-wavelength light using the ALMA telescope in Chile and calculated its disk’s rotation rate (specifically, by observing Doppler effects on the carbon-monoxide emission lines). HK Tau A is inclined at 43° ± 5°; HK Tau B is inclined at 85° ± 1°. Depending on which direction the disks are rotating in, Jensen & Akeson find that they’re inclined relative to each other at at least 60° and at most 68° (± 3° for both). So planets forming in their protoplanetary disks could end up with eccentric and tilted orbits around their individual stars as a result of being perturbed by gravitational effects from their neighbor. As Jensen told Centauri Dreams: “Our results show that the necessary conditions exist to modify planetary orbits and that these conditions are present at the time of planet formation, apparently due to the formation process of a binary star system. We can’t rule other theories out, but we can certainly rule in that a second star will do the job.”

References

Jensen, E. & Akeson, R., Misaligned protoplanetary disks in a young binary star system, Nature 511, 567–569 (31 July 2014). doi:10.1038/nature13521