Over the next several weeks we’re going to be saying a bit more about the various Herschel first science results that were discussed at the meeting in Madrid just before Christmas. These are very early results that have not been fully digested or understood, but they give a taste of what is to come once these various projects collect many times more data on their targets and the scientists working on them have had more time to understand the results.
The first set of new results we’re going to discuss are observations of Trans-Neptunian Objects – dwarf planets at the edges of our own solar system like Pluto, Makemake and Eris. My field is extragalactic astronomy and cosmology, so my professional work doesn’t usually involve such things, but I do find them interesting on several levels. Firstly, they’re the most distant objects in our solar system and the least affected by the Sun, so they’re the closest thing we have to the very earliest objects to form around the Sun. Secondly, they’re so distant that they could be thought of as being on the edge of interstellar space – they are, in a sense, the boarder guards of the solar system. Finally, and this isn’t really a scientific point, I’m working on some science fiction set in trans-Neptunian space and beyond so any new results have a bearing on what I’m going to write.
What has Herschel seen of TNOs? So far a handful have been observed with the SPIRE and PACS instruments. The aim is to better understand their basic properties – size, mass, albedo – by measuring the amount of far-IR light that they emit. Since they’re small, maybe 1000km in diameter, distant and cold, they are very faint and require a large telescope like Herschel for them to be detectable. The second picture shows what PACS saw when it observed Pluto. This is a ‘differencing’ observation, where the target is observed in one position on the detector, then moved to another and the two are subtracted. This process is repeated several times so you see a positive image at the centre and other positive and negative images at the reference positions.
PACS observed six TNOs, including Pluto. Two were fainter than predicted based on what we thought we knew about them, one was significantly brighter and three were about right. Meanwhile two TNOs were observed by SPIRE and here too things weren’t quite what was expected.
What does all this mean? We don’t know just yet, but since the results aren’t quite what was predicted we’re clearly learning something new about these objects. They might be more reflective – have higher albedo – than we thought, their surfaces might be more rocky, or their sizes might not be what is expected.
As with all the data we’ve seen from Herschel so far, there’s a lot more work to be done before we understand what the telescope is telling us. But it’s clear that Herschel is going to bring new light to our own solar system along with the rest of the observable universe!