AAS Monday: The Sun, and planets around other stars

Posted by Liz Kruesi
on Monday, June 02, 2014

The 224th meeting of the American Astronomical Society (AAS) officially began this morning, with a welcome address from the current president, David Helfand. Shortly afterward was the first press conference of the meeting, and it focused on the nearest star to Earth — the Sun. (The Solar Physics Division of the AAS is also meeting this week in Boston, so there are a number of Sun-related presentations and posters.)

The dark region, called the umbra, of a sunspot is cooler than its surroundings. Solar astrophysicists recorded outgoing shock waves from the umbra that travel at 45,000 mph (72,000 km/h) into the Sun’s high atmosphere. // photo by New Solar Telescope of the Big Bear Solar Observatory
Our star is a complex beast. AAS Press Secretary Rick Feinberg said it best: The Sun has material moving up and down from its surface to its atmosphere, away from sunspots and also toward sunspots, fast and slow, and on small and big scales. The Sun has a lot going on, and scientists are continuing to learn a great deal about it. They also have an arsenal of observatories at their disposal: the Solar Dynamics Observatory (SDO), Hinode, the Interface Region Imaging Spectrograph (IRIS), and Earth-based telescopes.

With these instruments, solar astronomers can see different resolutions, but more importantly, they can study the Sun’s surface or different regions within the Sun’s atmosphere. That specific area relates to the specific radiation wavelengths they observe, which corresponds to temperature; the Sun’s atmosphere is cooler in the lower levels and hottest in the outer corona. A major question in solar physics is what heats the corona to million-degree temperatures while the solar surface is much cooler, about 5800 kelvins (9980° Fahrenheit).

Alexander Kosovichev of the Big Bear Solar Observatory talked about his team’s observations using the New Solar Telescope (NST) of material moving toward sunspots. The 1.6-meter solar telescope captures 360,000 images per hour, and astronomers can use those photos to create movies of how a sunspot’s umbra (the darkest central region, which is actually the coldest material) changes. Kosovichev’s team saw material flowing into the sunspot for the first time. “Sunspots are not static structures; they are very dynamic,” he said during the press conference. The astronomers also watched shocks leaving a sunspot and traveling into the solar atmosphere at about 45,000 mph (72,000 km/h). The space-based IRIS then sees these shocks as ultraviolet flashes above the sunspot.

Thomas Berger of the National Solar Observatory closed the press conference with an overview of the future Daniel K. Inouye Solar Telescope. This instrument is in the construction stages now and should be up and running in mid-2019. The 4-meter telescope will be able to resolve details as small as 0.03 arcsecond on the sky, which corresponds to structures about 20 miles (30 kilometers) on the Sun. This future scope, with its larger collecting area, also will collect more photons from less luminous atmospheric layers. And this can help scientists piece together how energy transfers from the surface to the atmosphere.

Astronomers have found that Kepler-10c holds 17 times Earth’s mass in a sphere just 2.3 times Earth’s width. This solid planet, shown in the foreground of this illustration, challenges planet-formation theories. // photo by David A. Aguilar (CfA)
The afternoon press conference focused on exoplanet oddities. Over four years of staring at the same 150,000-plus stars, the Kepler spacecraft tracked small dips in light. Such a brightness drop could indicate a planet passing in front of the star. And from the amount of decreased light, astronomers can figure out how wide the crossing planet is. Kepler, however, doesn’t give scientists any information about those planets’ weights, so they can’t learn about their densities or compositions. But astronomers can rely on another method to figure out an exoplanet’s mass: by how much it yanks on its star.

Astronomers have known the star nicknamed Kepler-10 hosts two planets — one about 1.5 Earth-widths and another about 2.3 Earth-widths. They already knew, from studies a few years ago, that Kepler-10b is a rocky and metallic world and weighs about three times Earth. At today’s press conference, Dimiar Sasselov of the Harvard-Smithsonian Center for Astrophysics announced that his team made extremely precise measurements of how much the other planet, Kepler-10c, tugs on the star. They found that this world holds 17 Earth masses in a sphere just 2.3 times as wide as Earth. That’s an extremely heavy planet. Kepler-10c, therefore, has a density of 7.5 grams/cubic centimeter. (For comparison, Earth’s density is 5.5 grams/cubic centimeter.) The scientists say it probably has the same type of material that Earth does (rocky and metal) but because it’s so much more massive, its materials would be more compressed. Because Kepler-10c is a massive, solid world, astronomers have playfully categorized it as a "mega-Earth."

New exoplanets always make interesting news, but why are they important? It tests astronomers’ theories – and if they’re understanding the rules of planet formation. This world, Kepler-10c, doesn’t fit in with how scientists think planets form. It lies closer to its star than Mercury does to our Sun, so how could the evolving planet grab that much material? The leading theory of planet formation says that the process begins with a disk around a young star. That disk is 98–99 percent gas and just 1–2 percent solids. Small rocky conglomerates collect more solid material, which can interact gravitationally — meaning, collide and merge. The problem is that when you have objects the mass of multiple-Earths colliding, they don’t just stick together like putty. Instead, a lot of debris will fly out as they slam into one another. Astronomers have no idea how the dense rocky exoplanet could have formed.

Here are a few other discoveries described during the press conferences today:

  • Solar astrophysicists using the NST watched as granules (convecting modules of high-temperature gas called plasma) evolved on the Sun’s surface. As these granules warp they power a surge of energy. You can read more about this discovery in the news release.
  • Astronomers found two planets orbiting an aging star that will be eaten by the star in the relativity near future: 129 million years and 155 million years. This was the first time astronomers found a system with two exoplanets that will be destroyed by their star.
  • Exoplanets that lie in the habitable zones of their red dwarf parent stars may be so close in that the stars’ magnetic fields strip the worlds of their atmospheres. Without an atmosphere, a planet can’t host life.

This was just a taste of what’s going on at the AAS meeting in Boston this week. Tomorrow promises to be another exciting day.

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