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Warwick and MIT teams find two dwarf stars that appear to be eating their companions
Two Nature Astronomy studies report a red dwarf drawing material off a brown dwarf and a white dwarf apparently fed by a second-generation planet. Each rests on a single system, so together they show that such feeding happens and leave open how often it does.
The Scientist · Science desk
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What happened
- The Warwick team found unusual signatures of zinc, copper and niobium on the white dwarf, along with periodic dimming that points to an orbiting planet.
- The red dwarf appears to be pulling material off its brown dwarf companion, and the transfer could go on for millions of years.
- Scientific American reports that Householder's result suggests the sun might one day slowly strip away Earth's atmosphere instead of consuming the planet quickly.
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Why it matters
- constraint With one system per star type, the studies cannot supply the survival rate that any general statement about planets around dwarf stars would need.
- capability If planets can form around stars that have already died, white dwarfs become places to look for planets born after their star's death.
- precedent Searches for a star eating a planet, the pairing Householder wants to find next, now have a slow mode to look for, and a process that runs for millions of years gives them more time to catch one underway.
The white dwarf case is an inference from two kinds of observation read together: the star's unusual chemistry and its regular dimming [3][4]. From the pair, the Warwick team concludes that a planet formed after the star died and that the planet's atmosphere now feeds it [2][5]. Williams said the finding suggests "planetary systems can experience a rebirth," something astronomers have not seen in our own solar system [6].
The red dwarf's companion is a brown dwarf [8]. Scientific American's account groups brown dwarfs with the failed stars [13]. So neither study, as reported, follows a confirmed planet being consumed. One companion is a brown dwarf, and the other is a planet inferred from dimming and chemistry [14].
I think the red dwarf result is the more useful of the two for models of close companions. According to the account, such pairs usually go one of two ways: the star leaves its companion alone, or it devours it fast [9]. This one adds a slow, steady middle case [8]. A process that slow stays underway far longer than a rapid one. If such pairs are common, surveys have a better chance of catching them mid-transfer [18].
The sample is one system per star type [15]. One system shows an interaction can occur. It cannot give a rate, and whether companions usually survive around dwarf stars is a question about rates. The press account does not name either system or give a transfer rate, a planet mass or the period of the dimming. Householder, the MIT graduate student who led the red dwarf study, is already thinking about other pairings [11]. "We're excited to see if we can find a star eating a planet or maybe even two planets eating each other," he said [12].
Both lead authors apply their findings to the sun. The sun is a yellow dwarf, so it is neither of the types studied here [13]. Williams said a second-generation planet could in theory form after the sun dies [16], which carries a result from a single white dwarf over to a different kind of star [15]. "The key takeaway is that planets don't exist in isolation, but their evolution is directly tied to the star they orbit," he said [7].
What to watch
- A measured transfer rate for the red dwarf and brown dwarf pair, the figure needed to test whether the process really runs for millions of years.
- More white dwarfs showing both unusual metals such as zinc, copper and niobium and periodic dimming, enough to turn one rebirth case into a sample.
- Whether Householder's group finds a star consuming a planet, as opposed to a brown dwarf.