Since the first planets were discovered in the early 1990s, more have been found, and the number has now passed 5,000.
A team of astronomer is taking that number backwards. Three previously detected planets, and maybe a fourth, that aren't planets at all, have been found. They are stars.
Small stars, but still stars, are masquerading as planets in the observations.
Most planets are found using the transit method, where we see their edge-on and once per orbit they pass in front of their host star, making a mini-eclipse. We can't see the planet directly, but we can see the light from the star. A big planet blocks more light than a small one, so how much it dims depends on the size of the star and the planet. We can calculate the size of the planet if we know how big the star is and how dim it is. There are other methods for estimating the size of a star.
Thousands of planets were detected via their transits by NASA's Kepler space observatory, which stared at one patch of sky for years. NASA launched TESS to look for planets. It observed many of the same stars, but in the time between the missions, better numbers for many star distances have been measured.
The estimates for diameters of the majority of stars were very similar for both missions, but some stood out as pretty discrepant. The stars that were off were very far away. It's no surprise that stars that far away have hard to measure distances. The stars in reality are farther away than the distances used by the program.
The stars are bigger if they are farther away. If you look at the planets around those stars, they have to be bigger than their host stars.
They looked at the ratio of the sizes of the planets and their stars in the TESS data and compared them. Four planets jumped out as being much larger than they had been measured. The planets were named after them.
They looked at the data first. The host star became brighter and dimmer over time. The period was the same as the planet's. That is very suspicious, why would a star be brighter and dimmer at the same rate as the planet?
The star's brightness change is consistent with it not being round, but instead being egg or teardrop shaped. If the second object in the system is very large, the first object will be pulled into a shape. The second object must be over 100 times the mass of Jupiter to account for the star's shape.
Oof. An object more than 80 times Jupiter's mass can cause enough pressure in its core to ignite nuclear fusion. It's a low-mass star. It looks like it's a star, not a planet right away. The host star is three times the diameter of Jupiter and twice as large as previously thought. It's too big, planets can't get that size without becoming stars. Again, it must be a star.
They found similar results with all of them. The planets are bigger because the stars are farther away than first thought. There are arguments that the system must be a star because it drops in light a lot more than you would expect if it were a planet. If it is a star, the total light from the system will be dimmer than for a planet.
The star is four times bigger than thought, so the planet is a little under Jupiter's size. It isn't enough to completely rule it out as a planet, but it is close enough that they think its planetary status is questionable.
What does this mean for exoplanets? Are we about to find a lot more of them?
That is unlikely. The majority of the host stars that were looked at didn't change. The distances to stars are better known thanks to the observations of the Gaia mission. A billion.
The refs had to call back a few points if you were keeping an eye on the score. 3 or 4 out of 5,000 is a pretty low correction, and only sets us back a few days. We will hit that 1.6 kilometerstone very soon.
The sky is filled with planets, probably as many as stars. It's a lot of planets, and we can find out if a few aren't real. The whole point of science is to correct mistakes and learn from them. The lesson is not too painful.
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