It looks like Oracle removed Java Web Start from Java 8 for Solaris. javaws isn't available anymore in Java 8u51.
From the compatibility guide:
The 64-bit binaries do not contain deployment tools such as Java Web Start and Java Plug-in, therefore desktop integration is no longer required.
What do they mean with "required"? It's not possible anymore. It's not possible to launch a Web Start application on Solaris with Java 8.
Does anyone know an alternate way (standard, no 3rd party stuff) to start a Web Start application on Solaris with Java 8?
The "required" means that, as the build for the java 8 for solaris is only available in 64bit version; therefore, the Java Plug-in and Java Web Start is no longer required to be installed as those two things are not available for 64-bit version of Solaris (you can check the link here https://www.java.com/en/download/help/webstart_64bits.xml).
I'm afraid I cannot found any way to run Java Web Start in "standard" way on Java 8. The easiest workaround for this is, of course, to install the older version of 32-bit java such as version 7.